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     15   1.3  christos <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
     16   1.1  christos   <nav>
     17   1.1  christos     <ul>
     18   1.3  christos       <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
     19   1.3  christos 	  database</a></li>
     20   1.7  christos       <li><a href="#naming">Timezone identifiers</a></li>
     21   1.1  christos       <li><a href="#abbreviations">Time zone abbreviations</a></li>
     22   1.3  christos       <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
     23   1.3  christos 	  database</a></li>
     24   1.1  christos       <li><a href="#functions">Time and date functions</a></li>
     25   1.1  christos       <li><a href="#stability">Interface stability</a></li>
     26   1.9  christos       <li><a href="#leapsec">Leap seconds</a></li>
     27   1.1  christos       <li><a href="#calendar">Calendrical issues</a></li>
     28  1.18  christos       <li><a href="#planets">Time and time zones off earth</a></li>
     29   1.1  christos     </ul>
     30   1.1  christos   </nav>
     31   1.1  christos 
     32   1.3  christos <section>
     33   1.3  christos   <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
     34   1.1  christos <p>
     35   1.3  christos The <a
     36   1.3  christos href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
     37   1.3  christos database</a> attempts to record the history and predicted future of
     38  1.10  christos civil time scales.
     39   1.3  christos It organizes <a href="tz-link.html">time zone and daylight saving time
     40   1.3  christos data</a> by partitioning the world into <a
     41   1.4  christos href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a>
     42   1.3  christos whose clocks all agree about timestamps that occur after the <a
     43   1.3  christos href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
     44   1.3  christos (1970-01-01 00:00:00 <a
     45   1.3  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
     46   1.3  christos title="Coordinated Universal Time">UTC</abbr></a>).
     47   1.3  christos Although 1970 is a somewhat-arbitrary cutoff, there are significant
     48   1.3  christos challenges to moving the cutoff earlier even by a decade or two, due
     49   1.3  christos to the wide variety of local practices before computer timekeeping
     50   1.3  christos became prevalent.
     51  1.12  christos Most timezones correspond to a notable location and the database
     52  1.12  christos records all known clock transitions for that location;
     53  1.12  christos some timezones correspond instead to a fixed <abbr>UTC</abbr> offset.
     54   1.1  christos </p>
     55   1.1  christos 
     56   1.1  christos <p>
     57   1.4  christos Each timezone typically corresponds to a geographical region that is
     58   1.4  christos smaller than a traditional time zone, because clocks in a timezone
     59   1.4  christos all agree after 1970 whereas a traditional time zone merely
     60   1.4  christos specifies current standard time. For example, applications that deal
     61   1.4  christos with current and future timestamps in the traditional North
     62   1.4  christos American mountain time zone can choose from the timezones
     63   1.4  christos <code>America/Denver</code> which observes US-style daylight saving
     64  1.12  christos time (<abbr>DST</abbr>),
     65  1.12  christos and <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>.
     66   1.4  christos Applications that also deal with past timestamps in the mountain time
     67   1.4  christos zone can choose from over a dozen timezones, such as
     68   1.4  christos <code>America/Boise</code>, <code>America/Edmonton</code>, and
     69   1.4  christos <code>America/Hermosillo</code>, each of which currently uses mountain
     70   1.4  christos time but differs from other timezones for some timestamps after 1970.
     71   1.4  christos </p>
     72   1.4  christos 
     73   1.4  christos <p>
     74  1.12  christos Clock transitions before 1970 are recorded for location-based timezones,
     75   1.1  christos because most systems support timestamps before 1970 and could
     76   1.1  christos misbehave if data entries were omitted for pre-1970 transitions.
     77   1.1  christos However, the database is not designed for and does not suffice for
     78   1.1  christos applications requiring accurate handling of all past times everywhere,
     79   1.1  christos as it would take far too much effort and guesswork to record all
     80   1.1  christos details of pre-1970 civil timekeeping.
     81   1.3  christos Although some information outside the scope of the database is
     82   1.2  christos collected in a file <code>backzone</code> that is distributed along
     83   1.2  christos with the database proper, this file is less reliable and does not
     84   1.2  christos necessarily follow database guidelines.
     85   1.1  christos </p>
     86   1.1  christos 
     87   1.1  christos <p>
     88   1.3  christos As described below, reference source code for using the
     89   1.3  christos <code><abbr>tz</abbr></code> database is also available.
     90   1.3  christos The <code><abbr>tz</abbr></code> code is upwards compatible with <a
     91   1.3  christos href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
     92   1.3  christos standard for <a
     93   1.3  christos href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
     94  1.22  christos As of this writing, the current edition of POSIX is
     95  1.22  christos <a href="https://pubs.opengroup.org/onlinepubs/9799919799/">POSIX.1-2024</a>
     96  1.22  christos (The Open Group Base Specifications Issue 8, IEEE Std 1003.1-2024).
     97  1.22  christos Unlike its predecessors
     98  1.22  christos <a href="https://archive.org/details/POSIX.1-1988">POSIX.1-1988</a> through
     99  1.22  christos <a href="https://pubs.opengroup.org/onlinepubs/9699919799/">POSIX.1-2017</a>,
    100  1.22  christos POSIX.1-2024 requires support for the
    101  1.20  christos <code><abbr>tz</abbr></code> database, which has a
    102  1.20  christos model for describing civil time that is more complex than the
    103  1.22  christos standard and daylight saving times required by earlier POSIX editions.
    104   1.4  christos A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
    105   1.3  christos have more than two changes per year, these changes need not merely
    106   1.3  christos flip back and forth between two alternatives, and the rules themselves
    107   1.3  christos can change at times.
    108   1.9  christos Whether and when a timezone changes its clock,
    109  1.22  christos and even the timezones notional base offset from <abbr>UTC</abbr>,
    110   1.9  christos are variable.
    111  1.22  christos It does not always make sense to talk about a timezones
    112  1.22  christos base offset, which is not necessarily a single number.
    113   1.1  christos </p>
    114   1.1  christos 
    115   1.3  christos </section>
    116   1.1  christos 
    117   1.3  christos <section>
    118   1.7  christos   <h2 id="naming">Timezone identifiers</h2>
    119   1.1  christos <p>
    120   1.7  christos Each timezone has a name that uniquely identifies the timezone.
    121   1.1  christos Inexperienced users are not expected to select these names unaided.
    122   1.1  christos Distributors should provide documentation and/or a simple selection
    123   1.4  christos interface that explains each name via a map or via descriptive text like
    124  1.22  christos Czech Republic instead of the timezone name <code>Europe/Prague</code>.
    125   1.4  christos If geolocation information is available, a selection interface can
    126   1.4  christos locate the user on a timezone map or prioritize names that are
    127   1.4  christos geographically close. For an example selection interface, see the
    128   1.3  christos <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
    129  1.22  christos Unicodes <a href="https://cldr.unicode.org">Common Locale Data
    130  1.21  christos Repository (<abbr>CLDR</abbr>)</a>
    131  1.21  christos contains data that may be useful for other selection
    132  1.10  christos interfaces; it maps timezone names like <code>Europe/Prague</code> to
    133  1.22  christos locale-dependent strings like Prague, Praha, , and .
    134   1.1  christos </p>
    135   1.1  christos 
    136   1.1  christos <p>
    137   1.3  christos The naming conventions attempt to strike a balance
    138   1.1  christos among the following goals:
    139   1.1  christos </p>
    140   1.3  christos 
    141   1.1  christos <ul>
    142   1.1  christos   <li>
    143   1.4  christos     Uniquely identify every timezone where clocks have agreed since 1970.
    144   1.3  christos     This is essential for the intended use: static clocks keeping local
    145   1.3  christos     civil time.
    146   1.1  christos   </li>
    147   1.1  christos   <li>
    148  1.22  christos     Indicate to experts where the timezones clocks typically are.
    149   1.1  christos   </li>
    150   1.1  christos   <li>
    151   1.3  christos     Be robust in the presence of political changes.
    152   1.7  christos     For example, names are typically not tied to countries, to avoid
    153   1.3  christos     incompatibilities when countries change their name (e.g.,
    154  1.22  christos     SwazilandEswatini) or when locations change countries (e.g., Hong
    155   1.3  christos     Kong from UK colony to China).
    156   1.7  christos     There is no requirement that every country or national
    157   1.7  christos     capital must have a timezone name.
    158   1.1  christos   </li>
    159   1.1  christos   <li>
    160   1.3  christos     Be portable to a wide variety of implementations.
    161   1.1  christos   </li>
    162   1.1  christos   <li>
    163   1.3  christos     Use a consistent naming conventions over the entire world.
    164   1.1  christos   </li>
    165   1.1  christos </ul>
    166   1.3  christos 
    167   1.1  christos <p>
    168  1.20  christos Names normally have the format
    169   1.3  christos <var>AREA</var><code>/</code><var>LOCATION</var>, where
    170   1.4  christos <var>AREA</var> is a continent or ocean, and
    171   1.4  christos <var>LOCATION</var> is a specific location within the area.
    172  1.22  christos North and South America share the same area, <code>America</code>.
    173  1.22  christos Typical names are <code>Africa/Cairo</code>,
    174  1.22  christos <code>America/New_York</code>, and <code>Pacific/Honolulu</code>.
    175   1.3  christos Some names are further qualified to help avoid confusion; for example,
    176  1.22  christos <code>America/Indiana/Petersburg</code> distinguishes Petersburg,
    177   1.3  christos Indiana from other Petersburgs in America.
    178   1.1  christos </p>
    179   1.1  christos 
    180   1.1  christos <p>
    181   1.3  christos Here are the general guidelines used for
    182   1.4  christos choosing timezone names,
    183   1.1  christos in decreasing order of importance:
    184   1.1  christos </p>
    185   1.3  christos 
    186   1.1  christos <ul>
    187   1.1  christos   <li>
    188   1.3  christos     Use only valid POSIX file name components (i.e., the parts of
    189  1.22  christos     names other than "<code>/</code>").
    190  1.22  christos     Do not use the file name components "<code>.</code>" and
    191  1.22  christos     "<code>..</code>".
    192   1.3  christos     Within a file name component, use only <a
    193   1.3  christos     href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
    194  1.22  christos     "<code>.</code>", "<code>-</code>" and "<code>_</code>".
    195   1.3  christos     Do not use digits, as that might create an ambiguity with <a
    196  1.22  christos     href="https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html#tag_08_03">POSIXs
    197  1.22  christos     proleptic <code>TZ</code> strings</a>.
    198   1.3  christos     A file name component must not exceed 14 characters or start with
    199  1.22  christos     "<code>-</code>".
    200  1.12  christos     E.g., prefer <code>America/Noronha</code> to
    201  1.12  christos     <code>America/Fernando_de_Noronha</code>.
    202   1.3  christos     Exceptions: see the discussion of legacy names below.
    203   1.1  christos   </li>
    204   1.1  christos   <li>
    205  1.22  christos     A name must not be empty, or contain "<code>//</code>", or
    206  1.22  christos     start or end with "<code>/</code>".
    207  1.22  christos     Also, a name must not be "<code>Etc/Unknown</code>", as
    208  1.21  christos     <abbr>CLDR</abbr> uses that string for an unknown or invalid timezone.
    209   1.1  christos   </li>
    210   1.1  christos   <li>
    211   1.3  christos     Do not use names that differ only in case.
    212   1.3  christos     Although the reference implementation is case-sensitive, some
    213   1.3  christos     other implementations are not, and they would mishandle names
    214   1.3  christos     differing only in case.
    215   1.1  christos   </li>
    216   1.1  christos   <li>
    217   1.3  christos     If one name <var>A</var> is an initial prefix of another
    218   1.3  christos     name <var>AB</var> (ignoring case), then <var>B</var> must not
    219  1.22  christos     start with "<code>/</code>", as a regular file cannot have the
    220   1.3  christos     same name as a directory in POSIX.
