Home | History | Annotate | Line # | Download | only in time
theory.html revision 1.12 
      1   1.4  christos <!DOCTYPE html>
      2   1.1  christos <html lang="en">
      3   1.1  christos <head>
      4   1.1  christos   <title>Theory and pragmatics of the tz code and data</title>
      5   1.1  christos   <meta charset="UTF-8">
      6   1.4  christos   <style>
      7   1.4  christos     pre {margin-left: 2em; white-space: pre-wrap;}
      8   1.4  christos   </style>
      9   1.1  christos </head>
     10   1.1  christos 
     11   1.1  christos <body>
     12   1.3  christos <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
     13   1.1  christos   <h3>Outline</h3>
     14   1.1  christos   <nav>
     15   1.1  christos     <ul>
     16   1.3  christos       <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
     17   1.3  christos 	  database</a></li>
     18   1.7  christos       <li><a href="#naming">Timezone identifiers</a></li>
     19   1.1  christos       <li><a href="#abbreviations">Time zone abbreviations</a></li>
     20   1.3  christos       <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
     21   1.3  christos 	  database</a></li>
     22   1.1  christos       <li><a href="#functions">Time and date functions</a></li>
     23   1.1  christos       <li><a href="#stability">Interface stability</a></li>
     24   1.9  christos       <li><a href="#leapsec">Leap seconds</a></li>
     25   1.1  christos       <li><a href="#calendar">Calendrical issues</a></li>
     26   1.1  christos       <li><a href="#planets">Time and time zones on other planets</a></li>
     27   1.1  christos     </ul>
     28   1.1  christos   </nav>
     29   1.1  christos 
     30   1.3  christos <section>
     31   1.3  christos   <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
     32   1.1  christos <p>
     33   1.3  christos The <a
     34   1.3  christos href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
     35   1.3  christos database</a> attempts to record the history and predicted future of
     36  1.10  christos civil time scales.
     37   1.3  christos It organizes <a href="tz-link.html">time zone and daylight saving time
     38   1.3  christos data</a> by partitioning the world into <a
     39   1.4  christos href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a>
     40   1.3  christos whose clocks all agree about timestamps that occur after the <a
     41   1.3  christos href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
     42   1.3  christos (1970-01-01 00:00:00 <a
     43   1.3  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
     44   1.3  christos title="Coordinated Universal Time">UTC</abbr></a>).
     45   1.3  christos Although 1970 is a somewhat-arbitrary cutoff, there are significant
     46   1.3  christos challenges to moving the cutoff earlier even by a decade or two, due
     47   1.3  christos to the wide variety of local practices before computer timekeeping
     48   1.3  christos became prevalent.
     49  1.12  christos Most timezones correspond to a notable location and the database
     50  1.12  christos records all known clock transitions for that location;
     51  1.12  christos some timezones correspond instead to a fixed <abbr>UTC</abbr> offset.
     52   1.1  christos </p>
     53   1.1  christos 
     54   1.1  christos <p>
     55   1.4  christos Each timezone typically corresponds to a geographical region that is
     56   1.4  christos smaller than a traditional time zone, because clocks in a timezone
     57   1.4  christos all agree after 1970 whereas a traditional time zone merely
     58   1.4  christos specifies current standard time. For example, applications that deal
     59   1.4  christos with current and future timestamps in the traditional North
     60   1.4  christos American mountain time zone can choose from the timezones
     61   1.4  christos <code>America/Denver</code> which observes US-style daylight saving
     62  1.12  christos time (<abbr>DST</abbr>),
     63  1.12  christos <code>America/Mazatlan</code> which observes Mexican-style <abbr>DST</abbr>,
     64  1.12  christos and <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>.
     65   1.4  christos Applications that also deal with past timestamps in the mountain time
     66   1.4  christos zone can choose from over a dozen timezones, such as
     67   1.4  christos <code>America/Boise</code>, <code>America/Edmonton</code>, and
     68   1.4  christos <code>America/Hermosillo</code>, each of which currently uses mountain
     69   1.4  christos time but differs from other timezones for some timestamps after 1970.
     70   1.4  christos </p>
     71   1.4  christos 
     72   1.4  christos <p>
     73  1.12  christos Clock transitions before 1970 are recorded for location-based timezones,
     74   1.1  christos because most systems support timestamps before 1970 and could
     75   1.1  christos misbehave if data entries were omitted for pre-1970 transitions.
     76   1.1  christos However, the database is not designed for and does not suffice for
     77   1.1  christos applications requiring accurate handling of all past times everywhere,
     78   1.1  christos as it would take far too much effort and guesswork to record all
     79   1.1  christos details of pre-1970 civil timekeeping.
     80   1.3  christos Although some information outside the scope of the database is
     81   1.2  christos collected in a file <code>backzone</code> that is distributed along
     82   1.2  christos with the database proper, this file is less reliable and does not
     83   1.2  christos necessarily follow database guidelines.
     84   1.1  christos </p>
     85   1.1  christos 
     86   1.1  christos <p>
     87   1.3  christos As described below, reference source code for using the
     88   1.3  christos <code><abbr>tz</abbr></code> database is also available.
     89   1.3  christos The <code><abbr>tz</abbr></code> code is upwards compatible with <a
     90   1.3  christos href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
     91   1.3  christos standard for <a
     92   1.3  christos href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
     93   1.3  christos As of this writing, the current edition of POSIX is: <a
     94   1.8  christos href="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
     95   1.3  christos Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
     96   1.3  christos Edition.
     97   1.3  christos Because the database's scope encompasses real-world changes to civil
     98   1.3  christos timekeeping, its model for describing time is more complex than the
     99   1.3  christos standard and daylight saving times supported by POSIX.
    100   1.4  christos A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
    101   1.3  christos have more than two changes per year, these changes need not merely
    102   1.3  christos flip back and forth between two alternatives, and the rules themselves
    103   1.3  christos can change at times.
    104   1.9  christos Whether and when a timezone changes its clock,
    105   1.9  christos and even the timezone's notional base offset from <abbr>UTC</abbr>,
    106   1.9  christos are variable.
    107   1.4  christos It does not always make sense to talk about a timezone's
    108   1.4  christos "base offset", which is not necessarily a single number.
    109   1.1  christos </p>
    110   1.1  christos 
    111   1.3  christos </section>
    112   1.1  christos 
    113   1.3  christos <section>
    114   1.7  christos   <h2 id="naming">Timezone identifiers</h2>
    115   1.1  christos <p>
    116   1.7  christos Each timezone has a name that uniquely identifies the timezone.
    117   1.1  christos Inexperienced users are not expected to select these names unaided.
    118   1.1  christos Distributors should provide documentation and/or a simple selection
    119   1.4  christos interface that explains each name via a map or via descriptive text like
    120  1.10  christos "Czech Republic" instead of the timezone name "<code>Europe/Prague</code>".
    121   1.4  christos If geolocation information is available, a selection interface can
    122   1.4  christos locate the user on a timezone map or prioritize names that are
    123   1.4  christos geographically close. For an example selection interface, see the
    124   1.3  christos <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
    125  1.10  christos The <a href="http://cldr.unicode.org">Unicode Common Locale Data
    126   1.3  christos Repository</a> contains data that may be useful for other selection
    127  1.10  christos interfaces; it maps timezone names like <code>Europe/Prague</code> to
    128  1.10  christos locale-dependent strings like "Prague", "Praha", "", and "".
    129   1.1  christos </p>
    130   1.1  christos 
    131   1.1  christos <p>
    132   1.3  christos The naming conventions attempt to strike a balance
    133   1.1  christos among the following goals:
    134   1.1  christos </p>
    135   1.3  christos 
    136   1.1  christos <ul>
    137   1.1  christos   <li>
    138   1.4  christos     Uniquely identify every timezone where clocks have agreed since 1970.
    139   1.3  christos     This is essential for the intended use: static clocks keeping local
    140   1.3  christos     civil time.
    141   1.1  christos   </li>
    142   1.1  christos   <li>
    143   1.4  christos     Indicate to experts where the timezone's clocks typically are.
    144   1.1  christos   </li>
    145   1.1  christos   <li>
    146   1.3  christos     Be robust in the presence of political changes.
    147   1.7  christos     For example, names are typically not tied to countries, to avoid
    148   1.3  christos     incompatibilities when countries change their name (e.g.,
    149   1.7  christos     Swaziland&rarr;Eswatini) or when locations change countries (e.g., Hong
    150   1.3  christos     Kong from UK colony to China).
    151   1.7  christos     There is no requirement that every country or national
    152   1.7  christos     capital must have a timezone name.
    153   1.1  christos   </li>
    154   1.1  christos   <li>
    155   1.3  christos     Be portable to a wide variety of implementations.
    156   1.1  christos   </li>
    157   1.1  christos   <li>
    158   1.3  christos     Use a consistent naming conventions over the entire world.
    159   1.1  christos   </li>
    160   1.1  christos </ul>
    161   1.3  christos 
    162   1.1  christos <p>
    163   1.3  christos Names normally have the form
    164   1.3  christos <var>AREA</var><code>/</code><var>LOCATION</var>, where
    165   1.4  christos <var>AREA</var> is a continent or ocean, and
    166   1.4  christos <var>LOCATION</var> is a specific location within the area.
    167   1.3  christos North and South America share the same area, '<code>America</code>'.
    168   1.3  christos Typical names are '<code>Africa/Cairo</code>',
    169   1.3  christos '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
    170   1.3  christos Some names are further qualified to help avoid confusion; for example,
    171   1.3  christos '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
    172   1.3  christos Indiana from other Petersburgs in America.
    173   1.1  christos </p>
    174   1.1  christos 
    175   1.1  christos <p>
    176   1.3  christos Here are the general guidelines used for
    177   1.4  christos choosing timezone names,
    178   1.1  christos in decreasing order of importance:
    179   1.1  christos </p>
    180   1.3  christos 
    181   1.1  christos <ul>
    182   1.1  christos   <li>
    183   1.3  christos     Use only valid POSIX file name components (i.e., the parts of
    184   1.3  christos     names other than '<code>/</code>').
    185   1.3  christos     Do not use the file name components '<code>.</code>' and
    186   1.3  christos     '<code>..</code>'.
    187   1.3  christos     Within a file name component, use only <a
    188   1.3  christos     href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
    189   1.3  christos     '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
    190   1.3  christos     Do not use digits, as that might create an ambiguity with <a
    191   1.8  christos     href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
    192   1.3  christos     <code>TZ</code> strings</a>.