    221   1.3  christos     For example, <code>America/New_York</code> precludes
    222   1.3  christos     <code>America/New_York/Bronx</code>.
    223   1.1  christos   </li>
    224   1.1  christos   <li>
    225   1.3  christos     Uninhabited regions like the North Pole and Bouvet Island
    226   1.3  christos     do not need locations, since local time is not defined there.
    227   1.1  christos   </li>
    228   1.1  christos   <li>
    229  1.21  christos     If all clocks in a region have agreed since 1970,
    230  1.21  christos     give them just one name even if some of the clocks disagreed before 1970,
    231  1.21  christos     or reside in different countries or in notable or faraway locations.
    232   1.3  christos     Otherwise these tables would become annoyingly large.
    233  1.21  christos     For example, do not create a name <code>Indian/Crozet</code>
    234  1.21  christos     as a near-duplicate or alias of <code>Asia/Dubai</code>
    235  1.21  christos     merely because they are different countries or territories,
    236  1.21  christos     or their clocks disagreed before 1970, or the
    237  1.21  christos     <a href="https://en.wikipedia.org/wiki/Crozet_Islands">Crozet Islands</a>
    238  1.21  christos     are notable in their own right,
    239  1.21  christos     or the Crozet Islands are not adjacent to other locations
    240  1.21  christos     that use <code>Asia/Dubai</code>.
    241   1.1  christos   </li>
    242   1.1  christos   <li>
    243   1.7  christos     If boundaries between regions are fluid, such as during a war or
    244   1.7  christos     insurrection, do not bother to create a new timezone merely
    245   1.7  christos     because of yet another boundary change. This helps prevent table
    246   1.7  christos     bloat and simplifies maintenance.
    247   1.7  christos   </li>
    248   1.7  christos   <li>
    249   1.3  christos     If a name is ambiguous, use a less ambiguous alternative;
    250   1.3  christos     e.g., many cities are named San Jos and Georgetown, so
    251   1.3  christos     prefer <code>America/Costa_Rica</code> to
    252   1.3  christos     <code>America/San_Jose</code> and <code>America/Guyana</code>
    253   1.3  christos     to <code>America/Georgetown</code>.
    254   1.1  christos   </li>
    255   1.1  christos   <li>
    256   1.3  christos     Keep locations compact.
    257   1.3  christos     Use cities or small islands, not countries or regions, so that any
    258   1.3  christos     future changes do not split individual locations into different
    259   1.4  christos     timezones.
    260   1.3  christos     E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
    261   1.3  christos     since
    262   1.3  christos     <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
    263   1.3  christos     has had multiple time zones</a>.
    264   1.1  christos   </li>
    265   1.1  christos   <li>
    266   1.3  christos     Use mainstream English spelling, e.g., prefer
    267   1.4  christos     <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
    268   1.3  christos     prefer <code>Europe/Athens</code> to the Greek
    269   1.4  christos     <code>/</code> or the Romanized
    270   1.4  christos     <code>Evrpi/Athna</code>.
    271   1.3  christos     The POSIX file name restrictions encourage this guideline.
    272   1.1  christos   </li>
    273   1.1  christos   <li>
    274   1.3  christos     Use the most populous among locations in a region,
    275   1.3  christos     e.g., prefer <code>Asia/Shanghai</code> to
    276   1.3  christos     <code>Asia/Beijing</code>.
    277   1.3  christos     Among locations with similar populations, pick the best-known
    278   1.3  christos     location, e.g., prefer <code>Europe/Rome</code> to
    279   1.3  christos     <code>Europe/Milan</code>.
    280   1.1  christos   </li>
    281   1.1  christos   <li>
    282   1.3  christos     Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
    283   1.3  christos     <code>Atlantic/Canaries</code>.
    284   1.1  christos   </li>
    285   1.1  christos   <li>
    286  1.22  christos     Omit common suffixes like "<code>_Islands</code>" and
    287  1.22  christos     "<code>_City</code>", unless that would lead to ambiguity.
    288   1.3  christos     E.g., prefer <code>America/Cayman</code> to
    289   1.3  christos     <code>America/Cayman_Islands</code> and
    290   1.3  christos     <code>America/Guatemala</code> to
    291   1.3  christos     <code>America/Guatemala_City</code>, but prefer
    292   1.3  christos     <code>America/Mexico_City</code> to
    293   1.3  christos     <code>America/Mexico</code>
    294   1.3  christos     because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
    295   1.3  christos     country of Mexico has several time zones</a>.
    296   1.1  christos   </li>
    297   1.1  christos   <li>
    298  1.22  christos     Use "<code>_</code>" to represent a space.
    299   1.1  christos   </li>
    300   1.1  christos   <li>
    301  1.22  christos     Omit "<code>.</code>" from abbreviations in names.
    302   1.3  christos     E.g., prefer <code>Atlantic/St_Helena</code> to
    303   1.3  christos     <code>Atlantic/St._Helena</code>.
    304   1.1  christos   </li>
    305   1.1  christos   <li>
    306   1.3  christos     Do not change established names if they only marginally violate
    307   1.3  christos     the above guidelines.
    308   1.3  christos     For example, do not change the existing name <code>Europe/Rome</code> to
    309  1.22  christos     <code>Europe/Milan</code> merely because Milans population has grown
    310  1.22  christos     to be somewhat greater than Romes.
    311   1.1  christos   </li>
    312   1.1  christos   <li>
    313   1.3  christos     If a name is changed, put its old spelling in the
    314  1.22  christos     "<code>backward</code>" file as a link to the new spelling.
    315   1.3  christos     This means old spellings will continue to work.
    316   1.9  christos     Ordinarily a name change should occur only in the rare case when
    317  1.22  christos     a locations consensus English-language spelling changes; for example,
    318   1.9  christos     in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
    319   1.9  christos     due to long-time widespread use of the new city name instead of the old.
    320   1.1  christos   </li>
    321   1.1  christos </ul>
    322   1.1  christos 
    323   1.1  christos <p>
    324   1.7  christos Guidelines have evolved with time, and names following old versions of
    325   1.8  christos these guidelines might not follow the current version. When guidelines
    326   1.7  christos have changed, old names continue to be supported. Guideline changes
    327   1.7  christos have included the following:
    328   1.1  christos </p>
    329   1.1  christos 
    330   1.7  christos <ul>
    331   1.7  christos <li>
    332   1.7  christos Older versions of this package used a different naming scheme.
    333  1.22  christos See the file "<code>backward</code>" for most of these older names
    334  1.22  christos (e.g., <code>US/Eastern</code> instead of <code>America/New_York</code>).
    335   1.1  christos The other old-fashioned names still supported are
    336  1.22  christos <code>WET</code>, <code>CET</code>, <code>MET</code>, and
    337  1.22  christos <code>EET</code> (see the file "<code>europe</code>").
    338   1.7  christos </li>
    339   1.1  christos 
    340   1.7  christos <li>
    341   1.1  christos Older versions of this package defined legacy names that are
    342   1.3  christos incompatible with the first guideline of location names, but which are
    343   1.3  christos still supported.
    344   1.3  christos These legacy names are mostly defined in the file
    345  1.22  christos "<code>etcetera</code>".
    346  1.22  christos Also, the file "<code>backward</code>" defines the legacy names
    347  1.22  christos <code>Etc/GMT0</code>, <code>Etc/GMT-0</code>, <code>Etc/GMT+0</code>,
    348  1.22  christos <code>GMT0</code>, <code>GMT-0</code> and <code>GMT+0</code>,
    349  1.22  christos and the file "<code>northamerica</code>" defines the legacy names
    350  1.22  christos <code>EST5EDT</code>, <code>CST6CDT</code>,
    351  1.22  christos <code>MST7MDT</code>, and <code>PST8PDT</code>.
    352   1.7  christos </li>
    353   1.7  christos 
    354   1.7  christos <li>
    355   1.8  christos Older versions of these guidelines said that
    356   1.7  christos there should typically be at least one name for each <a
    357   1.7  christos href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
    358   1.7  christos title="International Organization for Standardization">ISO</abbr>
    359   1.7  christos 3166-1</a> officially assigned two-letter code for an inhabited
    360   1.7  christos country or territory.
    361   1.7  christos This old guideline has been dropped, as it was not needed to handle
    362   1.7  christos timestamps correctly and it increased maintenance burden.
    363   1.7  christos </li>
    364   1.7  christos </ul>
    365   1.7  christos 
    366   1.7  christos <p>
    367  1.13  christos The file <code>zone1970.tab</code> lists geographical locations used
    368   1.7  christos to name timezones.
    369   1.7  christos It is intended to be an exhaustive list of names for geographic
    370   1.7  christos regions as described above; this is a subset of the timezones in the data.
    371  1.22  christos Although a <code>zone1970.tab</code> locations
    372   1.7  christos <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
    373   1.7  christos corresponds to
    374   1.7  christos its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
    375  1.22  christos time (<abbr>LMT</abbr>)</a> offset with one hour for every 15
    376   1.7  christos east longitude, this relationship is not exact.
    377  1.13  christos The backward-compatibility file <code>zone.tab</code> is similar
    378  1.13  christos but conforms to the older-version guidelines related to <abbr>ISO</abbr> 3166-1;
    379  1.13  christos it lists only one country code per entry and unlike <code>zone1970.tab</code>
    380  1.13  christos it can list names defined in <code>backward</code>.
    381  1.19  christos Applications that process only timestamps from now on can instead use the file
    382  1.19  christos <code>zonenow.tab</code>, which partitions the world more coarsely,
    383  1.19  christos into regions where clocks agree now and in the predicted future;
    384  1.19  christos this file is smaller and simpler than <code>zone1970.tab</code>
    385  1.19  christos and <code>zone.tab</code>.
    386   1.1  christos </p>
    387   1.1  christos 
    388   1.1  christos <p>
    389  1.13  christos The database defines each timezone name to be a zone, or a link to a zone.
    390  1.13  christos The source file <code>backward</code> defines links for backward
    391  1.13  christos compatibility; it does not define zones.
    392  1.13  christos Although <code>backward</code> was originally designed to be optional,
    393  1.13  christos nowadays distributions typically use it
    394  1.13  christos and no great weight should be attached to whether a link
    395  1.13  christos is defined in <code>backward</code> or in some other file.
    396  1.13  christos The source file <code>etcetera</code> defines names that may be useful
    397  1.20  christos on platforms that do not support proleptic <code>TZ</code> strings
    398  1.20  christos like <code>&lt;+08&gt;-8</code>;
    399  1.13  christos no other source file other than <code>backward</code>
    400  1.13  christos contains links to its zones.
    401  1.22  christos One of <code>etcetera</code>s names is <code>Etc/UTC</code>,
    402  1.13  christos used by functions like <code>gmtime</code> to obtain leap
    403  1.13  christos second information on platforms that support leap seconds.
    404  1.14  christos Another <code>etcetera</code> name, <code>GMT</code>,
    405  1.14  christos is used by older code releases.
    406   1.1  christos </p>
    407   1.3  christos </section>
    408   1.1  christos 
    409   1.3  christos <section>
    410   1.3  christos   <h2 id="abbreviations">Time zone abbreviations</h2>
    411   1.1  christos <p>
    412   1.1  christos When this package is installed, it generates time zone abbreviations
    413  1.22  christos like <code>EST</code> to be compatible with human tradition and POSIX.
    414   1.3  christos Here are the general guidelines used for choosing time zone abbreviations,
    415   1.1  christos in decreasing order of importance:
    416   1.3  christos </p>
    417   1.3  christos 
    418   1.1  christos <ul>
    419   1.1  christos   <li>
    420   1.3  christos     Use three to six characters that are ASCII alphanumerics or
    421  1.22  christos     "<code>+</code>" or "<code>-</code>".