    193   1.3  christos     A file name component must not exceed 14 characters or start with
    194   1.3  christos     '<code>-</code>'.
    195  1.12  christos     E.g., prefer <code>America/Noronha</code> to
    196  1.12  christos     <code>America/Fernando_de_Noronha</code>.
    197   1.3  christos     Exceptions: see the discussion of legacy names below.
    198   1.1  christos   </li>
    199   1.1  christos   <li>
    200   1.3  christos     A name must not be empty, or contain '<code>//</code>', or
    201   1.3  christos     start or end with '<code>/</code>'.
    202   1.1  christos   </li>
    203   1.1  christos   <li>
    204   1.3  christos     Do not use names that differ only in case.
    205   1.3  christos     Although the reference implementation is case-sensitive, some
    206   1.3  christos     other implementations are not, and they would mishandle names
    207   1.3  christos     differing only in case.
    208   1.1  christos   </li>
    209   1.1  christos   <li>
    210   1.3  christos     If one name <var>A</var> is an initial prefix of another
    211   1.3  christos     name <var>AB</var> (ignoring case), then <var>B</var> must not
    212   1.3  christos     start with '<code>/</code>', as a regular file cannot have the
    213   1.3  christos     same name as a directory in POSIX.
    214   1.3  christos     For example, <code>America/New_York</code> precludes
    215   1.3  christos     <code>America/New_York/Bronx</code>.
    216   1.1  christos   </li>
    217   1.1  christos   <li>
    218   1.3  christos     Uninhabited regions like the North Pole and Bouvet Island
    219   1.3  christos     do not need locations, since local time is not defined there.
    220   1.1  christos   </li>
    221   1.1  christos   <li>
    222   1.4  christos     If all the clocks in a timezone have agreed since 1970,
    223   1.4  christos     do not bother to include more than one timezone
    224   1.4  christos     even if some of the clocks disagreed before 1970.
    225   1.3  christos     Otherwise these tables would become annoyingly large.
    226   1.1  christos   </li>
    227   1.1  christos   <li>
    228   1.7  christos     If boundaries between regions are fluid, such as during a war or
    229   1.7  christos     insurrection, do not bother to create a new timezone merely
    230   1.7  christos     because of yet another boundary change. This helps prevent table
    231   1.7  christos     bloat and simplifies maintenance.
    232   1.7  christos   </li>
    233   1.7  christos   <li>
    234   1.3  christos     If a name is ambiguous, use a less ambiguous alternative;
    235   1.3  christos     e.g., many cities are named San Jos and Georgetown, so
    236   1.3  christos     prefer <code>America/Costa_Rica</code> to
    237   1.3  christos     <code>America/San_Jose</code> and <code>America/Guyana</code>
    238   1.3  christos     to <code>America/Georgetown</code>.
    239   1.1  christos   </li>
    240   1.1  christos   <li>
    241   1.3  christos     Keep locations compact.
    242   1.3  christos     Use cities or small islands, not countries or regions, so that any
    243   1.3  christos     future changes do not split individual locations into different
    244   1.4  christos     timezones.
    245   1.3  christos     E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
    246   1.3  christos     since
    247   1.3  christos     <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
    248   1.3  christos     has had multiple time zones</a>.
    249   1.1  christos   </li>
    250   1.1  christos   <li>
    251   1.3  christos     Use mainstream English spelling, e.g., prefer
    252   1.4  christos     <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
    253   1.3  christos     prefer <code>Europe/Athens</code> to the Greek
    254   1.4  christos     <code>/</code> or the Romanized
    255   1.4  christos     <code>Evrpi/Athna</code>.
    256   1.3  christos     The POSIX file name restrictions encourage this guideline.
    257   1.1  christos   </li>
    258   1.1  christos   <li>
    259   1.3  christos     Use the most populous among locations in a region,
    260   1.3  christos     e.g., prefer <code>Asia/Shanghai</code> to
    261   1.3  christos     <code>Asia/Beijing</code>.
    262   1.3  christos     Among locations with similar populations, pick the best-known
    263   1.3  christos     location, e.g., prefer <code>Europe/Rome</code> to
    264   1.3  christos     <code>Europe/Milan</code>.
    265   1.1  christos   </li>
    266   1.1  christos   <li>
    267   1.3  christos     Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
    268   1.3  christos     <code>Atlantic/Canaries</code>.
    269   1.1  christos   </li>
    270   1.1  christos   <li>
    271   1.3  christos     Omit common suffixes like '<code>_Islands</code>' and
    272   1.3  christos     '<code>_City</code>', unless that would lead to ambiguity.
    273   1.3  christos     E.g., prefer <code>America/Cayman</code> to
    274   1.3  christos     <code>America/Cayman_Islands</code> and
    275   1.3  christos     <code>America/Guatemala</code> to
    276   1.3  christos     <code>America/Guatemala_City</code>, but prefer
    277   1.3  christos     <code>America/Mexico_City</code> to
    278   1.3  christos     <code>America/Mexico</code>
    279   1.3  christos     because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
    280   1.3  christos     country of Mexico has several time zones</a>.
    281   1.1  christos   </li>
    282   1.1  christos   <li>
    283   1.3  christos     Use '<code>_</code>' to represent a space.
    284   1.1  christos   </li>
    285   1.1  christos   <li>
    286   1.3  christos     Omit '<code>.</code>' from abbreviations in names.
    287   1.3  christos     E.g., prefer <code>Atlantic/St_Helena</code> to
    288   1.3  christos     <code>Atlantic/St._Helena</code>.
    289   1.1  christos   </li>
    290   1.1  christos   <li>
    291   1.3  christos     Do not change established names if they only marginally violate
    292   1.3  christos     the above guidelines.
    293   1.3  christos     For example, do not change the existing name <code>Europe/Rome</code> to
    294   1.3  christos     <code>Europe/Milan</code> merely because Milan's population has grown
    295   1.3  christos     to be somewhat greater than Rome's.
    296   1.1  christos   </li>
    297   1.1  christos   <li>
    298   1.3  christos     If a name is changed, put its old spelling in the
    299   1.3  christos     '<code>backward</code>' file.
    300   1.3  christos     This means old spellings will continue to work.
    301   1.9  christos     Ordinarily a name change should occur only in the rare case when
    302   1.9  christos     a location's consensus English-language spelling changes; for example,
    303   1.9  christos     in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
    304   1.9  christos     due to long-time widespread use of the new city name instead of the old.
    305   1.1  christos   </li>
    306   1.1  christos </ul>
    307   1.1  christos 
    308   1.1  christos <p>
    309   1.7  christos Guidelines have evolved with time, and names following old versions of
    310   1.8  christos these guidelines might not follow the current version. When guidelines
    311   1.7  christos have changed, old names continue to be supported. Guideline changes
    312   1.7  christos have included the following:
    313   1.1  christos </p>
    314   1.1  christos 
    315   1.7  christos <ul>
    316   1.7  christos <li>
    317   1.7  christos Older versions of this package used a different naming scheme.
    318   1.1  christos See the file '<code>backward</code>' for most of these older names
    319   1.1  christos (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
    320   1.1  christos The other old-fashioned names still supported are
    321   1.3  christos '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
    322   1.3  christos '<code>EET</code>' (see the file '<code>europe</code>').
    323   1.7  christos </li>
    324   1.1  christos 
    325   1.7  christos <li>
    326   1.1  christos Older versions of this package defined legacy names that are
    327   1.3  christos incompatible with the first guideline of location names, but which are
    328   1.3  christos still supported.
    329   1.3  christos These legacy names are mostly defined in the file
    330   1.3  christos '<code>etcetera</code>'.
    331   1.3  christos Also, the file '<code>backward</code>' defines the legacy names
    332   1.3  christos '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
    333   1.3  christos and the file '<code>northamerica</code>' defines the legacy names
    334   1.3  christos '<code>EST5EDT</code>', '<code>CST6CDT</code>',
    335   1.3  christos '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
    336   1.7  christos </li>
    337   1.7  christos 
    338   1.7  christos <li>
    339   1.8  christos Older versions of these guidelines said that
    340   1.7  christos there should typically be at least one name for each <a
    341   1.7  christos href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
    342   1.7  christos title="International Organization for Standardization">ISO</abbr>
    343   1.7  christos 3166-1</a> officially assigned two-letter code for an inhabited
    344   1.7  christos country or territory.
    345   1.7  christos This old guideline has been dropped, as it was not needed to handle
    346   1.7  christos timestamps correctly and it increased maintenance burden.
    347   1.7  christos </li>
    348   1.7  christos </ul>
    349   1.7  christos 
    350   1.7  christos <p>
    351   1.7  christos The file '<code>zone1970.tab</code>' lists geographical locations used
    352   1.7  christos to name timezones.
    353   1.7  christos It is intended to be an exhaustive list of names for geographic
    354   1.7  christos regions as described above; this is a subset of the timezones in the data.
    355   1.7  christos Although a '<code>zone1970.tab</code>' location's
    356   1.7  christos <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
    357   1.7  christos corresponds to
    358   1.7  christos its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
    359   1.7  christos time (<abbr>LMT</abbr>)</a> offset with one hour for every 15&deg;
    360   1.7  christos east longitude, this relationship is not exact.
    361   1.1  christos </p>
    362   1.1  christos 
    363   1.1  christos <p>
    364   1.3  christos Excluding '<code>backward</code>' should not affect the other data.
    365   1.3  christos If '<code>backward</code>' is excluded, excluding
    366   1.3  christos '<code>etcetera</code>' should not affect the remaining data.
    367   1.1  christos </p>
    368   1.3  christos </section>
    369   1.1  christos 
    370   1.3  christos <section>
    371   1.3  christos   <h2 id="abbreviations">Time zone abbreviations</h2>
    372   1.1  christos <p>
    373   1.1  christos When this package is installed, it generates time zone abbreviations
    374   1.1  christos like '<code>EST</code>' to be compatible with human tradition and POSIX.
    375   1.3  christos Here are the general guidelines used for choosing time zone abbreviations,
    376   1.1  christos in decreasing order of importance:
    377   1.3  christos </p>
    378   1.3  christos 
    379   1.1  christos <ul>
    380   1.1  christos   <li>
    381   1.3  christos     Use three to six characters that are ASCII alphanumerics or
    382   1.3  christos     '<code>+</code>' or '<code>-</code>'.