    422   1.3  christos     Previous editions of this database also used characters like
    423  1.22  christos     space and "<code>?</code>", but these characters have a
    424   1.3  christos     special meaning to the
    425   1.3  christos     <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
    426   1.3  christos     and cause commands like
    427  1.22  christos     "<code><a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#set">set</a>
    428  1.22  christos     `<a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/date.html">date</a>`</code>"
    429   1.3  christos     to have unexpected effects.
    430   1.3  christos     Previous editions of this guideline required upper-case letters, but the
    431   1.3  christos     Congressman who introduced
    432   1.3  christos     <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
    433  1.22  christos     Standard Time</a> preferred ChST, so lower-case letters are now allowed.
    434  1.22  christos     Also, POSIX from 2001 on relaxed the rule to allow "<code>-</code>",
    435  1.22  christos     "<code>+</code>", and alphanumeric characters from the portable
    436   1.3  christos     character set in the current locale.
    437  1.22  christos     In practice ASCII alphanumerics and "<code>+</code>" and
    438  1.22  christos     "<code>-</code>" are safe in all locales.
    439   1.3  christos 
    440   1.3  christos     <p>
    441   1.3  christos     In other words, in the C locale the POSIX extended regular
    442   1.3  christos     expression <code>[-+[:alnum:]]{3,6}</code> should match the
    443   1.3  christos     abbreviation.
    444  1.20  christos     This guarantees that all abbreviations could have been specified
    445  1.20  christos     explicitly by a POSIX proleptic <code>TZ</code> string.
    446   1.3  christos     </p>
    447   1.3  christos   </li>
    448   1.3  christos   <li>
    449   1.3  christos     Use abbreviations that are in common use among English-speakers,
    450  1.22  christos     e.g., EST for Eastern Standard Time in North America.
    451   1.3  christos     We assume that applications translate them to other languages
    452   1.3  christos     as part of the normal localization process; for example,
    453  1.22  christos     a French application might translate EST to HNE.
    454   1.3  christos 
    455   1.3  christos     <p>
    456   1.3  christos     <small>These abbreviations (for standard/daylight/etc. time) are:
    457   1.3  christos       ACST/ACDT Australian Central,
    458   1.3  christos       AST/ADT/APT/AWT/ADDT Atlantic,
    459   1.3  christos       AEST/AEDT Australian Eastern,
    460   1.3  christos       AHST/AHDT Alaska-Hawaii,
    461   1.3  christos       AKST/AKDT Alaska,
    462   1.3  christos       AWST/AWDT Australian Western,
    463   1.3  christos       BST/BDT Bering,
    464   1.3  christos       CAT/CAST Central Africa,
    465   1.3  christos       CET/CEST/CEMT Central European,
    466   1.3  christos       ChST Chamorro,
    467  1.18  christos       CST/CDT/CWT/CPT Central [North America],
    468   1.3  christos       CST/CDT China,
    469   1.3  christos       GMT/BST/IST/BDST Greenwich,
    470   1.3  christos       EAT East Africa,
    471  1.18  christos       EST/EDT/EWT/EPT Eastern [North America],
    472   1.3  christos       EET/EEST Eastern European,
    473   1.6  christos       GST/GDT Guam,
    474   1.5  christos       HST/HDT/HWT/HPT Hawaii,
    475   1.9  christos       HKT/HKST/HKWT Hong Kong,
    476   1.3  christos       IST India,
    477   1.3  christos       IST/GMT Irish,
    478   1.3  christos       IST/IDT/IDDT Israel,
    479   1.3  christos       JST/JDT Japan,
    480   1.3  christos       KST/KDT Korea,
    481   1.3  christos       MET/MEST Middle European (a backward-compatibility alias for
    482   1.3  christos 	Central European),
    483   1.3  christos       MSK/MSD Moscow,
    484  1.18  christos       MST/MDT/MWT/MPT Mountain,
    485   1.3  christos       NST/NDT/NWT/NPT/NDDT Newfoundland,
    486   1.3  christos       NST/NDT/NWT/NPT Nome,
    487   1.3  christos       NZMT/NZST New Zealand through 1945,
    488  1.22  christos       NZST/NZDT New Zealand 1946present,
    489   1.3  christos       PKT/PKST Pakistan,
    490  1.18  christos       PST/PDT/PWT/PPT Pacific,
    491   1.4  christos       PST/PDT Philippine,
    492   1.3  christos       SAST South Africa,
    493   1.3  christos       SST Samoa,
    494  1.14  christos       UTC Universal,
    495   1.3  christos       WAT/WAST West Africa,
    496   1.3  christos       WET/WEST/WEMT Western European,
    497   1.3  christos       WIB Waktu Indonesia Barat,
    498   1.3  christos       WIT Waktu Indonesia Timur,
    499   1.3  christos       WITA Waktu Indonesia Tengah,
    500   1.3  christos       YST/YDT/YWT/YPT/YDDT Yukon</small>.
    501   1.3  christos     </p>
    502   1.3  christos   </li>
    503   1.3  christos   <li>
    504   1.3  christos     <p>
    505  1.22  christos     For times taken from a citys longitude, use the
    506   1.3  christos     traditional <var>x</var>MT notation.
    507  1.22  christos     The only abbreviation like this in current use is <abbr>GMT</abbr>.
    508   1.3  christos     The others are for timestamps before 1960,
    509   1.3  christos     except that Monrovia Mean Time persisted until 1972.
    510  1.22  christos     Typically, numeric abbreviations (e.g., <code>-</code>004430 for
    511   1.3  christos     MMT) would cause trouble here, as the numeric strings would exceed
    512   1.3  christos     the POSIX length limit.
    513   1.3  christos     </p>
    514   1.3  christos 
    515   1.3  christos     <p>
    516   1.3  christos     <small>These abbreviations are:
    517  1.12  christos       AMT Asuncin, Athens;
    518  1.18  christos       BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogot,
    519  1.11  christos         Brussels, Bucharest;
    520  1.12  christos       CMT Calamarca, Caracas, Chisinau, Coln, Crdoba;
    521   1.3  christos       DMT Dublin/Dunsink;
    522   1.3  christos       EMT Easter;
    523   1.3  christos       FFMT Fort-de-France;
    524   1.3  christos       FMT Funchal;
    525   1.3  christos       GMT Greenwich;
    526   1.3  christos       HMT Havana, Helsinki, Horta, Howrah;
    527   1.3  christos       IMT Irkutsk, Istanbul;
    528   1.3  christos       JMT Jerusalem;
    529  1.14  christos       KMT Kaunas, Kyiv, Kingston;
    530  1.18  christos       LMT Lima, Lisbon, local;
    531   1.3  christos       MMT Macassar, Madras, Mal, Managua, Minsk, Monrovia, Montevideo,
    532   1.3  christos 	Moratuwa, Moscow;
    533   1.3  christos       PLMT Ph Lin;
    534   1.3  christos       PMT Paramaribo, Paris, Perm, Pontianak, Prague;
    535   1.3  christos       PMMT Port Moresby;
    536  1.18  christos       PPMT Port-au-Prince;
    537   1.3  christos       QMT Quito;
    538   1.3  christos       RMT Rangoon, Riga, Rome;
    539   1.3  christos       SDMT Santo Domingo;
    540   1.3  christos       SJMT San Jos;
    541   1.3  christos       SMT Santiago, Simferopol, Singapore, Stanley;
    542   1.3  christos       TBMT Tbilisi;
    543   1.3  christos       TMT Tallinn, Tehran;
    544  1.18  christos       WMT Warsaw.</small>
    545   1.3  christos     </p>
    546   1.3  christos 
    547   1.3  christos     <p>
    548   1.3  christos     <small>A few abbreviations also follow the pattern that
    549   1.4  christos     <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
    550   1.3  christos     They are:
    551  1.22  christos       BMT/BST for Bermuda 18901930,
    552   1.3  christos       CMT/BST for Calamarca Mean Time and Bolivian Summer Time
    553  1.22  christos 	18901932,
    554   1.3  christos       DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
    555  1.22  christos 	18801916,
    556  1.22  christos       MMT/MST/MDST for Moscow 18801919, and
    557  1.22  christos       RMT/LST for Riga Mean Time and Latvian Summer time 18801926.
    558  1.12  christos     </small>
    559   1.3  christos     </p>
    560   1.3  christos   </li>
    561   1.3  christos   <li>
    562  1.22  christos     Use <abbr>LMT</abbr> for local mean time of locations before the
    563  1.22  christos     introduction of standard time; see <a href="#scope">Scope of the
    564  1.22  christos     <code><abbr>tz</abbr></code> database</a>.
    565   1.3  christos   </li>
    566   1.3  christos   <li>
    567   1.3  christos     If there is no common English abbreviation, use numeric offsets like
    568   1.4  christos     <code>-</code>05 and <code>+</code>0530 that are generated
    569  1.22  christos     by <code>zic</code>s <code>%z</code> notation.
    570   1.3  christos   </li>
    571   1.3  christos   <li>
    572   1.3  christos     Use current abbreviations for older timestamps to avoid confusion.
    573   1.3  christos     For example, in 1910 a common English abbreviation for time
    574  1.22  christos     in central Europe was MEZ (short for both Middle European
    575  1.22  christos     Zone and for Mitteleuropische Zeit in German).
    576  1.22  christos     Nowadays CET (Central European Time) is more common in
    577  1.22  christos     English, and the database uses CET even for circa-1910
    578   1.3  christos     timestamps as this is less confusing for modern users and avoids
    579  1.22  christos     the need for determining when CET supplanted MEZ in common
    580   1.3  christos     usage.
    581   1.3  christos   </li>
    582   1.3  christos   <li>
    583  1.22  christos     Use a consistent style in a timezones history.
    584   1.4  christos     For example, if a history tends to use numeric
    585   1.3  christos     abbreviations and a particular entry could go either way, use a
    586   1.3  christos     numeric abbreviation.
    587   1.3  christos   </li>
    588   1.3  christos   <li>
    589   1.3  christos     Use
    590   1.3  christos     <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
    591  1.22  christos     (<abbr>UT</abbr>) (with time zone abbreviation <code>-</code>00) for
    592   1.3  christos     locations while uninhabited.
    593  1.22  christos     The leading "<code>-</code>" is a flag that the <abbr>UT</abbr> offset is in
    594   1.3  christos     some sense undefined; this notation is derived
    595  1.21  christos     from <a href="https://www.rfc-editor.org/rfc/rfc3339">Internet
    596   1.4  christos     <abbr title="Request For Comments">RFC</abbr> 3339</a>.
    597  1.22  christos     (The abbreviation Z that
    598  1.21  christos     <a href="https://www.rfc-editor.org/rfc/rfc9557">Internet
    599  1.20  christos     <abbr>RFC</abbr> 9557</a> uses for this concept
    600  1.20  christos     would violate the POSIX requirement
    601  1.20  christos     of at least three characters in an abbreviation.)
    602   1.1  christos   </li>
    603   1.1  christos </ul>
    604   1.3  christos 
    605   1.1  christos <p>
    606   1.1  christos Application writers should note that these abbreviations are ambiguous
    607  1.22  christos in practice: e.g., CST means one thing in China and something else
    608  1.22  christos in North America, and IST can refer to time in India, Ireland or
    609   1.3  christos Israel.
    610   1.3  christos To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
    611  1.22  christos <code>-</code>0600 instead of time zone abbreviations like CST.
    612   1.1  christos </p>
    613   1.3  christos </section>
    614   1.1  christos 
    615   1.3  christos <section>
    616   1.3  christos   <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
    617   1.1  christos <p>
    618   1.3  christos The <code><abbr>tz</abbr></code> database is not authoritative, and it
    619   1.3  christos surely has errors.
    620   1.2  christos Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
    621   1.1  christos Users requiring authoritative data should consult national standards
    622  1.22  christos bodies and the references cited in the databases comments.
    623   1.1  christos </p>
    624   1.1  christos 
    625   1.1  christos <p>
    626   1.3  christos Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
    627   1.1  christos </p>
    628   1.3  christos 
    629   1.1  christos <ul>
    630   1.1  christos   <li>
    631   1.3  christos     The <code><abbr>tz</abbr></code> database predicts future
    632   1.3  christos     timestamps, and current predictions
    633   1.3  christos     will be incorrect after future governments change the rules.
    634   1.3  christos     For example, if today someone schedules a meeting for 13:00 next
    635   1.3  christos     October 1, Casablanca time, and tomorrow Morocco changes its
    636   1.3  christos     daylight saving rules, software can mess up after the rule change
    637   1.3  christos     if it blithely relies on conversions made before the change.
    638   1.3  christos   </li>
    639   1.3  christos   <li>
    640   1.3  christos     The pre-1970 entries in this database cover only a tiny sliver of how
    641   1.3  christos     clocks actually behaved; the vast majority of the necessary
    642   1.3  christos     information was lost or never recorded.
    643   1.4  christos     Thousands more timezones would be needed if
    644  1.22  christos     the <code><abbr>tz</abbr></code> databases scope were extended to
    645   1.3  christos     cover even just the known or guessed history of standard time; for
    646   1.3  christos     example, the current single entry for France would need to split
    647   1.3  christos     into dozens of entries, perhaps hundreds.
    648   1.3  christos     And in most of the world even this approach would be misleading
    649   1.3  christos     due to widespread disagreement or indifference about what times
    650   1.3  christos     should be observed.
    651   1.3  christos     In her 2015 book
    652   1.3  christos     <cite><a
    653  1.23  christos     href="https://www.hup.harvard.edu/books/9780674286146">The
    654  1.22  christos     Global Transformation of Time, 18701950</a></cite>,
    655   1.3  christos     Vanessa Ogle writes
    656  1.22  christos     Outside of Europe and North America there was no system of time
    657   1.3  christos     zones at all, often not even a stable landscape of mean times,
    658  1.22  christos     prior to the middle decades of the twentieth century.
    659   1.3  christos     See: Timothy Shenk, <a
    660  1.23  christos href="https://dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle/">Booked:
    661   1.3  christos       A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
    662   1.3  christos   </li>
    663   1.3  christos   <li>
    664   1.3  christos     Most of the pre-1970 data entries come from unreliable sources, often
    665   1.3  christos     astrology books that lack citations and whose compilers evidently
    666   1.3  christos     invented entries when the true facts were unknown, without
    667   1.3  christos     reporting which entries were known and which were invented.
    668   1.3  christos     These books often contradict each other or give implausible entries,
    669   1.3  christos     and on the rare occasions when they are checked they are
    670   1.3  christos     typically found to be incorrect.
    671   1.3  christos   </li>
    672   1.3  christos   <li>
    673   1.3  christos     For the UK the <code><abbr>tz</abbr></code> database relies on
    674   1.3  christos     years of first-class work done by
    675   1.3  christos     Joseph Myers and others; see
    676  1.22  christos     <a href="https://www.polyomino.org.uk/british-time/">History of
    677  1.22  christos     legal time in Britain</a>.
    678   1.3  christos     Other countries are not done nearly as well.
    679   1.3  christos   </li>
    680   1.3  christos   <li>
    681   1.3  christos     Sometimes, different people in the same city maintain clocks
    682   1.3  christos     that differ significantly.
    683   1.3  christos     Historically, railway time was used by railroad companies (which
    684   1.3  christos     did not always
    685   1.3  christos     agree with each other), church-clock time was used for birth
    686   1.3  christos     certificates, etc.
    687   1.3  christos     More recently, competing political groups might disagree about
    688   1.3  christos     clock settings. Often this is merely common practice, but
    689   1.3  christos     sometimes it is set by law.
    690   1.3  christos     For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
    691   1.3  christos     was legally <abbr>UT</abbr> +00:09:21 outside train stations and
    692   1.3  christos     <abbr>UT</abbr> +00:04:21 inside. Other examples include
    693   1.3  christos     Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
    694   1.3  christos     rmqi to this day.
    695   1.3  christos   </li>
    696   1.3  christos   <li>
    697   1.3  christos     Although a named location in the <code><abbr>tz</abbr></code>
    698   1.3  christos     database stands for the containing region, its pre-1970 data
    699   1.3  christos     entries are often accurate for only a small subset of that region.
    700   1.3  christos     For example, <code>Europe/London</code> stands for the United
    701   1.3  christos     Kingdom, but its pre-1847 times are valid only for locations that
    702  1.22  christos     have Londons exact meridian, and its 1847 transition
    703   1.3  christos     to <abbr>GMT</abbr> is known to be valid only for the L&amp;NW and
    704   1.3  christos     the Caledonian railways.
    705   1.3  christos   </li>
    706   1.3  christos   <li>
    707   1.3  christos     The <code><abbr>tz</abbr></code> database does not record the
    708  1.22  christos     earliest time for which a timezones
    709   1.3  christos     data entries are thereafter valid for every location in the region.
    710   1.3  christos     For example, <code>Europe/London</code> is valid for all locations
    711   1.3  christos     in its region after <abbr>GMT</abbr> was made the standard time,
    712   1.3  christos     but the date of standardization (1880-08-02) is not in the
    713   1.3  christos     <code><abbr>tz</abbr></code> database, other than in commentary.
    714   1.4  christos     For many timezones the earliest time of
    715   1.3  christos     validity is unknown.
    716   1.3  christos   </li>
    717   1.3  christos   <li>
    718   1.3  christos     The <code><abbr>tz</abbr></code> database does not record a
    719  1.22  christos     regions boundaries, and in many cases the boundaries are not known.
    720   1.4  christos     For example, the timezone
    721   1.3  christos     <code>America/Kentucky/Louisville</code> represents a region
    722   1.3  christos     around the city of Louisville, the boundaries of which are
    723   1.3  christos     unclear.
    724   1.3  christos   </li>
    725   1.3  christos   <li>
    726   1.3  christos     Changes that are modeled as instantaneous transitions in the
    727   1.3  christos     <code><abbr>tz</abbr></code>
    728   1.3  christos     database were often spread out over hours, days, or even decades.
    729   1.3  christos   </li>
    730   1.3  christos   <li>
    731   1.3  christos     Even if the time is specified by law, locations sometimes
    732   1.3  christos     deliberately flout the law.
    733   1.3  christos   </li>
    734   1.3  christos   <li>
    735   1.3  christos     Early timekeeping practices, even assuming perfect clocks, were
    736   1.3  christos     often not specified to the accuracy that the
    737   1.3  christos     <code><abbr>tz</abbr></code> database requires.
    738   1.3  christos   </li>
    739   1.3  christos   <li>
    740  1.10  christos     The <code><abbr>tz</abbr></code> database cannot represent stopped clocks.
    741  1.10  christos     However, on 1911-03-11 at 00:00, some public-facing French clocks
    742  1.10  christos     were changed by stopping them for a few minutes to effect a transition.
    743  1.10  christos     The <code><abbr>tz</abbr></code> database models this via a
    744  1.10  christos     backward transition; the relevant French legislation does not
    745  1.10  christos     specify exactly how the transition was to occur.
    746  1.10  christos   </li>
    747  1.10  christos   <li>
    748   1.3  christos     Sometimes historical timekeeping was specified more precisely
    749   1.3  christos     than what the <code><abbr>tz</abbr></code> code can handle.
    750  1.12  christos     For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean
    751   1.3  christos     Time (estimated to be <abbr>UT</abbr>
    752  1.22  christos     00:25:21.1); although the <code><abbr>tz</abbr></code>
    753  1.14  christos     source data can represent the .1 second, TZif files and the code cannot.
    754   1.3  christos     In practice these old specifications were rarely if ever
    755   1.3  christos     implemented to subsecond precision.
    756   1.3  christos   </li>
    757   1.3  christos   <li>
    758   1.3  christos     Even when all the timestamp transitions recorded by the
    759   1.3  christos     <code><abbr>tz</abbr></code> database are correct, the
    760   1.3  christos     <code><abbr>tz</abbr></code> rules that generate them may not
    761   1.3  christos     faithfully reflect the historical rules.
    762   1.3  christos     For example, from 1922 until World War II the UK moved clocks
    763   1.3  christos     forward the day following the third Saturday in April unless that
    764   1.3  christos     was Easter, in which case it moved clocks forward the previous
    765   1.3  christos     Sunday.
    766   1.3  christos     Because the <code><abbr>tz</abbr></code> database has no
    767   1.3  christos     way to specify Easter, these exceptional years are entered as
    768   1.3  christos     separate <code><abbr>tz</abbr> Rule</code> lines, even though the
    769   1.3  christos     legal rules did not change.
    770   1.4  christos     When transitions are known but the historical rules behind them are not,
    771   1.4  christos     the database contains <code>Zone</code> and <code>Rule</code>
    772   1.4  christos     entries that are intended to represent only the generated
    773   1.4  christos     transitions, not any underlying historical rules; however, this
    774   1.4  christos     intent is recorded at best only in commentary.
    775   1.3  christos   </li>
    776   1.3  christos   <li>
    777   1.4  christos     The <code><abbr>tz</abbr></code> database models time
    778   1.3  christos     using the <a
    779   1.3  christos     href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
    780   1.4  christos     Gregorian calendar</a> with days containing 24 equal-length hours
    781   1.4  christos     numbered 00 through 23, except when clock transitions occur.
    782   1.4  christos     Pre-standard time is modeled as local mean time.
    783   1.4  christos     However, historically many people used other calendars and other timescales.
    784   1.3  christos     For example, the Roman Empire used
    785   1.3  christos     the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
    786   1.3  christos     calendar</a>,
    787   1.3  christos     and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
    788   1.3  christos     timekeeping</a> had twelve varying-length daytime hours with a
    789   1.3  christos     non-hour-based system at night.
    790   1.4  christos     And even today, some local practices diverge from the Gregorian
    791   1.4  christos     calendar with 24-hour days. These divergences range from
    792  1.22  christos     relatively minor, such as Japanese bars giving times like 24:30 for the
    793   1.4  christos     wee hours of the morning, to more-significant differences such as <a
    794  1.23  christos     href="https://theworld.org/stories/2015/01/30/ethiopian-time">the
    795   1.4  christos     east African practice of starting the day at dawn</a>, renumbering
    796   1.4  christos     the Western 06:00 to be 12:00. These practices are largely outside
    797   1.4  christos     the scope of the <code><abbr>tz</abbr></code> code and data, which
    798   1.4  christos     provide only limited support for date and time localization
    799  1.20  christos     such as that required by POSIX.
    800  1.12  christos     If <abbr>DST</abbr> is not used a different time zone
    801   1.4  christos     can often do the trick; for example, in Kenya a <code>TZ</code> setting
    802   1.4  christos     like <code>&lt;-03&gt;3</code> or <code>America/Cayenne</code> starts
    803   1.4  christos     the day six hours later than <code>Africa/Nairobi</code> does.
    804   1.3  christos   </li>
    805   1.3  christos   <li>
    806   1.3  christos     Early clocks were less reliable, and data entries do not represent
    807   1.3  christos     clock error.
    808   1.3  christos   </li>
    809   1.3  christos   <li>
    810   1.3  christos     The <code><abbr>tz</abbr></code> database assumes Universal Time
    811   1.3  christos     (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
    812   1.3  christos     standardized for older timestamps.