    383   1.3  christos     Previous editions of this database also used characters like
    384   1.3  christos     space and '<code>?</code>', but these characters have a
    385   1.3  christos     special meaning to the
    386   1.3  christos     <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
    387   1.3  christos     and cause commands like
    388   1.8  christos     '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
    389   1.8  christos     `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
    390   1.3  christos     to have unexpected effects.
    391   1.3  christos     Previous editions of this guideline required upper-case letters, but the
    392   1.3  christos     Congressman who introduced
    393   1.3  christos     <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
    394   1.3  christos     Standard Time</a> preferred "ChST", so lower-case letters are now
    395   1.3  christos     allowed.
    396   1.3  christos     Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
    397   1.3  christos     '<code>+</code>', and alphanumeric characters from the portable
    398   1.3  christos     character set in the current locale.
    399   1.3  christos     In practice ASCII alphanumerics and '<code>+</code>' and
    400   1.3  christos     '<code>-</code>' are safe in all locales.
    401   1.3  christos 
    402   1.3  christos     <p>
    403   1.3  christos     In other words, in the C locale the POSIX extended regular
    404   1.3  christos     expression <code>[-+[:alnum:]]{3,6}</code> should match the
    405   1.3  christos     abbreviation.
    406   1.3  christos     This guarantees that all abbreviations could have been specified by a
    407   1.3  christos     POSIX <code>TZ</code> string.
    408   1.3  christos     </p>
    409   1.3  christos   </li>
    410   1.3  christos   <li>
    411   1.3  christos     Use abbreviations that are in common use among English-speakers,
    412   1.3  christos     e.g., 'EST' for Eastern Standard Time in North America.
    413   1.3  christos     We assume that applications translate them to other languages
    414   1.3  christos     as part of the normal localization process; for example,
    415   1.3  christos     a French application might translate 'EST' to 'HNE'.
    416   1.3  christos 
    417   1.3  christos     <p>
    418   1.3  christos     <small>These abbreviations (for standard/daylight/etc. time) are:
    419   1.3  christos       ACST/ACDT Australian Central,
    420   1.3  christos       AST/ADT/APT/AWT/ADDT Atlantic,
    421   1.3  christos       AEST/AEDT Australian Eastern,
    422   1.3  christos       AHST/AHDT Alaska-Hawaii,
    423   1.3  christos       AKST/AKDT Alaska,
    424   1.3  christos       AWST/AWDT Australian Western,
    425   1.3  christos       BST/BDT Bering,
    426   1.3  christos       CAT/CAST Central Africa,
    427   1.3  christos       CET/CEST/CEMT Central European,
    428   1.3  christos       ChST Chamorro,
    429   1.3  christos       CST/CDT/CWT/CPT/CDDT Central [North America],
    430   1.3  christos       CST/CDT China,
    431   1.3  christos       GMT/BST/IST/BDST Greenwich,
    432   1.3  christos       EAT East Africa,
    433   1.3  christos       EST/EDT/EWT/EPT/EDDT Eastern [North America],
    434   1.3  christos       EET/EEST Eastern European,
    435   1.6  christos       GST/GDT Guam,
    436   1.5  christos       HST/HDT/HWT/HPT Hawaii,
    437   1.9  christos       HKT/HKST/HKWT Hong Kong,
    438   1.3  christos       IST India,
    439   1.3  christos       IST/GMT Irish,
    440   1.3  christos       IST/IDT/IDDT Israel,
    441   1.3  christos       JST/JDT Japan,
    442   1.3  christos       KST/KDT Korea,
    443   1.3  christos       MET/MEST Middle European (a backward-compatibility alias for
    444   1.3  christos 	Central European),
    445   1.3  christos       MSK/MSD Moscow,
    446   1.3  christos       MST/MDT/MWT/MPT/MDDT Mountain,
    447   1.3  christos       NST/NDT/NWT/NPT/NDDT Newfoundland,
    448   1.3  christos       NST/NDT/NWT/NPT Nome,
    449   1.3  christos       NZMT/NZST New Zealand through 1945,
    450   1.3  christos       NZST/NZDT New Zealand 1946&ndash;present,
    451   1.3  christos       PKT/PKST Pakistan,
    452   1.3  christos       PST/PDT/PWT/PPT/PDDT Pacific,
    453   1.4  christos       PST/PDT Philippine,
    454   1.3  christos       SAST South Africa,
    455   1.3  christos       SST Samoa,
    456   1.3  christos       WAT/WAST West Africa,
    457   1.3  christos       WET/WEST/WEMT Western European,
    458   1.3  christos       WIB Waktu Indonesia Barat,
    459   1.3  christos       WIT Waktu Indonesia Timur,
    460   1.3  christos       WITA Waktu Indonesia Tengah,
    461   1.3  christos       YST/YDT/YWT/YPT/YDDT Yukon</small>.
    462   1.3  christos     </p>
    463   1.3  christos   </li>
    464   1.3  christos   <li>
    465   1.3  christos     <p>
    466   1.3  christos     For times taken from a city's longitude, use the
    467   1.3  christos     traditional <var>x</var>MT notation.
    468   1.3  christos     The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
    469   1.3  christos     The others are for timestamps before 1960,
    470   1.3  christos     except that Monrovia Mean Time persisted until 1972.
    471   1.3  christos     Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
    472   1.3  christos     MMT) would cause trouble here, as the numeric strings would exceed
    473   1.3  christos     the POSIX length limit.
    474   1.3  christos     </p>
    475   1.3  christos 
    476   1.3  christos     <p>
    477   1.3  christos     <small>These abbreviations are:
    478  1.12  christos       AMT Asuncin, Athens;
    479  1.11  christos       BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogot, Bridgetown,
    480  1.11  christos         Brussels, Bucharest;
    481  1.12  christos       CMT Calamarca, Caracas, Chisinau, Coln, Crdoba;
    482   1.3  christos       DMT Dublin/Dunsink;
    483   1.3  christos       EMT Easter;
    484   1.3  christos       FFMT Fort-de-France;
    485   1.3  christos       FMT Funchal;
    486   1.3  christos       GMT Greenwich;
    487   1.3  christos       HMT Havana, Helsinki, Horta, Howrah;
    488   1.3  christos       IMT Irkutsk, Istanbul;
    489   1.3  christos       JMT Jerusalem;
    490   1.3  christos       KMT Kaunas, Kiev, Kingston;
    491   1.3  christos       LMT Lima, Lisbon, local, Luanda;
    492   1.3  christos       MMT Macassar, Madras, Mal, Managua, Minsk, Monrovia, Montevideo,
    493   1.3  christos 	Moratuwa, Moscow;
    494   1.3  christos       PLMT Ph Lin;
    495   1.3  christos       PMT Paramaribo, Paris, Perm, Pontianak, Prague;
    496   1.3  christos       PMMT Port Moresby;
    497   1.3  christos       QMT Quito;
    498   1.3  christos       RMT Rangoon, Riga, Rome;
    499   1.3  christos       SDMT Santo Domingo;
    500   1.3  christos       SJMT San Jos;
    501   1.3  christos       SMT Santiago, Simferopol, Singapore, Stanley;
    502   1.3  christos       TBMT Tbilisi;
    503   1.3  christos       TMT Tallinn, Tehran;
    504  1.12  christos       WMT Warsaw;
    505  1.12  christos       ZMT Zomba.</small>
    506   1.3  christos     </p>
    507   1.3  christos 
    508   1.3  christos     <p>
    509   1.3  christos     <small>A few abbreviations also follow the pattern that
    510   1.4  christos     <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
    511   1.3  christos     They are:
    512  1.11  christos       BMT/BST for Bermuda 1890&ndash;1930,
    513   1.3  christos       CMT/BST for Calamarca Mean Time and Bolivian Summer Time
    514   1.3  christos 	1890&ndash;1932,
    515   1.3  christos       DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
    516   1.3  christos 	1880&ndash;1916,
    517   1.3  christos       MMT/MST/MDST for Moscow 1880&ndash;1919, and
    518   1.3  christos       RMT/LST for Riga Mean Time and Latvian Summer time 1880&ndash;1926.
    519  1.12  christos     </small>
    520   1.3  christos     </p>
    521   1.3  christos   </li>
    522   1.3  christos   <li>
    523   1.3  christos     Use '<abbr>LMT</abbr>' for local mean time of locations before the
    524   1.3  christos     introduction of standard time; see "<a href="#scope">Scope of the
    525   1.3  christos     <code><abbr>tz</abbr></code> database</a>".
    526   1.3  christos   </li>
    527   1.3  christos   <li>
    528   1.3  christos     If there is no common English abbreviation, use numeric offsets like
    529   1.4  christos     <code>-</code>05 and <code>+</code>0530 that are generated
    530   1.3  christos     by <code>zic</code>'s <code>%z</code> notation.
    531   1.3  christos   </li>
    532   1.3  christos   <li>
    533   1.3  christos     Use current abbreviations for older timestamps to avoid confusion.
    534   1.3  christos     For example, in 1910 a common English abbreviation for time
    535   1.3  christos     in central Europe was 'MEZ' (short for both "Middle European
    536   1.3  christos     Zone" and for "Mitteleuropische Zeit" in German).
    537   1.3  christos     Nowadays 'CET' ("Central European Time") is more common in
    538   1.3  christos     English, and the database uses 'CET' even for circa-1910
    539   1.3  christos     timestamps as this is less confusing for modern users and avoids
    540   1.3  christos     the need for determining when 'CET' supplanted 'MEZ' in common
    541   1.3  christos     usage.
    542   1.3  christos   </li>
    543   1.3  christos   <li>
    544   1.4  christos     Use a consistent style in a timezone's history.
    545   1.4  christos     For example, if a history tends to use numeric
    546   1.3  christos     abbreviations and a particular entry could go either way, use a
    547   1.3  christos     numeric abbreviation.
    548   1.3  christos   </li>
    549   1.3  christos   <li>
    550   1.3  christos     Use
    551   1.3  christos     <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
    552   1.3  christos     (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
    553   1.3  christos     locations while uninhabited.
    554   1.3  christos     The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
    555   1.3  christos     some sense undefined; this notation is derived
    556   1.3  christos     from <a href="https://tools.ietf.org/html/rfc3339">Internet
    557   1.4  christos     <abbr title="Request For Comments">RFC</abbr> 3339</a>.