    813   1.3  christos     In the <code><abbr>tz</abbr></code> database commentary,
    814   1.3  christos     <abbr>UT</abbr> denotes a family of time standards that includes
    815   1.3  christos     Coordinated Universal Time (<abbr>UTC</abbr>) along with other
    816   1.3  christos     variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
    817   1.3  christos     with days starting at midnight.
    818   1.3  christos     Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
    819   1.3  christos     timestamps, <abbr>UTC</abbr> was not defined until 1960, so
    820  1.18  christos     commentary uses the more general abbreviation <abbr>UT</abbr> for
    821   1.3  christos     timestamps that might predate 1960.
    822   1.3  christos     Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
    823   1.3  christos     and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
    824   1.3  christos     interpretation of older timestamps can be problematic when
    825   1.3  christos     subsecond accuracy is needed.
    826   1.3  christos   </li>
    827   1.3  christos   <li>
    828   1.3  christos     Civil time was not based on atomic time before 1972, and we do not
    829   1.3  christos     know the history of
    830  1.22  christos     <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earths
    831   1.3  christos     rotation</a> accurately enough to map <a
    832   1.3  christos     href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
    833   1.3  christos     title="International System of Units">SI</abbr></a> seconds to
    834   1.3  christos     historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
    835   1.3  christos     to more than about one-hour accuracy.
    836  1.22  christos     See: Morrison LV, Stephenson FR, Hohenkerk CY, Zawilski M.
    837  1.22  christos     <a href="https://doi.org/10.1098/rspa.2020.0776">Addendum 2020
    838  1.22  christos     to Measurement of the Earths rotation: 720 BC to AD 2015</a>.
    839  1.22  christos     <cite>Proc Royal Soc A</cite>. 2021;477:20200776.
    840   1.3  christos     Also see: Espenak F. <a
    841   1.3  christos     href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
    842   1.3  christos     in Delta T (T)</a>.
    843   1.3  christos   </li>
    844   1.3  christos   <li>
    845   1.3  christos     The relationship between POSIX time (that is, <abbr>UTC</abbr> but
    846   1.3  christos     ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
    847  1.18  christos     seconds</a>) and <abbr>UTC</abbr> is not agreed upon.
    848  1.22  christos     This affects time stamps during the leap second era (19722035).
    849   1.3  christos     Although the POSIX
    850   1.3  christos     clock officially stops during an inserted leap second, at least one
    851   1.3  christos     proposed standard has it jumping back a second instead; and in
    852   1.3  christos     practice POSIX clocks more typically either progress glacially during
    853   1.3  christos     a leap second, or are slightly slowed while near a leap second.
    854   1.3  christos   </li>
    855   1.3  christos   <li>
    856   1.3  christos     The <code><abbr>tz</abbr></code> database does not represent how
    857   1.3  christos     uncertain its information is.
    858   1.3  christos     Ideally it would contain information about when data entries are
    859   1.3  christos     incomplete or dicey.
    860   1.3  christos     Partial temporal knowledge is a field of active research, though,
    861   1.3  christos     and it is not clear how to apply it here.
    862   1.1  christos   </li>
    863   1.1  christos </ul>
    864   1.1  christos 
    865   1.1  christos <p>
    866   1.3  christos In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
    867  1.22  christos databases pre-1970 and future timestamps are either wrong or
    868   1.3  christos misleading.
    869   1.3  christos Any attempt to pass the
    870   1.3  christos <code><abbr>tz</abbr></code> database off as the definition of time
    871   1.3  christos should be unacceptable to anybody who cares about the facts.
    872  1.22  christos In particular, the <code><abbr>tz</abbr></code> databases
    873   1.3  christos <abbr>LMT</abbr> offsets should not be considered meaningful, and
    874   1.4  christos should not prompt creation of timezones
    875   1.3  christos merely because two locations
    876   1.3  christos differ in <abbr>LMT</abbr> or transitioned to standard time at
    877   1.3  christos different dates.
    878   1.3  christos </p>
    879   1.3  christos </section>
    880   1.3  christos 
    881   1.3  christos <section>
    882   1.3  christos   <h2 id="functions">Time and date functions</h2>
    883   1.3  christos <p>
    884   1.3  christos The <code><abbr>tz</abbr></code> code contains time and date functions
    885   1.3  christos that are upwards compatible with those of POSIX.
    886   1.3  christos Code compatible with this package is already
    887   1.3  christos <a href="tz-link.html#tzdb">part of many platforms</a>, where the
    888   1.3  christos primary use of this package is to update obsolete time-related files.
    889   1.3  christos To do this, you may need to compile the time zone compiler
    890  1.20  christos <code>zic</code> supplied with this package instead of using the
    891  1.22  christos system <code>zic</code>, since the format of <code>zic</code>s
    892   1.3  christos input is occasionally extended, and a platform may still be shipping
    893   1.3  christos an older <code>zic</code>.
    894   1.1  christos </p>
    895   1.1  christos 
    896  1.20  christos <p>
    897  1.20  christos In POSIX, time display in a process is controlled by the
    898  1.20  christos environment variable <code>TZ</code>, which can have two forms:
    899  1.20  christos </p>
    900   1.1  christos <ul>
    901   1.1  christos   <li>
    902  1.20  christos     A <dfn>proleptic <code>TZ</code></dfn> value
    903  1.20  christos     like <code>CET-1CEST,M3.5.0,M10.5.0/3</code> uses a complex
    904  1.20  christos     notation that specifies a single standard time along with daylight
    905  1.20  christos     saving rules that apply to all years past, present, and future.
    906  1.20  christos   </li>
    907  1.20  christos   <li>
    908  1.20  christos     A <dfn>geographical <code>TZ</code></dfn> value
    909  1.20  christos     like <code>Europe/Berlin</code> names a location that stands for
    910  1.20  christos     civil time near that location, which can have more than
    911  1.20  christos     one standard time and more than one set of daylight saving rules,
    912  1.20  christos     to record timekeeping practice more accurately.
    913  1.20  christos     These names are defined by the <code><abbr>tz</abbr></code> database.
    914  1.20  christos   </li>
    915  1.20  christos </ul>
    916  1.20  christos 
    917  1.20  christos <h3 id="POSIX.1-2017">POSIX.1-2017 properties and limitations</h3>
    918  1.20  christos <p>
    919  1.22  christos Some platforms support only the features required by POSIX.1-2017
    920  1.22  christos and earlier editions,
    921  1.20  christos and have not yet upgraded to POSIX.1-2024.
    922  1.20  christos Code intended to be portable to these platforms must deal
    923  1.20  christos with problems that were fixed in later POSIX editions.
    924  1.20  christos </p>
    925  1.20  christos 
    926  1.20  christos <ul>
    927  1.20  christos   <li>
    928  1.20  christos     POSIX.1-2017 does not require support for geographical <code>TZ</code>,
    929  1.20  christos     and there is no convenient and efficient way to determine
    930  1.20  christos     the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
    931  1.20  christos     timestamps, particularly for timezones
    932  1.20  christos     that do not fit into the POSIX model.
    933  1.20  christos   </li>
    934  1.20  christos   <li>
    935   1.1  christos     <p>
    936  1.20  christos     The proleptic <code>TZ</code> string,
    937  1.20  christos     which is all that POSIX.1-2017 requires,
    938  1.20  christos     has a format that is hard to describe and is error-prone in practice.
    939  1.20  christos     Also, proleptic <code>TZ</code> strings cannot deal with daylight
    940   1.3  christos     saving time rules not based on the Gregorian calendar (as in
    941  1.18  christos     Morocco), or with situations where more than two time zone
    942   1.3  christos     abbreviations or <abbr>UT</abbr> offsets are used in an area.
    943   1.1  christos     </p>
    944   1.3  christos 
    945   1.1  christos     <p>
    946  1.20  christos     A proleptic <code>TZ</code> string has the following format:
    947   1.1  christos     </p>
    948   1.3  christos 
    949   1.1  christos     <p>
    950   1.3  christos     <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]]
    951   1.1  christos     </p>
    952   1.3  christos 
    953   1.1  christos     <p>
    954   1.3  christos     where:
    955   1.3  christos     </p>
    956   1.3  christos 
    957   1.1  christos     <dl>
    958   1.1  christos       <dt><var>std</var> and <var>dst</var></dt><dd>
    959   1.3  christos 	are 3 or more characters specifying the standard
    960   1.4  christos 	and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
    961   1.3  christos 	Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
    962  1.22  christos 	may also be quoted in angle brackets, like <code>&lt;+09&gt;</code>;
    963   1.3  christos 	this allows "<code>+</code>" and "<code>-</code>" in the names.
    964   1.1  christos       </dd>
    965   1.1  christos       <dt><var>offset</var></dt><dd>
    966   1.3  christos 	is of the form
    967  1.22  christos 	<code>[]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>
    968   1.3  christos 	and specifies the offset west of <abbr>UT</abbr>.
    969  1.22  christos 	<var>hh</var> may be a single digit;
    970   1.3  christos 	0&le;<var>hh</var>&le;24.
    971   1.3  christos 	The default <abbr>DST</abbr> offset is one hour ahead of
    972   1.3  christos 	standard time.
    973   1.1  christos       </dd>
    974   1.1  christos       <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
    975   1.3  christos 	specifies the beginning and end of <abbr>DST</abbr>.
    976   1.3  christos 	If this is absent, the system supplies its own ruleset
    977  1.18  christos 	for <abbr>DST</abbr>, typically	current <abbr>US</abbr>
    978  1.18  christos 	<abbr>DST</abbr> rules.
    979   1.1  christos       </dd>
    980   1.1  christos       <dt><var>time</var></dt><dd>
    981   1.3  christos 	takes the form
    982  1.22  christos 	<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]
    983   1.3  christos 	and defaults to 02:00.
    984   1.3  christos 	This is the same format as the offset, except that a
    985  1.22  christos 	leading "<code>+</code>" or "<code>-</code>" is not allowed.
    986   1.1  christos       </dd>
    987   1.1  christos       <dt><var>date</var></dt><dd>
    988   1.3  christos 	takes one of the following forms:
    989   1.1  christos 	<dl>
    990   1.1  christos 	  <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
    991   1.3  christos 	    origin-1 day number not counting February 29
    992   1.3  christos 	  </dd>
    993   1.1  christos 	  <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
    994   1.3  christos 	    origin-0 day number counting February 29 if present
    995   1.3  christos 	  </dd>
    996   1.3  christos 	  <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
    997   1.3  christos 	    (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5,
    998   1.3  christos 	    1&le;<var>m</var>&le;12)</dt><dd>
    999   1.3  christos 	    for the <var>d</var>th day of week <var>n</var> of
   1000   1.3  christos 	    month <var>m</var> of the year, where week 1 is the first
   1001   1.3  christos 	    week in which day <var>d</var> appears, and
   1002  1.22  christos 	    "<code>5</code>" stands for the last week in which
   1003   1.3  christos 	    day <var>d</var> appears (which may be either the 4th or
   1004   1.3  christos 	    5th week).
   1005   1.3  christos 	    Typically, this is the only useful form; the <var>n</var>
   1006   1.3  christos 	    and <code>J</code><var>n</var> forms are rarely used.
   1007   1.1  christos 	  </dd>
   1008   1.3  christos 	</dl>
   1009   1.3  christos       </dd>
   1010   1.3  christos     </dl>
   1011   1.3  christos 
   1012   1.3  christos     <p>
   1013  1.20  christos     Here is an example proleptic <code>TZ</code> string for New
   1014   1.3  christos     Zealand after 2007.