    558   1.1  christos   </li>
    559   1.1  christos </ul>
    560   1.3  christos 
    561   1.1  christos <p>
    562   1.1  christos Application writers should note that these abbreviations are ambiguous
    563   1.2  christos in practice: e.g., 'CST' means one thing in China and something else
    564   1.2  christos in North America, and 'IST' can refer to time in India, Ireland or
    565   1.3  christos Israel.
    566   1.3  christos To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
    567   1.2  christos '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
    568   1.1  christos </p>
    569   1.3  christos </section>
    570   1.1  christos 
    571   1.3  christos <section>
    572   1.3  christos   <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
    573   1.1  christos <p>
    574   1.3  christos The <code><abbr>tz</abbr></code> database is not authoritative, and it
    575   1.3  christos surely has errors.
    576   1.2  christos Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
    577   1.1  christos Users requiring authoritative data should consult national standards
    578   1.1  christos bodies and the references cited in the database's comments.
    579   1.1  christos </p>
    580   1.1  christos 
    581   1.1  christos <p>
    582   1.3  christos Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
    583   1.1  christos </p>
    584   1.3  christos 
    585   1.1  christos <ul>
    586   1.1  christos   <li>
    587   1.3  christos     The <code><abbr>tz</abbr></code> database predicts future
    588   1.3  christos     timestamps, and current predictions
    589   1.3  christos     will be incorrect after future governments change the rules.
    590   1.3  christos     For example, if today someone schedules a meeting for 13:00 next
    591   1.3  christos     October 1, Casablanca time, and tomorrow Morocco changes its
    592   1.3  christos     daylight saving rules, software can mess up after the rule change
    593   1.3  christos     if it blithely relies on conversions made before the change.
    594   1.3  christos   </li>
    595   1.3  christos   <li>
    596   1.3  christos     The pre-1970 entries in this database cover only a tiny sliver of how
    597   1.3  christos     clocks actually behaved; the vast majority of the necessary
    598   1.3  christos     information was lost or never recorded.
    599   1.4  christos     Thousands more timezones would be needed if
    600   1.3  christos     the <code><abbr>tz</abbr></code> database's scope were extended to
    601   1.3  christos     cover even just the known or guessed history of standard time; for
    602   1.3  christos     example, the current single entry for France would need to split
    603   1.3  christos     into dozens of entries, perhaps hundreds.
    604   1.3  christos     And in most of the world even this approach would be misleading
    605   1.3  christos     due to widespread disagreement or indifference about what times
    606   1.3  christos     should be observed.
    607   1.3  christos     In her 2015 book
    608   1.3  christos     <cite><a
    609   1.3  christos     href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
    610   1.3  christos     Global Transformation of Time, 1870&ndash;1950</a></cite>,
    611   1.3  christos     Vanessa Ogle writes
    612   1.3  christos     "Outside of Europe and North America there was no system of time
    613   1.3  christos     zones at all, often not even a stable landscape of mean times,
    614   1.3  christos     prior to the middle decades of the twentieth century".
    615   1.3  christos     See: Timothy Shenk, <a
    616   1.3  christos href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
    617   1.3  christos       A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
    618   1.3  christos   </li>
    619   1.3  christos   <li>
    620   1.3  christos     Most of the pre-1970 data entries come from unreliable sources, often
    621   1.3  christos     astrology books that lack citations and whose compilers evidently
    622   1.3  christos     invented entries when the true facts were unknown, without
    623   1.3  christos     reporting which entries were known and which were invented.
    624   1.3  christos     These books often contradict each other or give implausible entries,
    625   1.3  christos     and on the rare occasions when they are checked they are
    626   1.3  christos     typically found to be incorrect.
    627   1.3  christos   </li>
    628   1.3  christos   <li>
    629   1.3  christos     For the UK the <code><abbr>tz</abbr></code> database relies on
    630   1.3  christos     years of first-class work done by
    631   1.3  christos     Joseph Myers and others; see
    632   1.3  christos     "<a href="https://www.polyomino.org.uk/british-time/">History of
    633   1.3  christos     legal time in Britain</a>".
    634   1.3  christos     Other countries are not done nearly as well.
    635   1.3  christos   </li>
    636   1.3  christos   <li>
    637   1.3  christos     Sometimes, different people in the same city maintain clocks
    638   1.3  christos     that differ significantly.
    639   1.3  christos     Historically, railway time was used by railroad companies (which
    640   1.3  christos     did not always
    641   1.3  christos     agree with each other), church-clock time was used for birth
    642   1.3  christos     certificates, etc.
    643   1.3  christos     More recently, competing political groups might disagree about
    644   1.3  christos     clock settings. Often this is merely common practice, but
    645   1.3  christos     sometimes it is set by law.
    646   1.3  christos     For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
    647   1.3  christos     was legally <abbr>UT</abbr> +00:09:21 outside train stations and
    648   1.3  christos     <abbr>UT</abbr> +00:04:21 inside. Other examples include
    649   1.3  christos     Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
    650   1.3  christos     rmqi to this day.
    651   1.3  christos   </li>
    652   1.3  christos   <li>
    653   1.3  christos     Although a named location in the <code><abbr>tz</abbr></code>
    654   1.3  christos     database stands for the containing region, its pre-1970 data
    655   1.3  christos     entries are often accurate for only a small subset of that region.
    656   1.3  christos     For example, <code>Europe/London</code> stands for the United
    657   1.3  christos     Kingdom, but its pre-1847 times are valid only for locations that
    658   1.3  christos     have London's exact meridian, and its 1847 transition
    659   1.3  christos     to <abbr>GMT</abbr> is known to be valid only for the L&amp;NW and
    660   1.3  christos     the Caledonian railways.
    661   1.3  christos   </li>
    662   1.3  christos   <li>
    663   1.3  christos     The <code><abbr>tz</abbr></code> database does not record the
    664   1.4  christos     earliest time for which a timezone's
    665   1.3  christos     data entries are thereafter valid for every location in the region.
    666   1.3  christos     For example, <code>Europe/London</code> is valid for all locations
    667   1.3  christos     in its region after <abbr>GMT</abbr> was made the standard time,
    668   1.3  christos     but the date of standardization (1880-08-02) is not in the
    669   1.3  christos     <code><abbr>tz</abbr></code> database, other than in commentary.
    670   1.4  christos     For many timezones the earliest time of
    671   1.3  christos     validity is unknown.
    672   1.3  christos   </li>
    673   1.3  christos   <li>
    674   1.3  christos     The <code><abbr>tz</abbr></code> database does not record a
    675   1.3  christos     region's boundaries, and in many cases the boundaries are not known.
    676   1.4  christos     For example, the timezone
    677   1.3  christos     <code>America/Kentucky/Louisville</code> represents a region
    678   1.3  christos     around the city of Louisville, the boundaries of which are
    679   1.3  christos     unclear.
    680   1.3  christos   </li>
    681   1.3  christos   <li>
    682   1.3  christos     Changes that are modeled as instantaneous transitions in the
    683   1.3  christos     <code><abbr>tz</abbr></code>
    684   1.3  christos     database were often spread out over hours, days, or even decades.
    685   1.3  christos   </li>
    686   1.3  christos   <li>
    687   1.3  christos     Even if the time is specified by law, locations sometimes
    688   1.3  christos     deliberately flout the law.
    689   1.3  christos   </li>
    690   1.3  christos   <li>
    691   1.3  christos     Early timekeeping practices, even assuming perfect clocks, were
    692   1.3  christos     often not specified to the accuracy that the
    693   1.3  christos     <code><abbr>tz</abbr></code> database requires.
    694   1.3  christos   </li>
    695   1.3  christos   <li>
    696  1.10  christos     The <code><abbr>tz</abbr></code> database cannot represent stopped clocks.
    697  1.10  christos     However, on 1911-03-11 at 00:00, some public-facing French clocks
    698  1.10  christos     were changed by stopping them for a few minutes to effect a transition.
    699  1.10  christos     The <code><abbr>tz</abbr></code> database models this via a
    700  1.10  christos     backward transition; the relevant French legislation does not
    701  1.10  christos     specify exactly how the transition was to occur.
    702  1.10  christos   </li>
    703  1.10  christos   <li>
    704   1.3  christos     Sometimes historical timekeeping was specified more precisely
    705   1.3  christos     than what the <code><abbr>tz</abbr></code> code can handle.
    706  1.12  christos     For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean
    707   1.3  christos     Time (estimated to be <abbr>UT</abbr>
    708  1.12  christos     &minus;00:25:21.1), but the <code><abbr>tz</abbr></code>
    709   1.3  christos     code cannot represent the fractional second.
    710   1.3  christos     In practice these old specifications were rarely if ever
    711   1.3  christos     implemented to subsecond precision.
    712   1.3  christos   </li>
    713   1.3  christos   <li>
    714   1.3  christos     Even when all the timestamp transitions recorded by the
    715   1.3  christos     <code><abbr>tz</abbr></code> database are correct, the
    716   1.3  christos     <code><abbr>tz</abbr></code> rules that generate them may not
    717   1.3  christos     faithfully reflect the historical rules.
    718   1.3  christos     For example, from 1922 until World War II the UK moved clocks
    719   1.3  christos     forward the day following the third Saturday in April unless that
    720   1.3  christos     was Easter, in which case it moved clocks forward the previous
    721   1.3  christos     Sunday.
    722   1.3  christos     Because the <code><abbr>tz</abbr></code> database has no
    723   1.3  christos     way to specify Easter, these exceptional years are entered as
    724   1.3  christos     separate <code><abbr>tz</abbr> Rule</code> lines, even though the
    725   1.3  christos     legal rules did not change.
    726   1.4  christos     When transitions are known but the historical rules behind them are not,
    727   1.4  christos     the database contains <code>Zone</code> and <code>Rule</code>
    728   1.4  christos     entries that are intended to represent only the generated
    729   1.4  christos     transitions, not any underlying historical rules; however, this
    730   1.4  christos     intent is recorded at best only in commentary.
    731   1.3  christos   </li>
    732   1.3  christos   <li>
    733   1.4  christos     The <code><abbr>tz</abbr></code> database models time
    734   1.3  christos     using the <a
    735   1.3  christos     href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
    736   1.4  christos     Gregorian calendar</a> with days containing 24 equal-length hours
    737   1.4  christos     numbered 00 through 23, except when clock transitions occur.
    738   1.4  christos     Pre-standard time is modeled as local mean time.
    739   1.4  christos     However, historically many people used other calendars and other timescales.
    740   1.3  christos     For example, the Roman Empire used
    741   1.3  christos     the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
    742   1.3  christos     calendar</a>,
    743   1.3  christos     and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
    744   1.3  christos     timekeeping</a> had twelve varying-length daytime hours with a
    745   1.3  christos     non-hour-based system at night.
    746   1.4  christos     And even today, some local practices diverge from the Gregorian
    747   1.4  christos     calendar with 24-hour days. These divergences range from
    748   1.4  christos     relatively minor, such as Japanese bars giving times like "24:30" for the
    749   1.4  christos     wee hours of the morning, to more-significant differences such as <a
    750   1.4  christos     href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the
    751   1.4  christos     east African practice of starting the day at dawn</a>, renumbering
    752   1.4  christos     the Western 06:00 to be 12:00. These practices are largely outside
    753   1.4  christos     the scope of the <code><abbr>tz</abbr></code> code and data, which
    754   1.4  christos     provide only limited support for date and time localization
    755  1.12  christos     such as that required by POSIX.
    756  1.12  christos     If <abbr>DST</abbr> is not used a different time zone
    757   1.4  christos     can often do the trick; for example, in Kenya a <code>TZ</code> setting
    758   1.4  christos     like <code>&lt;-03&gt;3</code> or <code>America/Cayenne</code> starts
    759   1.4  christos     the day six hours later than <code>Africa/Nairobi</code> does.
    760   1.3  christos   </li>
    761   1.3  christos   <li>
    762   1.3  christos     Early clocks were less reliable, and data entries do not represent
    763   1.3  christos     clock error.
    764   1.3  christos   </li>
    765   1.3  christos   <li>
    766   1.3  christos     The <code><abbr>tz</abbr></code> database assumes Universal Time
    767   1.3  christos     (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
    768   1.3  christos     standardized for older timestamps.
    769   1.3  christos     In the <code><abbr>tz</abbr></code> database commentary,
    770   1.3  christos     <abbr>UT</abbr> denotes a family of time standards that includes
    771   1.3  christos     Coordinated Universal Time (<abbr>UTC</abbr>) along with other
    772   1.3  christos     variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
    773   1.3  christos     with days starting at midnight.
    774   1.3  christos     Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
    775   1.3  christos     timestamps, <abbr>UTC</abbr> was not defined until 1960, so
    776   1.3  christos     commentary uses the more-general abbreviation <abbr>UT</abbr> for
    777   1.3  christos     timestamps that might predate 1960.
    778   1.3  christos     Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
    779   1.3  christos     and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
    780   1.3  christos     interpretation of older timestamps can be problematic when
    781   1.3  christos     subsecond accuracy is needed.
    782   1.3  christos   </li>
    783   1.3  christos   <li>
    784   1.3  christos     Civil time was not based on atomic time before 1972, and we do not
    785   1.3  christos     know the history of
    786   1.3  christos     <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
    787   1.3  christos     rotation</a> accurately enough to map <a
    788   1.3  christos     href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
    789   1.3  christos     title="International System of Units">SI</abbr></a> seconds to
    790   1.3  christos     historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
    791   1.3  christos     to more than about one-hour accuracy.
    792   1.3  christos     See: Stephenson FR, Morrison LV, Hohenkerk CY.
    793   1.4  christos     <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
    794   1.3  christos     the Earth's rotation: 720 BC to AD 2015</a>.
    795   1.3  christos     <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
    796   1.3  christos     Also see: Espenak F. <a
    797   1.3  christos     href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
    798   1.3  christos     in Delta T (T)</a>.
    799   1.3  christos   </li>
    800   1.3  christos   <li>
    801   1.3  christos     The relationship between POSIX time (that is, <abbr>UTC</abbr> but
    802   1.3  christos     ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
    803   1.3  christos     seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
    804   1.3  christos     Although the POSIX
    805   1.3  christos     clock officially stops during an inserted leap second, at least one
    806   1.3  christos     proposed standard has it jumping back a second instead; and in
    807   1.3  christos     practice POSIX clocks more typically either progress glacially during
    808   1.3  christos     a leap second, or are slightly slowed while near a leap second.
    809   1.3  christos   </li>
    810   1.3  christos   <li>
    811   1.3  christos     The <code><abbr>tz</abbr></code> database does not represent how
    812   1.3  christos     uncertain its information is.
    813   1.3  christos     Ideally it would contain information about when data entries are
    814   1.3  christos     incomplete or dicey.
    815   1.3  christos     Partial temporal knowledge is a field of active research, though,
    816   1.3  christos     and it is not clear how to apply it here.
    817   1.1  christos   </li>
    818   1.1  christos </ul>
    819   1.1  christos 
    820   1.1  christos <p>
    821   1.3  christos In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
    822   1.3  christos database's pre-1970 and future timestamps are either wrong or
    823   1.3  christos misleading.
    824   1.3  christos Any attempt to pass the
    825   1.3  christos <code><abbr>tz</abbr></code> database off as the definition of time
    826   1.3  christos should be unacceptable to anybody who cares about the facts.
    827   1.3  christos In particular, the <code><abbr>tz</abbr></code> database's
    828   1.3  christos <abbr>LMT</abbr> offsets should not be considered meaningful, and
    829   1.4  christos should not prompt creation of timezones
    830   1.3  christos merely because two locations
    831   1.3  christos differ in <abbr>LMT</abbr> or transitioned to standard time at
    832   1.3  christos different dates.
    833   1.3  christos </p>
    834   1.3  christos </section>
    835   1.3  christos 
    836   1.3  christos <section>
    837   1.3  christos   <h2 id="functions">Time and date functions</h2>
    838   1.3  christos <p>
    839   1.3  christos The <code><abbr>tz</abbr></code> code contains time and date functions
    840   1.3  christos that are upwards compatible with those of POSIX.
    841   1.3  christos Code compatible with this package is already
    842   1.3  christos <a href="tz-link.html#tzdb">part of many platforms</a>, where the
    843   1.3  christos primary use of this package is to update obsolete time-related files.
    844   1.3  christos To do this, you may need to compile the time zone compiler
    845   1.3  christos '<code>zic</code>' supplied with this package instead of using the
    846   1.3  christos system '<code>zic</code>', since the format of <code>zic</code>'s
    847   1.3  christos input is occasionally extended, and a platform may still be shipping
    848   1.3  christos an older <code>zic</code>.
    849   1.1  christos </p>
    850   1.1  christos 
    851   1.3  christos <h3 id="POSIX">POSIX properties and limitations</h3>
    852   1.1  christos <ul>
    853   1.1  christos   <li>
    854   1.1  christos     <p>
    855   1.3  christos     In POSIX, time display in a process is controlled by the
    856   1.3  christos     environment variable <code>TZ</code>.
    857   1.3  christos     Unfortunately, the POSIX
    858   1.3  christos     <code>TZ</code> string takes a form that is hard to describe and
    859   1.3  christos     is error-prone in practice.
    860   1.3  christos     Also, POSIX <code>TZ</code> strings cannot deal with daylight
    861   1.3  christos     saving time rules not based on the Gregorian calendar (as in
    862   1.3  christos     Iran), or with situations where more than two time zone
    863   1.3  christos     abbreviations or <abbr>UT</abbr> offsets are used in an area.
    864   1.1  christos     </p>
    865   1.3  christos 
    866   1.1  christos     <p>
    867   1.3  christos     The POSIX <code>TZ</code> string takes the following form:
    868   1.1  christos     </p>
    869   1.3  christos 
    870   1.1  christos     <p>
    871   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>]]]
    872   1.1  christos     </p>
    873   1.3  christos 
    874   1.1  christos     <p>
    875   1.3  christos     where:
    876   1.3  christos     </p>
    877   1.3  christos 
    878   1.1  christos     <dl>
    879   1.1  christos       <dt><var>std</var> and <var>dst</var></dt><dd>
    880   1.3  christos 	are 3 or more characters specifying the standard
    881   1.4  christos 	and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
    882   1.3  christos 	Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
    883   1.3  christos 	may also be in a quoted form like '<code>&lt;+09&gt;</code>';
    884   1.3  christos 	this allows "<code>+</code>" and "<code>-</code>" in the names.
    885   1.1  christos       </dd>
    886   1.1  christos       <dt><var>offset</var></dt><dd>
    887   1.3  christos 	is of the form
    888   1.3  christos 	'<code>[&plusmn;]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
    889   1.3  christos 	and specifies the offset west of <abbr>UT</abbr>.
    890   1.3  christos 	'<var>hh</var>' may be a single digit;
    891   1.3  christos 	0&le;<var>hh</var>&le;24.
    892   1.3  christos 	The default <abbr>DST</abbr> offset is one hour ahead of
    893   1.3  christos 	standard time.
    894   1.1  christos       </dd>
    895   1.1  christos       <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
    896   1.3  christos 	specifies the beginning and end of <abbr>DST</abbr>.
    897   1.3  christos 	If this is absent, the system supplies its own ruleset
    898   1.3  christos 	for <abbr>DST</abbr>, and its rules can differ from year to year;
    899   1.3  christos 	typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
    900   1.1  christos       </dd>
    901   1.1  christos       <dt><var>time</var></dt><dd>
    902   1.3  christos 	takes the form
    903   1.3  christos 	'<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
    904   1.3  christos 	and defaults to 02:00.
    905   1.3  christos 	This is the same format as the offset, except that a
    906   1.3  christos 	leading '<code>+</code>' or '<code>-</code>' is not allowed.
    907   1.1  christos       </dd>
    908   1.1  christos       <dt><var>date</var></dt><dd>
    909   1.3  christos 	takes one of the following forms:
    910   1.1  christos 	<dl>
    911   1.1  christos 	  <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
    912   1.3  christos 	    origin-1 day number not counting February 29
    913   1.3  christos 	  </dd>
    914   1.1  christos 	  <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
    915   1.3  christos 	    origin-0 day number counting February 29 if present
    916   1.3  christos 	  </dd>
    917   1.3  christos 	  <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
    918   1.3  christos 	    (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5,
    919   1.3  christos 	    1&le;<var>m</var>&le;12)</dt><dd>
    920   1.3  christos 	    for the <var>d</var>th day of week <var>n</var> of
    921   1.3  christos 	    month <var>m</var> of the year, where week 1 is the first
    922   1.3  christos 	    week in which day <var>d</var> appears, and
    923   1.3  christos 	    '<code>5</code>' stands for the last week in which
    924   1.3  christos 	    day <var>d</var> appears (which may be either the 4th or
    925   1.3  christos 	    5th week).