   1015   1.3  christos     It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
   1016   1.3  christos     of <abbr>UT</abbr>, and that daylight saving time
   1017  1.22  christos     (<abbr>NZDT</abbr>) is observed from Septembers last Sunday at
   1018  1.22  christos     02:00 until Aprils first Sunday at 03:00:
   1019   1.3  christos     </p>
   1020   1.3  christos 
   1021   1.3  christos     <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
   1022   1.3  christos 
   1023   1.3  christos     <p>
   1024  1.20  christos     This proleptic <code>TZ</code> string is hard to remember, and
   1025   1.3  christos     mishandles some timestamps before 2008.
   1026  1.20  christos     With this package you can use a geographical <code>TZ</code> instead:
   1027   1.3  christos     </p>
   1028   1.3  christos 
   1029   1.3  christos     <pre><code>TZ='Pacific/Auckland'</code></pre>
   1030   1.3  christos   </li>
   1031  1.20  christos </ul>
   1032  1.20  christos 
   1033  1.20  christos <p>
   1034  1.20  christos POSIX.1-2017 also has the limitations of POSIX.1-2024,
   1035  1.20  christos discussed in the next section.
   1036  1.20  christos </p>
   1037  1.20  christos 
   1038  1.20  christos <h3 id="POSIX.1-2024">POSIX.1-2024 properties and limitations</h3>
   1039  1.20  christos <p>
   1040  1.20  christos POSIX.1-2024 extends POSIX.1-2017 in the following significant ways:
   1041  1.20  christos </p>
   1042  1.20  christos <ul>
   1043  1.20  christos   <li>
   1044  1.20  christos     POSIX.1-2024 requires support for geographical <code>TZ</code>.
   1045  1.20  christos     Earlier POSIX editions require support only for proleptic <code>TZ</code>.
   1046  1.20  christos   </li>
   1047  1.20  christos   <li>
   1048  1.20  christos     POSIX.1-2024 requires <code>struct tm</code>
   1049  1.20  christos     to have a <abbr>UT</abbr> offset member <code>tm_gmtoff</code>
   1050  1.20  christos     and a time zone abbreviation member <code>tm_zone</code>.
   1051  1.20  christos     Earlier POSIX editions lack this requirement.
   1052  1.20  christos   </li>
   1053   1.3  christos   <li>
   1054  1.22  christos     DST transition times can range from 167:59:59
   1055  1.20  christos     to 167:59:59 instead of merely from 00:00:00 to 24:59:59.
   1056  1.20  christos     This allows for proleptic TZ strings
   1057  1.20  christos     like <code>"&lt;-02&gt;2&lt;-01&gt;,M3.5.0/-1,M10.5.0/0"</code>
   1058  1.22  christos     where the transition time 1:00 means 23:00 the previous day.
   1059   1.3  christos   </li>
   1060  1.20  christos </ul>
   1061  1.20  christos <p>
   1062  1.20  christos However POSIX.1-2024, like earlier POSIX editions, has some limitations:
   1063  1.20  christos <ul>
   1064   1.3  christos   <li>
   1065   1.3  christos     The <code>TZ</code> environment variable is process-global, which
   1066   1.3  christos     makes it hard to write efficient, thread-safe applications that
   1067   1.4  christos     need access to multiple timezones.
   1068   1.3  christos   </li>
   1069   1.3  christos   <li>
   1070   1.3  christos     In POSIX, there is no tamper-proof way for a process to learn the
   1071  1.22  christos     systems best idea of local (wall clock) time.
   1072   1.4  christos     This is important for applications that an administrator wants
   1073  1.22  christos     used only at certain times  without regard to whether the
   1074   1.3  christos     user has fiddled the
   1075   1.3  christos     <code>TZ</code> environment variable.
   1076  1.22  christos     While an administrator can do everything in <abbr>UT</abbr> to
   1077   1.3  christos     get around the problem, doing so is inconvenient and precludes
   1078  1.22  christos     handling daylight saving time shifts  as might be required to
   1079   1.4  christos     limit phone calls to off-peak hours.
   1080   1.3  christos   </li>
   1081   1.3  christos   <li>
   1082  1.20  christos     POSIX requires that <code>time_t</code> clock counts exclude leap
   1083  1.20  christos     seconds.
   1084   1.3  christos   </li>
   1085   1.3  christos   <li>
   1086  1.20  christos     POSIX does not define the <abbr>DST</abbr> transitions
   1087  1.22  christos     for settings like <code>TZ='EST5EDT'</code>.
   1088  1.20  christos     Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
   1089  1.20  christos     were used to interpret such values, but this meant that the
   1090  1.20  christos     <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
   1091  1.20  christos     time conversion package, and when
   1092  1.20  christos     <abbr>US</abbr> time conversion rules changed (as in the United
   1093  1.20  christos     States in 1987 and again in 2007), all packages that
   1094  1.20  christos     interpreted <code>TZ</code> values had to be updated
   1095  1.20  christos     to ensure proper results.
   1096   1.3  christos   </li>
   1097  1.20  christos </ul>
   1098  1.20  christos 
   1099  1.20  christos <h3 id="POSIX-extensions">Extensions to POSIX in the
   1100  1.20  christos <code><abbr>tz</abbr></code> code</h3>
   1101  1.20  christos <p>
   1102  1.20  christos   The <code><abbr>tz</abbr></code> code defines some properties
   1103  1.20  christos   left unspecified by POSIX, and attempts to support some
   1104  1.20  christos   extensions to POSIX.
   1105  1.20  christos </p>
   1106  1.20  christos 
   1107  1.20  christos <ul>
   1108   1.3  christos   <li>
   1109   1.3  christos     The <code><abbr>tz</abbr></code> code attempts to support all the
   1110   1.3  christos     <code>time_t</code> implementations allowed by POSIX.
   1111   1.3  christos     The <code>time_t</code> type represents a nonnegative count of seconds
   1112   1.3  christos     since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
   1113   1.3  christos     In practice, <code>time_t</code> is usually a signed 64- or 32-bit
   1114   1.3  christos     integer; 32-bit signed <code>time_t</code> values stop working after
   1115   1.3  christos     2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
   1116   1.3  christos     days typically use a signed 64-bit integer.
   1117   1.3  christos     Unsigned 32-bit integers are used on one or two platforms, and 36-bit
   1118   1.3  christos     and 40-bit integers are also used occasionally.
   1119   1.3  christos     Although earlier POSIX versions allowed <code>time_t</code> to be a
   1120   1.4  christos     floating-point type, this was not supported by any practical system,
   1121  1.20  christos     and POSIX.1-2013+ and the <code><abbr>tz</abbr></code> code both
   1122   1.3  christos     require <code>time_t</code> to be an integer type.
   1123   1.1  christos   </li>
   1124   1.1  christos   <li>
   1125   1.1  christos     <p>
   1126  1.20  christos     If the <code>TZ</code> environment variable uses the geographical format,
   1127  1.20  christos     it is used in generating
   1128  1.20  christos     the name of a file from which time-related information is read.
   1129  1.22  christos     The files format is <dfn><abbr>TZif</abbr></dfn>,
   1130   1.7  christos     a timezone information format that contains binary data; see
   1131  1.23  christos     <a href="https://www.rfc-editor.org/rfc/rfc9636">Internet
   1132  1.21  christos     <abbr>RFC</abbr> 9636</a>.
   1133   1.3  christos     The daylight saving time rules to be used for a
   1134   1.4  christos     particular timezone are encoded in the
   1135   1.4  christos     <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>,
   1136   1.4  christos     Australian, and other rules to be encoded, and
   1137   1.3  christos     allows for situations where more than two time zone
   1138   1.3  christos     abbreviations are used.
   1139   1.1  christos     </p>
   1140   1.1  christos     <p>
   1141  1.20  christos     When the <code><abbr>tz</abbr></code> code was developed in the 1980s,
   1142  1.20  christos     it was recognized that allowing the <code>TZ</code> environment
   1143  1.22  christos     variable to take on values such as <code>America/New_York</code>
   1144  1.22  christos     might cause old programs (that expect <code>TZ</code> to have a
   1145  1.20  christos     certain format) to operate incorrectly; consideration was given to using
   1146   1.3  christos     some other environment variable (for example, <code>TIMEZONE</code>)
   1147  1.22  christos     to hold the string used to generate the <abbr>TZif</abbr> files name.
   1148   1.3  christos     In the end, however, it was decided to continue using
   1149   1.3  christos     <code>TZ</code>: it is widely used for time zone purposes;
   1150   1.3  christos     separately maintaining both <code>TZ</code>
   1151   1.3  christos     and <code>TIMEZONE</code> seemed a nuisance; and systems where
   1152  1.22  christos     new forms of <code>TZ</code> might cause problems can simply
   1153  1.22  christos     use legacy settings such as <code>TZ='EST5EDT'</code> which
   1154  1.22  christos     can be used by new programs as well as by old programs that
   1155   1.4  christos     assume pre-POSIX <code>TZ</code> values.
   1156   1.1  christos     </p>
   1157   1.3  christos   </li>
   1158   1.3  christos   <li>
   1159   1.3  christos     Functions <code>tzalloc</code>, <code>tzfree</code>,
   1160   1.3  christos     <code>localtime_rz</code>, and <code>mktime_z</code> for
   1161   1.3  christos     more-efficient thread-safe applications that need to use multiple
   1162   1.4  christos     timezones.
   1163   1.3  christos     The <code>tzalloc</code> and <code>tzfree</code> functions
   1164   1.3  christos     allocate and free objects of type <code>timezone_t</code>,
   1165   1.3  christos     and <code>localtime_rz</code> and <code>mktime_z</code> are
   1166   1.3  christos     like <code>localtime_r</code> and <code>mktime</code> with an
   1167   1.3  christos     extra <code>timezone_t</code> argument.
   1168  1.20  christos     The functions were inspired by <a href="https://netbsd.org">NetBSD</a>.
   1169   1.3  christos   </li>
   1170   1.3  christos   <li>
   1171   1.3  christos     Negative <code>time_t</code> values are supported, on systems
   1172   1.3  christos     where <code>time_t</code> is signed.
   1173   1.3  christos   </li>
   1174   1.3  christos   <li>
   1175   1.9  christos     These functions can account for leap seconds;
   1176   1.9  christos     see <a href="#leapsec">Leap seconds</a> below.
   1177   1.3  christos   </li>
   1178   1.1  christos </ul>
   1179   1.3  christos 
   1180   1.3  christos <h3 id="vestigial">POSIX features no longer needed</h3>
   1181   1.1  christos <p>
   1182   1.3  christos POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
   1183   1.3  christos define some <a href="https://en.wikipedia.org/wiki/API"><abbr
   1184   1.3  christos title="application programming interface">API</abbr>s</a> that are vestigial:
   1185   1.3  christos they are not needed, and are relics of a too-simple model that does
   1186   1.3  christos not suffice to handle many real-world timestamps.
   1187   1.3  christos Although the <code><abbr>tz</abbr></code> code supports these
   1188   1.3  christos vestigial <abbr>API</abbr>s for backwards compatibility, they should
   1189   1.3  christos be avoided in portable applications.
   1190   1.3  christos The vestigial <abbr>API</abbr>s are:
   1191   1.1  christos </p>
   1192   1.1  christos <ul>
   1193   1.1  christos   <li>
   1194   1.3  christos     The POSIX <code>tzname</code> variable does not suffice and is no
   1195   1.3  christos     longer needed.
   1196  1.20  christos     It is planned to be removed in a future edition of POSIX.
   1197  1.22  christos     To get a timestamps time zone abbreviation, consult
   1198   1.3  christos     the <code>tm_zone</code> member if available; otherwise,
   1199  1.22  christos     use <code>strftime</code>s <code>"%Z"</code> conversion
   1200   1.3  christos     specification.