    926   1.3  christos 	    Typically, this is the only useful form; the <var>n</var>
    927   1.3  christos 	    and <code>J</code><var>n</var> forms are rarely used.
    928   1.1  christos 	  </dd>
    929   1.3  christos 	</dl>
    930   1.3  christos       </dd>
    931   1.3  christos     </dl>
    932   1.3  christos 
    933   1.3  christos     <p>
    934   1.3  christos     Here is an example POSIX <code>TZ</code> string for New
    935   1.3  christos     Zealand after 2007.
    936   1.3  christos     It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
    937   1.3  christos     of <abbr>UT</abbr>, and that daylight saving time
    938   1.3  christos     (<abbr>NZDT</abbr>) is observed from September's last Sunday at
    939   1.3  christos     02:00 until April's first Sunday at 03:00:
    940   1.3  christos     </p>
    941   1.3  christos 
    942   1.3  christos     <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
    943   1.3  christos 
    944   1.3  christos     <p>
    945   1.3  christos     This POSIX <code>TZ</code> string is hard to remember, and
    946   1.3  christos     mishandles some timestamps before 2008.
    947   1.3  christos     With this package you can use this instead:
    948   1.3  christos     </p>
    949   1.3  christos 
    950   1.3  christos     <pre><code>TZ='Pacific/Auckland'</code></pre>
    951   1.3  christos   </li>
    952   1.3  christos   <li>
    953   1.4  christos     POSIX does not define the <abbr>DST</abbr> transitions
    954   1.4  christos     for <code>TZ</code> values like
    955   1.3  christos     "<code>EST5EDT</code>".
    956   1.4  christos     Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
    957   1.4  christos     were used to interpret such values, but this meant that the
    958   1.4  christos     <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
    959   1.4  christos     program that did time conversion. This meant that when
    960   1.4  christos     <abbr>US</abbr> time conversion rules changed (as in the United
    961   1.4  christos     States in 1987), all programs that did time conversion had to be
    962   1.3  christos     recompiled to ensure proper results.
    963   1.3  christos   </li>
    964   1.3  christos   <li>
    965   1.3  christos     The <code>TZ</code> environment variable is process-global, which
    966   1.3  christos     makes it hard to write efficient, thread-safe applications that
    967   1.4  christos     need access to multiple timezones.
    968   1.3  christos   </li>
    969   1.3  christos   <li>
    970   1.3  christos     In POSIX, there is no tamper-proof way for a process to learn the
    971   1.8  christos     system's best idea of local (wall clock) time.
    972   1.4  christos     This is important for applications that an administrator wants
    973   1.3  christos     used only at certain times &ndash; without regard to whether the
    974   1.3  christos     user has fiddled the
    975   1.3  christos     <code>TZ</code> environment variable.
    976   1.3  christos     While an administrator can "do everything in <abbr>UT</abbr>" to
    977   1.3  christos     get around the problem, doing so is inconvenient and precludes
    978   1.4  christos     handling daylight saving time shifts &ndash; as might be required to
    979   1.4  christos     limit phone calls to off-peak hours.
    980   1.3  christos   </li>
    981   1.3  christos   <li>
    982   1.3  christos     POSIX provides no convenient and efficient way to determine
    983   1.3  christos     the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
    984   1.4  christos     timestamps, particularly for timezones
    985   1.3  christos     that do not fit into the POSIX model.
    986   1.3  christos   </li>
    987   1.3  christos   <li>
    988   1.9  christos     POSIX requires that <code>time_t</code> clock counts exclude leap
    989   1.9  christos     seconds.
    990   1.3  christos   </li>
    991   1.3  christos   <li>
    992   1.3  christos     The <code><abbr>tz</abbr></code> code attempts to support all the
    993   1.3  christos     <code>time_t</code> implementations allowed by POSIX.
    994   1.3  christos     The <code>time_t</code> type represents a nonnegative count of seconds
    995   1.3  christos     since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
    996   1.3  christos     In practice, <code>time_t</code> is usually a signed 64- or 32-bit
    997   1.3  christos     integer; 32-bit signed <code>time_t</code> values stop working after
    998   1.3  christos     2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
    999   1.3  christos     days typically use a signed 64-bit integer.
   1000   1.3  christos     Unsigned 32-bit integers are used on one or two platforms, and 36-bit
   1001   1.3  christos     and 40-bit integers are also used occasionally.
   1002   1.3  christos     Although earlier POSIX versions allowed <code>time_t</code> to be a
   1003   1.4  christos     floating-point type, this was not supported by any practical system,
   1004   1.3  christos     and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
   1005   1.3  christos     require <code>time_t</code> to be an integer type.
   1006   1.1  christos   </li>
   1007   1.1  christos </ul>
   1008   1.3  christos 
   1009   1.3  christos <h3 id="POSIX-extensions">Extensions to POSIX in the
   1010   1.3  christos <code><abbr>tz</abbr></code> code</h3>
   1011   1.1  christos <ul>
   1012   1.1  christos   <li>
   1013   1.1  christos     <p>
   1014   1.3  christos     The <code>TZ</code> environment variable is used in generating
   1015   1.4  christos     the name of a file from which time-related information is read
   1016   1.3  christos     (or is interpreted  la POSIX); <code>TZ</code> is no longer
   1017   1.4  christos     constrained to be a string containing abbreviations
   1018   1.4  christos     and numeric data as described <a href="#POSIX">above</a>.
   1019   1.4  christos     The file's format is <dfn><abbr>TZif</abbr></dfn>,
   1020   1.7  christos     a timezone information format that contains binary data; see
   1021   1.7  christos     <a href="https://tools.ietf.org/html/8536">Internet
   1022   1.7  christos     <abbr>RFC</abbr> 8536</a>.
   1023   1.3  christos     The daylight saving time rules to be used for a
   1024   1.4  christos     particular timezone are encoded in the
   1025   1.4  christos     <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>,
   1026   1.4  christos     Australian, and other rules to be encoded, and
   1027   1.3  christos     allows for situations where more than two time zone
   1028   1.3  christos     abbreviations are used.
   1029   1.1  christos     </p>
   1030   1.1  christos     <p>
   1031   1.3  christos     It was recognized that allowing the <code>TZ</code> environment
   1032   1.3  christos     variable to take on values such as '<code>America/New_York</code>'
   1033   1.3  christos     might cause "old" programs (that expect <code>TZ</code> to have a
   1034   1.3  christos     certain form) to operate incorrectly; consideration was given to using
   1035   1.3  christos     some other environment variable (for example, <code>TIMEZONE</code>)
   1036   1.4  christos     to hold the string used to generate the <abbr>TZif</abbr> file's name.
   1037   1.3  christos     In the end, however, it was decided to continue using
   1038   1.3  christos     <code>TZ</code>: it is widely used for time zone purposes;
   1039   1.3  christos     separately maintaining both <code>TZ</code>
   1040   1.3  christos     and <code>TIMEZONE</code> seemed a nuisance; and systems where
   1041   1.3  christos     "new" forms of <code>TZ</code> might cause problems can simply
   1042   1.4  christos     use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which
   1043   1.4  christos     can be used by "new" programs as well as by "old" programs that
   1044   1.4  christos     assume pre-POSIX <code>TZ</code> values.
   1045   1.1  christos     </p>
   1046   1.3  christos   </li>
   1047   1.3  christos   <li>
   1048   1.3  christos     The code supports platforms with a <abbr>UT</abbr> offset member
   1049   1.3  christos     in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
   1050   1.3  christos   </li>
   1051   1.3  christos   <li>
   1052   1.3  christos     The code supports platforms with a time zone abbreviation member in
   1053   1.3  christos     <code>struct tm</code>, e.g., <code>tm_zone</code>.
   1054   1.3  christos   </li>
   1055   1.3  christos   <li>
   1056   1.3  christos     Functions <code>tzalloc</code>, <code>tzfree</code>,
   1057   1.3  christos     <code>localtime_rz</code>, and <code>mktime_z</code> for
   1058   1.3  christos     more-efficient thread-safe applications that need to use multiple
   1059   1.4  christos     timezones.
   1060   1.3  christos     The <code>tzalloc</code> and <code>tzfree</code> functions
   1061   1.3  christos     allocate and free objects of type <code>timezone_t</code>,
   1062   1.3  christos     and <code>localtime_rz</code> and <code>mktime_z</code> are
   1063   1.3  christos     like <code>localtime_r</code> and <code>mktime</code> with an
   1064   1.3  christos     extra <code>timezone_t</code> argument.
   1065   1.3  christos     The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
   1066   1.3  christos   </li>
   1067   1.3  christos   <li>
   1068   1.3  christos     Negative <code>time_t</code> values are supported, on systems
   1069   1.3  christos     where <code>time_t</code> is signed.
   1070   1.3  christos   </li>
   1071   1.3  christos   <li>
   1072   1.9  christos     These functions can account for leap seconds;
   1073   1.9  christos     see <a href="#leapsec">Leap seconds</a> below.
   1074   1.3  christos   </li>
   1075   1.1  christos </ul>
   1076   1.3  christos 
   1077   1.3  christos <h3 id="vestigial">POSIX features no longer needed</h3>
   1078   1.1  christos <p>
   1079   1.3  christos POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
   1080   1.3  christos define some <a href="https://en.wikipedia.org/wiki/API"><abbr
   1081   1.3  christos title="application programming interface">API</abbr>s</a> that are vestigial:
   1082   1.3  christos they are not needed, and are relics of a too-simple model that does
   1083   1.3  christos not suffice to handle many real-world timestamps.
   1084   1.3  christos Although the <code><abbr>tz</abbr></code> code supports these
   1085   1.3  christos vestigial <abbr>API</abbr>s for backwards compatibility, they should
   1086   1.3  christos be avoided in portable applications.
   1087   1.3  christos The vestigial <abbr>API</abbr>s are:
   1088   1.1  christos </p>
   1089   1.1  christos <ul>
   1090   1.1  christos   <li>
   1091   1.3  christos     The POSIX <code>tzname</code> variable does not suffice and is no
   1092   1.3  christos     longer needed.
   1093   1.3  christos     To get a timestamp's time zone abbreviation, consult
   1094   1.3  christos     the <code>tm_zone</code> member if available; otherwise,
   1095   1.3  christos     use <code>strftime</code>'s <code>"%Z"</code> conversion
   1096   1.3  christos     specification.
   1097   1.3  christos   </li>
   1098   1.3  christos   <li>
   1099   1.3  christos     The POSIX <code>daylight</code> and <code>timezone</code>
   1100   1.3  christos     variables do not suffice and are no longer needed.
   1101   1.3  christos     To get a timestamp's <abbr>UT</abbr> offset, consult
   1102   1.3  christos     the <code>tm_gmtoff</code> member if available; otherwise,
   1103   1.3  christos     subtract values returned by <code>localtime</code>
   1104   1.3  christos     and <code>gmtime</code> using the rules of the Gregorian calendar,
   1105   1.3  christos     or use <code>strftime</code>'s <code>"%z"</code> conversion
   1106   1.3  christos     specification if a string like <code>"+0900"</code> suffices.
   1107   1.3  christos   </li>
   1108   1.3  christos   <li>
   1109   1.3  christos     The <code>tm_isdst</code> member is almost never needed and most of
   1110   1.3  christos     its uses should be discouraged in favor of the abovementioned
   1111   1.3  christos     <abbr>API</abbr>s.
   1112   1.3  christos     Although it can still be used in arguments to
   1113   1.3  christos     <code>mktime</code> to disambiguate timestamps near
   1114   1.3  christos     a <abbr>DST</abbr> transition when the clock jumps back, this
   1115   1.3  christos     disambiguation does not work when standard time itself jumps back,
   1116   1.3  christos     which can occur when a location changes to a time zone with a
   1117   1.3  christos     lesser <abbr>UT</abbr> offset.
   1118   1.3  christos   </li>
   1119   1.3  christos </ul>
   1120   1.3  christos 
   1121   1.3  christos <h3 id="other-portability">Other portability notes</h3>
   1122   1.3  christos <ul>
   1123   1.3  christos   <li>
   1124   1.3  christos     The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
   1125   1.3  christos     UNIX</a> <code>timezone</code> function is not present in this
   1126   1.3  christos     package; it is impossible to reliably map <code>timezone</code>'s
   1127   1.3  christos     arguments (a "minutes west of <abbr>GMT</abbr>" value and a
   1128   1.3  christos     "daylight saving time in effect" flag) to a time zone
   1129   1.3  christos     abbreviation, and we refuse to guess.
   1130   1.3  christos     Programs that in the past used the <code>timezone</code> function
   1131   1.3  christos     may now examine <code>localtime(&amp;clock)-&gt;tm_zone</code>
   1132   1.3  christos     (if <code>TM_ZONE</code> is defined) or
   1133   1.3  christos     <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code>
   1134   1.9  christos     (if <code>HAVE_TZNAME</code> is nonzero) to learn the correct time
   1135   1.3  christos     zone abbreviation to use.
   1136   1.3  christos   </li>
   1137   1.3  christos   <li>
   1138   1.3  christos     The <a
   1139   1.3  christos     href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
   1140   1.3  christos     <code>gettimeofday</code> function is not
   1141   1.3  christos     used in this package.
   1142   1.3  christos     This formerly let users obtain the current <abbr>UTC</abbr> offset
   1143   1.3  christos     and <abbr>DST</abbr> flag, but this functionality was removed in
   1144   1.3  christos     later versions of <abbr>BSD</abbr>.
   1145   1.3  christos   </li>
   1146   1.3  christos   <li>
   1147   1.3  christos     In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
   1148   1.3  christos     near-maximum <code>time_t</code> values when doing conversions
   1149   1.3  christos     for places that do not use <abbr>UT</abbr>.
   1150   1.3  christos     This package takes care to do these conversions correctly.
   1151   1.3  christos     A comment in the source code tells how to get compatibly wrong
   1152   1.3  christos     results.
   1153   1.3  christos   </li>
   1154   1.3  christos   <li>
   1155   1.3  christos     The functions that are conditionally compiled
   1156   1.3  christos     if <code>STD_INSPIRED</code> is defined should, at this point, be
   1157   1.3  christos     looked on primarily as food for thought.
   1158   1.3  christos     They are not in any sense "standard compatible" &ndash; some are
   1159   1.3  christos     not, in fact, specified in <em>any</em> standard.
   1160   1.3  christos     They do, however, represent responses of various authors to
   1161   1.3  christos     standardization proposals.
   1162   1.3  christos   </li>
   1163   1.3  christos   <li>
   1164   1.4  christos     Other time conversion proposals, in particular those supported by the
   1165   1.4  christos     <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
   1166   1.4  christos     Database Parser</a>, offer a wider selection of functions
   1167   1.3  christos     that provide capabilities beyond those provided here.
   1168   1.3  christos     The absence of such functions from this package is not meant to
   1169   1.3  christos     discourage the development, standardization, or use of such
   1170   1.3  christos     functions.
   1171   1.3  christos     Rather, their absence reflects the decision to make this package
   1172   1.3  christos     contain valid extensions to POSIX, to ensure its broad
   1173   1.3  christos     acceptability.
   1174   1.3  christos     If more powerful time conversion functions can be standardized, so
   1175   1.3  christos     much the better.
   1176   1.1  christos   </li>
   1177   1.1  christos </ul>
   1178   1.3  christos </section>
   1179   1.1  christos 
   1180   1.3  christos <section>
   1181   1.3  christos   <h2 id="stability">Interface stability</h2>
   1182   1.1  christos <p>
   1183   1.3  christos The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
   1184   1.1  christos </p>
   1185   1.1  christos 
   1186   1.1  christos <ul>
   1187   1.1  christos   <li>
   1188   1.4  christos     A set of timezone names as per
   1189   1.7  christos       "<a href="#naming">Timezone identifiers</a>" above.
   1190   1.1  christos   </li>
   1191   1.1  christos   <li>
   1192   1.3  christos     Library functions described in "<a href="#functions">Time and date
   1193   1.3  christos       functions</a>" above.
   1194   1.1  christos   </li>
   1195   1.1  christos   <li>
   1196   1.3  christos     The programs <code>tzselect</code>, <code>zdump</code>,
   1197   1.3  christos     and <code>zic</code>, documented in their man pages.
   1198   1.1  christos   </li>
   1199   1.1  christos   <li>
   1200   1.3  christos     The format of <code>zic</code> input files, documented in
   1201   1.3  christos     the <code>zic</code> man page.
   1202   1.1  christos   </li>
   1203   1.1  christos   <li>
   1204   1.3  christos     The format of <code>zic</code> output files, documented in
   1205   1.3  christos     the <code>tzfile</code> man page.
   1206   1.1  christos   </li>
   1207   1.1  christos   <li>
   1208   1.3  christos     The format of zone table files, documented in <code>zone1970.tab</code>.
   1209   1.1  christos   </li>
   1210   1.1  christos   <li>
   1211   1.3  christos     The format of the country code file, documented in <code>iso3166.tab</code>.
   1212   1.1  christos   </li>
   1213   1.1  christos   <li>
   1214   1.3  christos     The version number of the code and data, as the first line of
   1215   1.3  christos     the text file '<code>version</code>' in each release.
   1216   1.1  christos   </li>
   1217   1.1  christos </ul>
   1218   1.3  christos 
   1219   1.1  christos <p>
   1220   1.1  christos Interface changes in a release attempt to preserve compatibility with
   1221   1.3  christos recent releases.
   1222   1.3  christos For example, <code><abbr>tz</abbr></code> data files typically do not
   1223   1.3  christos rely on recently-added <code>zic</code> features, so that users can
   1224   1.3  christos run older <code>zic</code> versions to process newer data files.
   1225   1.3  christos <a href="tz-link.html#download">Downloading
   1226   1.3  christos the <code><abbr>tz</abbr></code> database</a> describes how releases
   1227   1.3  christos are tagged and distributed.
   1228   1.1  christos </p>
   1229   1.1  christos 
   1230   1.1  christos <p>
   1231   1.3  christos Interfaces not listed above are less stable.
   1232   1.3  christos For example, users should not rely on particular <abbr>UT</abbr>
   1233   1.3  christos offsets or abbreviations for timestamps, as data entries are often
   1234   1.3  christos based on guesswork and these guesses may be corrected or improved.
   1235   1.1  christos </p>
   1236   1.7  christos 
   1237   1.7  christos <p>
   1238   1.7  christos Timezone boundaries are not part of the stable interface.
   1239   1.7  christos For example, even though the <samp>Asia/Bangkok</samp> timezone
   1240   1.7  christos currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part
   1241   1.7  christos of the stable interface and the timezone can split at any time.
   1242   1.7  christos If a calendar application records a future event in some location other
   1243   1.7  christos than Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record,
   1244   1.7  christos the application should be robust in the presence of timezone splits
   1245   1.7  christos between now and the future time.
   1246   1.7  christos </p>
   1247   1.3  christos </section>
   1248   1.1  christos 
   1249   1.3  christos <section>
   1250   1.9  christos   <h2 id="leapsec">Leap seconds</h2>
   1251   1.9  christos <p>
   1252   1.9  christos The <code><abbr>tz</abbr></code> code and data can account for leap seconds,
   1253   1.9  christos thanks to code contributed by Bradley White.
   1254   1.9  christos However, the leap second support of this package is rarely used directly
   1255   1.9  christos because POSIX requires leap seconds to be excluded and many
   1256   1.9  christos software packages would mishandle leap seconds if they were present.
   1257   1.9  christos Instead, leap seconds are more commonly handled by occasionally adjusting
   1258   1.9  christos the operating system kernel clock as described in
   1259   1.9  christos <a href="tz-link.html#precision">Precision timekeeping</a>,
   1260   1.9  christos and this package by default installs a <samp>leapseconds</samp> file
   1261   1.9  christos commonly used by
   1262   1.9  christos <a href="http://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a>
   1263   1.9  christos software that adjusts the kernel clock.
   1264   1.9  christos However, kernel-clock twiddling approximates UTC only roughly,
   1265   1.9  christos and systems needing more-precise UTC can use this package's leap
   1266   1.9  christos second support directly.
   1267   1.9  christos </p>
   1268   1.9  christos 
   1269   1.9  christos <p>
   1270   1.9  christos The directly-supported mechanism assumes that <code>time_t</code>
   1271   1.9  christos counts of seconds since the POSIX epoch normally include leap seconds,
   1272   1.9  christos as opposed to POSIX <code>time_t</code> counts which exclude leap seconds.