   1201   1.3  christos   </li>
   1202   1.3  christos   <li>
   1203   1.3  christos     The POSIX <code>daylight</code> and <code>timezone</code>
   1204   1.3  christos     variables do not suffice and are no longer needed.
   1205  1.20  christos     They are planned to be removed in a future edition of POSIX.
   1206  1.22  christos     To get a timestamps <abbr>UT</abbr> offset, consult
   1207   1.3  christos     the <code>tm_gmtoff</code> member if available; otherwise,
   1208   1.3  christos     subtract values returned by <code>localtime</code>
   1209   1.3  christos     and <code>gmtime</code> using the rules of the Gregorian calendar,
   1210  1.22  christos     or use <code>strftime</code>s <code>"%z"</code> conversion
   1211   1.3  christos     specification if a string like <code>"+0900"</code> suffices.
   1212   1.3  christos   </li>
   1213   1.3  christos   <li>
   1214   1.3  christos     The <code>tm_isdst</code> member is almost never needed and most of
   1215   1.3  christos     its uses should be discouraged in favor of the abovementioned
   1216   1.3  christos     <abbr>API</abbr>s.
   1217  1.21  christos     It was intended as an index into the <code>tzname</code> variable,
   1218  1.21  christos     but as mentioned previously that usage is obsolete.
   1219   1.3  christos     Although it can still be used in arguments to
   1220   1.3  christos     <code>mktime</code> to disambiguate timestamps near
   1221  1.15  christos     a <abbr>DST</abbr> transition when the clock jumps back on
   1222  1.15  christos     platforms lacking <code>tm_gmtoff</code>, this
   1223  1.21  christos     disambiguation works only for proleptic <code>TZ</code> strings;
   1224  1.21  christos     it does not work in general for geographical timezones,
   1225  1.21  christos     such as when a location changes to a time zone with a
   1226   1.3  christos     lesser <abbr>UT</abbr> offset.
   1227   1.3  christos   </li>
   1228   1.3  christos </ul>
   1229   1.3  christos 
   1230   1.3  christos <h3 id="other-portability">Other portability notes</h3>
   1231   1.3  christos <ul>
   1232   1.3  christos   <li>
   1233   1.3  christos     The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
   1234   1.3  christos     UNIX</a> <code>timezone</code> function is not present in this
   1235  1.22  christos     package; it is impossible to reliably map <code>timezone</code>s
   1236  1.22  christos     arguments (a minutes west of <abbr>GMT</abbr> value and a
   1237  1.22  christos     daylight saving time in effect flag) to a time zone
   1238   1.3  christos     abbreviation, and we refuse to guess.
   1239   1.3  christos     Programs that in the past used the <code>timezone</code> function
   1240   1.3  christos     may now examine <code>localtime(&amp;clock)-&gt;tm_zone</code>
   1241   1.3  christos     (if <code>TM_ZONE</code> is defined) or
   1242  1.21  christos     use <code>strftime</code> with a <code>%Z</code> conversion specification
   1243  1.21  christos     to learn the correct time
   1244   1.3  christos     zone abbreviation to use.
   1245   1.3  christos   </li>
   1246   1.3  christos   <li>
   1247   1.3  christos     The <a
   1248   1.3  christos     href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
   1249   1.3  christos     <code>gettimeofday</code> function is not
   1250   1.3  christos     used in this package.
   1251   1.3  christos     This formerly let users obtain the current <abbr>UTC</abbr> offset
   1252   1.3  christos     and <abbr>DST</abbr> flag, but this functionality was removed in
   1253   1.3  christos     later versions of <abbr>BSD</abbr>.
   1254   1.3  christos   </li>
   1255   1.3  christos   <li>
   1256   1.3  christos     In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
   1257   1.3  christos     near-maximum <code>time_t</code> values when doing conversions
   1258   1.3  christos     for places that do not use <abbr>UT</abbr>.
   1259   1.3  christos     This package takes care to do these conversions correctly.
   1260   1.3  christos     A comment in the source code tells how to get compatibly wrong
   1261   1.3  christos     results.
   1262   1.3  christos   </li>
   1263   1.3  christos   <li>
   1264   1.3  christos     The functions that are conditionally compiled
   1265  1.18  christos     if <code>STD_INSPIRED</code> is nonzero should, at this point, be
   1266   1.3  christos     looked on primarily as food for thought.
   1267  1.22  christos     They are not in any sense standard compatible  some are
   1268   1.3  christos     not, in fact, specified in <em>any</em> standard.
   1269   1.3  christos     They do, however, represent responses of various authors to
   1270   1.3  christos     standardization proposals.
   1271   1.3  christos   </li>
   1272   1.3  christos   <li>
   1273   1.4  christos     Other time conversion proposals, in particular those supported by the
   1274   1.4  christos     <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
   1275   1.4  christos     Database Parser</a>, offer a wider selection of functions
   1276   1.3  christos     that provide capabilities beyond those provided here.
   1277   1.3  christos     The absence of such functions from this package is not meant to
   1278   1.3  christos     discourage the development, standardization, or use of such
   1279   1.3  christos     functions.
   1280   1.3  christos     Rather, their absence reflects the decision to make this package
   1281   1.3  christos     contain valid extensions to POSIX, to ensure its broad
   1282   1.3  christos     acceptability.
   1283   1.3  christos     If more powerful time conversion functions can be standardized, so
   1284   1.3  christos     much the better.
   1285   1.1  christos   </li>
   1286   1.1  christos </ul>
   1287   1.3  christos </section>
   1288   1.1  christos 
   1289   1.3  christos <section>
   1290   1.3  christos   <h2 id="stability">Interface stability</h2>
   1291   1.1  christos <p>
   1292   1.3  christos The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
   1293   1.1  christos </p>
   1294   1.1  christos 
   1295   1.1  christos <ul>
   1296   1.1  christos   <li>
   1297   1.4  christos     A set of timezone names as per
   1298  1.22  christos       <a href="#naming">Timezone identifiers</a> above.
   1299   1.1  christos   </li>
   1300   1.1  christos   <li>
   1301  1.22  christos     Library functions described in <a href="#functions">Time and date
   1302  1.22  christos       functions</a> above.
   1303   1.1  christos   </li>
   1304   1.1  christos   <li>
   1305   1.3  christos     The programs <code>tzselect</code>, <code>zdump</code>,
   1306   1.3  christos     and <code>zic</code>, documented in their man pages.
   1307   1.1  christos   </li>
   1308   1.1  christos   <li>
   1309   1.3  christos     The format of <code>zic</code> input files, documented in
   1310   1.3  christos     the <code>zic</code> man page.
   1311   1.1  christos   </li>
   1312   1.1  christos   <li>
   1313   1.3  christos     The format of <code>zic</code> output files, documented in
   1314   1.3  christos     the <code>tzfile</code> man page.
   1315   1.1  christos   </li>
   1316   1.1  christos   <li>
   1317   1.3  christos     The format of zone table files, documented in <code>zone1970.tab</code>.
   1318   1.1  christos   </li>
   1319   1.1  christos   <li>
   1320   1.3  christos     The format of the country code file, documented in <code>iso3166.tab</code>.
   1321   1.1  christos   </li>
   1322   1.1  christos   <li>
   1323   1.3  christos     The version number of the code and data, as the first line of
   1324  1.22  christos     the text file "<code>version</code>" in each release.
   1325   1.1  christos   </li>
   1326   1.1  christos </ul>
   1327   1.3  christos 
   1328   1.1  christos <p>
   1329   1.1  christos Interface changes in a release attempt to preserve compatibility with
   1330   1.3  christos recent releases.
   1331   1.3  christos For example, <code><abbr>tz</abbr></code> data files typically do not
   1332  1.18  christos rely on recently added <code>zic</code> features, so that users can
   1333   1.3  christos run older <code>zic</code> versions to process newer data files.
   1334   1.3  christos <a href="tz-link.html#download">Downloading
   1335   1.3  christos the <code><abbr>tz</abbr></code> database</a> describes how releases
   1336   1.3  christos are tagged and distributed.
   1337   1.1  christos </p>
   1338   1.1  christos 
   1339   1.1  christos <p>
   1340   1.3  christos Interfaces not listed above are less stable.
   1341   1.3  christos For example, users should not rely on particular <abbr>UT</abbr>
   1342   1.3  christos offsets or abbreviations for timestamps, as data entries are often
   1343   1.3  christos based on guesswork and these guesses may be corrected or improved.
   1344   1.1  christos </p>
   1345   1.7  christos 
   1346   1.7  christos <p>
   1347   1.7  christos Timezone boundaries are not part of the stable interface.
   1348   1.7  christos For example, even though the <samp>Asia/Bangkok</samp> timezone
   1349   1.7  christos currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part
   1350   1.7  christos of the stable interface and the timezone can split at any time.
   1351   1.7  christos If a calendar application records a future event in some location other
   1352  1.22  christos than Bangkok by putting <samp>Asia/Bangkok</samp> in the events record,
   1353   1.7  christos the application should be robust in the presence of timezone splits
   1354   1.7  christos between now and the future time.
   1355   1.7  christos </p>
   1356   1.3  christos </section>
   1357   1.1  christos 
   1358   1.3  christos <section>
   1359   1.9  christos   <h2 id="leapsec">Leap seconds</h2>
   1360   1.9  christos <p>
   1361  1.18  christos Leap seconds were introduced in 1972 to accommodate the
   1362  1.18  christos difference between atomic time and the less regular rotation of the earth.
   1363  1.20  christos Unfortunately they have caused so many problems with civil
   1364  1.20  christos timekeeping that there are
   1365  1.20  christos <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">plans
   1366  1.20  christos to discontinue them by 2035</a>.
   1367  1.20  christos Even if these plans come to fruition, a record of leap seconds will still be
   1368  1.20  christos needed to resolve timestamps from 1972 through 2035,
   1369  1.20  christos and there may also be a need to record whatever mechanism replaces them.
   1370  1.18  christos </p>
   1371  1.18  christos 
   1372  1.18  christos <p>
   1373   1.9  christos The <code><abbr>tz</abbr></code> code and data can account for leap seconds,
   1374   1.9  christos thanks to code contributed by Bradley White.
   1375   1.9  christos However, the leap second support of this package is rarely used directly
   1376   1.9  christos because POSIX requires leap seconds to be excluded and many
   1377   1.9  christos software packages would mishandle leap seconds if they were present.
   1378   1.9  christos Instead, leap seconds are more commonly handled by occasionally adjusting
   1379   1.9  christos the operating system kernel clock as described in
   1380   1.9  christos <a href="tz-link.html#precision">Precision timekeeping</a>,
   1381   1.9  christos and this package by default installs a <samp>leapseconds</samp> file
   1382   1.9  christos commonly used by
   1383  1.15  christos <a href="https://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a>
   1384   1.9  christos software that adjusts the kernel clock.
   1385   1.9  christos However, kernel-clock twiddling approximates UTC only roughly,
   1386  1.22  christos and systems needing more precise UTC can use this packages leap
   1387   1.9  christos second support directly.
   1388   1.9  christos </p>
   1389   1.9  christos 
   1390   1.9  christos <p>
   1391  1.18  christos The directly supported mechanism assumes that <code>time_t</code>
   1392   1.9  christos counts of seconds since the POSIX epoch normally include leap seconds,
   1393   1.9  christos as opposed to POSIX <code>time_t</code> counts which exclude leap seconds.
   1394   1.9  christos This modified timescale is converted to <abbr>UTC</abbr>
   1395  1.12  christos at the same point that time zone and <abbr>DST</abbr>
   1396  1.22  christos adjustments are applied 
   1397  1.22  christos namely, at calls to <code>localtime</code> and analogous functions 
   1398   1.9  christos and the process is driven by leap second information
   1399   1.9  christos stored in alternate versions of the <abbr>TZif</abbr> files.