   1273   1.9  christos This modified timescale is converted to <abbr>UTC</abbr>
   1274  1.12  christos at the same point that time zone and <abbr>DST</abbr>
   1275  1.12  christos adjustments are applied &ndash;
   1276   1.9  christos namely, at calls to <code>localtime</code> and analogous functions &ndash;
   1277   1.9  christos and the process is driven by leap second information
   1278   1.9  christos stored in alternate versions of the <abbr>TZif</abbr> files.
   1279   1.9  christos Because a leap second adjustment may be needed even
   1280   1.9  christos if no time zone correction is desired,
   1281   1.9  christos calls to <code>gmtime</code>-like functions
   1282   1.9  christos also need to consult a <abbr>TZif</abbr> file,
   1283   1.9  christos conventionally named <samp><abbr>GMT</abbr></samp>,
   1284   1.9  christos to see whether leap second corrections are needed.
   1285   1.9  christos To convert an application's <code>time_t</code> timestamps to or from
   1286   1.9  christos POSIX <code>time_t</code> timestamps (for use when, say,
   1287   1.9  christos embedding or interpreting timestamps in portable
   1288   1.9  christos <a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a>
   1289   1.9  christos files),
   1290   1.9  christos the application can call the utility functions
   1291   1.9  christos <code>time2posix</code> and <code>posix2time</code>
   1292   1.9  christos included with this package.
   1293   1.9  christos </p>
   1294   1.9  christos 
   1295   1.9  christos <p>
   1296   1.9  christos If the POSIX-compatible <abbr>TZif</abbr> file set is installed
   1297   1.9  christos in a directory whose basename is <samp>zoneinfo</samp>, the
   1298   1.9  christos leap-second-aware file set is by default installed in a separate
   1299   1.9  christos directory <samp>zoneinfo-leaps</samp>.
   1300   1.9  christos Although each process can have its own time zone by setting
   1301   1.9  christos its <code>TZ</code> environment variable, there is no support for some
   1302   1.9  christos processes being leap-second aware while other processes are
   1303   1.9  christos POSIX-compatible; the leap-second choice is system-wide.
   1304   1.9  christos So if you configure your kernel to count leap seconds, you should also
   1305   1.9  christos discard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp>
   1306   1.9  christos to <samp>zoneinfo</samp>.
   1307   1.9  christos Alternatively, you can install just one set of <abbr>TZif</abbr> files
   1308   1.9  christos in the first place; see the <code>REDO</code> variable in this package's
   1309   1.9  christos <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>.
   1310   1.9  christos </p>
   1311   1.9  christos </section>
   1312   1.9  christos 
   1313   1.9  christos <section>
   1314   1.3  christos   <h2 id="calendar">Calendrical issues</h2>
   1315   1.1  christos <p>
   1316   1.1  christos Calendrical issues are a bit out of scope for a time zone database,
   1317   1.1  christos but they indicate the sort of problems that we would run into if we
   1318   1.3  christos extended the time zone database further into the past.
   1319   1.3  christos An excellent resource in this area is Edward M. Reingold
   1320   1.3  christos and Nachum Dershowitz, <cite><a
   1321   1.3  christos href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
   1322   1.3  christos Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
   1323   1.3  christos Other information and sources are given in the file '<code>calendars</code>'
   1324   1.3  christos in the <code><abbr>tz</abbr></code> distribution.
   1325   1.3  christos They sometimes disagree.
   1326   1.3  christos </p>
   1327   1.3  christos </section>
   1328   1.3  christos 
   1329   1.3  christos <section>
   1330   1.3  christos   <h2 id="planets">Time and time zones on other planets</h2>
   1331   1.3  christos <p>
   1332  1.10  christos Some people's work schedules have used
   1333  1.10  christos <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
   1334   1.3  christos Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
   1335   1.3  christos and off during the
   1336   1.6  christos <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
   1337  1.10  christos Pathfinder</a> mission (1997).
   1338   1.3  christos Some of their family members also adapted to Mars time.
   1339   1.3  christos Dozens of special Mars watches were built for JPL workers who kept
   1340  1.10  christos Mars time during the
   1341  1.10  christos <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
   1342  1.10  christos Exploration Rovers (MER)</a> mission (2004&ndash;2018).
   1343  1.10  christos These timepieces looked like normal Seikos and Citizens but were adjusted
   1344  1.10  christos to use Mars seconds rather than terrestrial seconds, although
   1345  1.10  christos unfortunately the adjusted watches were unreliable and appear to have
   1346  1.10  christos had only limited use.
   1347   1.1  christos </p>
   1348   1.1  christos 
   1349   1.1  christos <p>
   1350   1.1  christos A Mars solar day is called a "sol" and has a mean period equal to
   1351   1.3  christos about 24 hours 39 minutes 35.244 seconds in terrestrial time.
   1352   1.3  christos It is divided into a conventional 24-hour clock, so each Mars second
   1353   1.3  christos equals about 1.02749125 terrestrial seconds.
   1354  1.10  christos (One MER worker noted, "If I am working Mars hours, and Mars hours are
   1355  1.10  christos 2.5% more than Earth hours, shouldn't I get an extra 2.5% pay raise?")
   1356   1.1  christos </p>
   1357   1.1  christos 
   1358   1.1  christos <p>
   1359   1.3  christos The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
   1360   1.3  christos meridian</a> of Mars goes through the center of the crater
   1361   1.3  christos <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
   1362   1.3  christos honor of the British astronomer who built the Greenwich telescope that
   1363   1.3  christos defines Earth's prime meridian.
   1364   1.3  christos Mean solar time on the Mars prime meridian is
   1365   1.6  christos called Mars Coordinated Time (<abbr>MTC</abbr>).
   1366   1.1  christos </p>
   1367   1.1  christos 
   1368   1.1  christos <p>
   1369   1.1  christos Each landed mission on Mars has adopted a different reference for
   1370   1.4  christos solar timekeeping, so there is no real standard for Mars time zones.
   1371  1.10  christos For example, the MER mission defined two time zones "Local
   1372   1.3  christos Solar Time A" and "Local Solar Time B" for its two missions, each zone
   1373   1.3  christos designed so that its time equals local true solar time at
   1374   1.3  christos approximately the middle of the nominal mission.
   1375  1.10  christos The A and B zones differ enough so that an MER worker assigned to
   1376  1.10  christos the A zone might suffer "Mars lag" when switching to work in the B zone.
   1377   1.3  christos Such a "time zone" is not particularly suited for any application
   1378   1.3  christos other than the mission itself.
   1379   1.1  christos </p>
   1380   1.1  christos 
   1381   1.1  christos <p>
   1382   1.1  christos Many calendars have been proposed for Mars, but none have achieved
   1383   1.3  christos wide acceptance.
   1384   1.3  christos Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
   1385   1.1  christos sequential count of Mars solar days elapsed since about 1873-12-29
   1386   1.3  christos 12:00 <abbr>GMT</abbr>.
   1387   1.1  christos </p>
   1388   1.1  christos 
   1389   1.1  christos <p>
   1390   1.1  christos In our solar system, Mars is the planet with time and calendar most
   1391   1.3  christos like Earth's.
   1392   1.3  christos On other planets, Sun-based time and calendars would work quite
   1393   1.3  christos differently.
   1394   1.3  christos For example, although Mercury's
   1395   1.3  christos <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
   1396   1.3  christos rotation period</a> is 58.646 Earth days, Mercury revolves around the
   1397   1.3  christos Sun so rapidly that an observer on Mercury's equator would see a
   1398   1.3  christos sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
   1399   1.3  christos Mercury day.
   1400   1.3  christos Venus is more complicated, partly because its rotation is slightly
   1401   1.3  christos <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
   1402   1.3  christos its year is 1.92 of its days.
   1403   1.3  christos Gas giants like Jupiter are trickier still, as their polar and
   1404   1.3  christos equatorial regions rotate at different rates, so that the length of a
   1405   1.3  christos day depends on latitude.
   1406   1.3  christos This effect is most pronounced on Neptune, where the day is about 12
   1407   1.3  christos hours at the poles and 18 hours at the equator.
   1408   1.1  christos </p>
   1409   1.1  christos 
   1410   1.1  christos <p>
   1411   1.3  christos Although the <code><abbr>tz</abbr></code> database does not support
   1412   1.3  christos time on other planets, it is documented here in the hopes that support
   1413   1.3  christos will be added eventually.
   1414   1.1  christos </p>
   1415   1.1  christos 
   1416   1.1  christos <p>
   1417   1.3  christos Sources for time on other planets:
   1418   1.1  christos </p>
   1419   1.3  christos 
   1420   1.1  christos <ul>
   1421   1.1  christos   <li>
   1422   1.3  christos     Michael Allison and Robert Schmunk,
   1423   1.3  christos     "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
   1424   1.3  christos       Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
   1425  1.10  christos     (2020-03-08).
   1426  1.10  christos   </li>
   1427  1.10  christos   <li>
   1428  1.10  christos     Zara Mirmalek,
   1429  1.10  christos     <em><a href="https://mitpress.mit.edu/books/making-time-mars">Making
   1430  1.10  christos 	Time on Mars</a></em>, MIT Press (March 2020), ISBN 978-0262043854.
   1431   1.1  christos   </li>
   1432   1.1  christos   <li>
   1433   1.3  christos     Jia-Rui Chong,
   1434   1.8  christos     "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
   1435   1.3  christos     Fit for a Martian</a>", <cite>Los Angeles Times</cite>
   1436   1.3  christos     (2004-01-14), pp A1, A20&ndash;A21.
   1437   1.1  christos   </li>
   1438   1.1  christos   <li>
   1439   1.3  christos     Tom Chmielewski,
   1440   1.3  christos     "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
   1441   1.3  christos     Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
   1442   1.1  christos   </li>
   1443   1.1  christos   <li>
   1444   1.3  christos     Matt Williams,
   1445   1.3  christos     "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
   1446   1.3  christos     long is a day on the other planets of the solar system?</a>"
   1447   1.4  christos     (2016-01-20).
   1448   1.1  christos   </li>
   1449   1.1  christos </ul>
   1450   1.3  christos </section>
   1451   1.1  christos 
   1452   1.3  christos <footer>
   1453   1.3  christos   <hr>
   1454   1.3  christos   This file is in the public domain, so clarified as of 2009-05-17 by
   1455   1.3  christos   Arthur David Olson.
   1456   1.3  christos </footer>
   1457   1.1  christos </body>
   1458   1.1  christos </html>
   1459