   1400   1.9  christos Because a leap second adjustment may be needed even
   1401   1.9  christos if no time zone correction is desired,
   1402   1.9  christos calls to <code>gmtime</code>-like functions
   1403   1.9  christos also need to consult a <abbr>TZif</abbr> file,
   1404  1.14  christos conventionally named <samp><abbr>Etc/UTC</abbr></samp>
   1405  1.14  christos (<samp><abbr>GMT</abbr></samp> in previous versions),
   1406   1.9  christos to see whether leap second corrections are needed.
   1407  1.22  christos To convert an applications <code>time_t</code> timestamps to or from
   1408   1.9  christos POSIX <code>time_t</code> timestamps (for use when, say,
   1409   1.9  christos embedding or interpreting timestamps in portable
   1410   1.9  christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a>
   1411   1.9  christos files),
   1412   1.9  christos the application can call the utility functions
   1413   1.9  christos <code>time2posix</code> and <code>posix2time</code>
   1414   1.9  christos included with this package.
   1415   1.9  christos </p>
   1416   1.9  christos 
   1417   1.9  christos <p>
   1418   1.9  christos If the POSIX-compatible <abbr>TZif</abbr> file set is installed
   1419   1.9  christos in a directory whose basename is <samp>zoneinfo</samp>, the
   1420   1.9  christos leap-second-aware file set is by default installed in a separate
   1421   1.9  christos directory <samp>zoneinfo-leaps</samp>.
   1422   1.9  christos Although each process can have its own time zone by setting
   1423   1.9  christos its <code>TZ</code> environment variable, there is no support for some
   1424   1.9  christos processes being leap-second aware while other processes are
   1425   1.9  christos POSIX-compatible; the leap-second choice is system-wide.
   1426   1.9  christos So if you configure your kernel to count leap seconds, you should also
   1427   1.9  christos discard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp>
   1428   1.9  christos to <samp>zoneinfo</samp>.
   1429   1.9  christos Alternatively, you can install just one set of <abbr>TZif</abbr> files
   1430  1.22  christos in the first place; see the <code>REDO</code> variable in this packages
   1431   1.9  christos <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>.
   1432   1.9  christos </p>
   1433   1.9  christos </section>
   1434   1.9  christos 
   1435   1.9  christos <section>
   1436   1.3  christos   <h2 id="calendar">Calendrical issues</h2>
   1437   1.1  christos <p>
   1438   1.1  christos Calendrical issues are a bit out of scope for a time zone database,
   1439   1.1  christos but they indicate the sort of problems that we would run into if we
   1440   1.3  christos extended the time zone database further into the past.
   1441   1.3  christos An excellent resource in this area is Edward M. Reingold
   1442   1.3  christos and Nachum Dershowitz, <cite><a
   1443  1.23  christos href="https://www.cambridge.org/fr/universitypress/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
   1444   1.3  christos Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
   1445  1.22  christos Other information and sources are given in the file "<code>calendars</code>"
   1446   1.3  christos in the <code><abbr>tz</abbr></code> distribution.
   1447   1.3  christos They sometimes disagree.
   1448   1.3  christos </p>
   1449   1.3  christos </section>
   1450   1.3  christos 
   1451   1.3  christos <section>
   1452  1.18  christos   <h2 id="planets">Time and time zones off Earth</h2>
   1453  1.18  christos <p>
   1454  1.18  christos The European Space Agency is <a
   1455  1.23  christos href="https://www.esa.int/Applications/Satellite_navigation/Telling_time_on_the_Moon">considering</a>
   1456  1.18  christos the establishment of a reference timescale for the Moon, which has
   1457  1.18  christos days roughly equivalent to 29.5 Earth days, and where relativistic
   1458  1.18  christos effects cause clocks to tick slightly faster than on Earth.
   1459  1.20  christos Also, <abbr title="National Aeronautics and Space Administration">NASA</abbr>
   1460  1.20  christos has been <a
   1461  1.23  christos href="https://bidenwhitehouse.archives.gov/wp-content/uploads/2024/04/Celestial-Time-Standardization-Policy.pdf">ordered</a>
   1462  1.20  christos to consider the establishment of Coordinated Lunar Time (<abbr>LTC</abbr>).
   1463  1.20  christos It is not yet known whether the US and European efforts will result in
   1464  1.20  christos multiple timescales on the Moon.
   1465  1.18  christos </p>
   1466  1.18  christos 
   1467   1.3  christos <p>
   1468  1.22  christos Some peoples work schedules have used
   1469  1.10  christos <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
   1470   1.3  christos Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
   1471   1.3  christos and off during the
   1472   1.6  christos <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
   1473  1.10  christos Pathfinder</a> mission (1997).
   1474   1.3  christos Some of their family members also adapted to Mars time.
   1475   1.3  christos Dozens of special Mars watches were built for JPL workers who kept
   1476  1.10  christos Mars time during the
   1477  1.10  christos <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
   1478  1.22  christos Exploration Rovers (MER)</a> mission (20042018).
   1479  1.10  christos These timepieces looked like normal Seikos and Citizens but were adjusted
   1480  1.10  christos to use Mars seconds rather than terrestrial seconds, although
   1481  1.10  christos unfortunately the adjusted watches were unreliable and appear to have
   1482  1.10  christos had only limited use.
   1483   1.1  christos </p>
   1484   1.1  christos 
   1485   1.1  christos <p>
   1486  1.22  christos A Mars solar day is called a sol and has a mean period equal to
   1487   1.3  christos about 24 hours 39 minutes 35.244 seconds in terrestrial time.
   1488   1.3  christos It is divided into a conventional 24-hour clock, so each Mars second
   1489   1.3  christos equals about 1.02749125 terrestrial seconds.
   1490  1.22  christos (One MER worker noted, If I am working Mars hours, and Mars hours are
   1491  1.22  christos 2.5% more than Earth hours, shouldnt I get an extra 2.5% pay raise?)
   1492   1.1  christos </p>
   1493   1.1  christos 
   1494   1.1  christos <p>
   1495   1.3  christos The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
   1496   1.3  christos meridian</a> of Mars goes through the center of the crater
   1497   1.3  christos <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
   1498   1.3  christos honor of the British astronomer who built the Greenwich telescope that
   1499  1.22  christos defines Earths prime meridian.
   1500   1.3  christos Mean solar time on the Mars prime meridian is
   1501   1.6  christos called Mars Coordinated Time (<abbr>MTC</abbr>).
   1502   1.1  christos </p>
   1503   1.1  christos 
   1504   1.1  christos <p>
   1505   1.1  christos Each landed mission on Mars has adopted a different reference for
   1506   1.4  christos solar timekeeping, so there is no real standard for Mars time zones.
   1507  1.22  christos For example, the MER mission defined two time zones Local
   1508  1.22  christos Solar Time A and Local Solar Time B for its two missions, each zone
   1509   1.3  christos designed so that its time equals local true solar time at
   1510   1.3  christos approximately the middle of the nominal mission.
   1511  1.10  christos The A and B zones differ enough so that an MER worker assigned to
   1512  1.22  christos the A zone might suffer Mars lag when switching to work in the B zone.
   1513  1.22  christos Such a time zone is not particularly suited for any application
   1514   1.3  christos other than the mission itself.
   1515   1.1  christos </p>
   1516   1.1  christos 
   1517   1.1  christos <p>
   1518   1.1  christos Many calendars have been proposed for Mars, but none have achieved
   1519   1.3  christos wide acceptance.
   1520   1.3  christos Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
   1521   1.1  christos sequential count of Mars solar days elapsed since about 1873-12-29
   1522   1.3  christos 12:00 <abbr>GMT</abbr>.
   1523   1.1  christos </p>
   1524   1.1  christos 
   1525   1.1  christos <p>
   1526   1.1  christos In our solar system, Mars is the planet with time and calendar most
   1527  1.22  christos like Earths.
   1528   1.3  christos On other planets, Sun-based time and calendars would work quite
   1529   1.3  christos differently.
   1530  1.22  christos For example, although Mercurys
   1531   1.3  christos <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
   1532   1.3  christos rotation period</a> is 58.646 Earth days, Mercury revolves around the
   1533  1.22  christos Sun so rapidly that an observer on Mercurys equator would see a
   1534   1.3  christos sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
   1535   1.3  christos Mercury day.
   1536   1.3  christos Venus is more complicated, partly because its rotation is slightly
   1537   1.3  christos <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
   1538   1.3  christos its year is 1.92 of its days.
   1539   1.3  christos Gas giants like Jupiter are trickier still, as their polar and
   1540   1.3  christos equatorial regions rotate at different rates, so that the length of a
   1541   1.3  christos day depends on latitude.
   1542   1.3  christos This effect is most pronounced on Neptune, where the day is about 12
   1543   1.3  christos hours at the poles and 18 hours at the equator.
   1544   1.1  christos </p>
   1545   1.1  christos 
   1546   1.1  christos <p>
   1547   1.3  christos Although the <code><abbr>tz</abbr></code> database does not support
   1548   1.3  christos time on other planets, it is documented here in the hopes that support
   1549   1.3  christos will be added eventually.
   1550   1.1  christos </p>
   1551   1.1  christos 
   1552   1.1  christos <p>
   1553   1.3  christos Sources for time on other planets:
   1554   1.1  christos </p>
   1555   1.3  christos 
   1556   1.1  christos <ul>
   1557   1.1  christos   <li>
   1558   1.3  christos     Michael Allison and Robert Schmunk,
   1559  1.22  christos     <a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
   1560  1.22  christos       Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>
   1561  1.10  christos     (2020-03-08).
   1562  1.10  christos   </li>
   1563  1.10  christos   <li>
   1564  1.10  christos     Zara Mirmalek,
   1565  1.10  christos     <em><a href="https://mitpress.mit.edu/books/making-time-mars">Making
   1566  1.10  christos 	Time on Mars</a></em>, MIT Press (March 2020), ISBN 978-0262043854.
   1567   1.1  christos   </li>
   1568   1.1  christos   <li>
   1569   1.3  christos     Jia-Rui Chong,
   1570  1.22  christos     <a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
   1571  1.22  christos     Fit for a Martian</a>, <cite>Los Angeles Times</cite>
   1572  1.22  christos     (2004-01-14), pp A1, A20A21.
   1573   1.1  christos   </li>
   1574   1.1  christos   <li>
   1575   1.3  christos     Tom Chmielewski,
   1576  1.22  christos     <a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
   1577  1.22  christos     Lag Is Worse on Mars</a>, <cite>The Atlantic</cite> (2015-02-26)
   1578   1.1  christos   </li>
   1579   1.1  christos   <li>
   1580   1.3  christos     Matt Williams,
   1581  1.23  christos     <a href="https://www.universetoday.com/articles/days-of-the-planets">How
   1582  1.22  christos     long is a day on the other planets of the solar system?</a>
   1583   1.4  christos     (2016-01-20).
   1584   1.1  christos   </li>
   1585   1.1  christos </ul>
   1586   1.3  christos </section>
   1587   1.1  christos 
   1588   1.3  christos <footer>
   1589  1.22  christos <hr>
   1590  1.22  christos This web page is in the public domain, so clarified as of
   1591  1.22  christos 2009-05-17 by Arthur David Olson.
   1592  1.22  christos <br>
   1593  1.22  christos Please send corrections to this web page to the
   1594  1.22  christos <a href="mailto:tz (a] iana.org">time zone mailing list</a>.
   1595  1.22  christos The mailing list and its archives are public,
   1596  1.22  christos so please do not send confidential information.
   1597   1.3  christos </footer>
   1598   1.1  christos </body>
   1599   1.1  christos </html>
   1600