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      2  1.1  christos <head>
      3  1.1  christos   <title>Theory and pragmatics of the tz code and data</title>
      4  1.1  christos   <meta charset="UTF-8">
      5  1.1  christos </head>
      6  1.1  christos 
      7  1.1  christos <body>
      8  1.3  christos <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
      9  1.1  christos   <h3>Outline</h3>
     10  1.1  christos   <nav>
     11  1.1  christos     <ul>
     12  1.3  christos       <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
     13  1.3  christos 	  database</a></li>
     14  1.3  christos       <li><a href="#naming">Names of time zone rulesets</a></li>
     15  1.1  christos       <li><a href="#abbreviations">Time zone abbreviations</a></li>
     16  1.3  christos       <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
     17  1.3  christos 	  database</a></li>
     18  1.1  christos       <li><a href="#functions">Time and date functions</a></li>
     19  1.1  christos       <li><a href="#stability">Interface stability</a></li>
     20  1.1  christos       <li><a href="#calendar">Calendrical issues</a></li>
     21  1.1  christos       <li><a href="#planets">Time and time zones on other planets</a></li>
     22  1.1  christos     </ul>
     23  1.1  christos   </nav>
     24  1.1  christos 
     25  1.3  christos <section>
     26  1.3  christos   <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
     27  1.1  christos <p>
     28  1.3  christos The <a
     29  1.3  christos href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
     30  1.3  christos database</a> attempts to record the history and predicted future of
     31  1.3  christos all computer-based clocks that track civil time.
     32  1.3  christos It organizes <a href="tz-link.html">time zone and daylight saving time
     33  1.3  christos data</a> by partitioning the world into <a
     34  1.3  christos href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones">regions</a>
     35  1.3  christos whose clocks all agree about timestamps that occur after the <a
     36  1.3  christos href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
     37  1.3  christos (1970-01-01 00:00:00 <a
     38  1.3  christos href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
     39  1.3  christos title="Coordinated Universal Time">UTC</abbr></a>).
     40  1.3  christos The database labels each such region with a notable location and
     41  1.3  christos records all known clock transitions for that location.
     42  1.3  christos Although 1970 is a somewhat-arbitrary cutoff, there are significant
     43  1.3  christos challenges to moving the cutoff earlier even by a decade or two, due
     44  1.3  christos to the wide variety of local practices before computer timekeeping
     45  1.3  christos became prevalent.
     46  1.1  christos </p>
     47  1.1  christos 
     48  1.1  christos <p>
     49  1.1  christos Clock transitions before 1970 are recorded for each such location,
     50  1.1  christos because most systems support timestamps before 1970 and could
     51  1.1  christos misbehave if data entries were omitted for pre-1970 transitions.
     52  1.1  christos However, the database is not designed for and does not suffice for
     53  1.1  christos applications requiring accurate handling of all past times everywhere,
     54  1.1  christos as it would take far too much effort and guesswork to record all
     55  1.1  christos details of pre-1970 civil timekeeping.
     56  1.3  christos Although some information outside the scope of the database is
     57  1.2  christos collected in a file <code>backzone</code> that is distributed along
     58  1.2  christos with the database proper, this file is less reliable and does not
     59  1.2  christos necessarily follow database guidelines.
     60  1.1  christos </p>
     61  1.1  christos 
     62  1.1  christos <p>
     63  1.3  christos As described below, reference source code for using the
     64  1.3  christos <code><abbr>tz</abbr></code> database is also available.
     65  1.3  christos The <code><abbr>tz</abbr></code> code is upwards compatible with <a
     66  1.3  christos href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
     67  1.3  christos standard for <a
     68  1.3  christos href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
     69  1.3  christos As of this writing, the current edition of POSIX is: <a
     70  1.3  christos href="http://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
     71  1.3  christos Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
     72  1.3  christos Edition.
     73  1.3  christos Because the database's scope encompasses real-world changes to civil
     74  1.3  christos timekeeping, its model for describing time is more complex than the
     75  1.3  christos standard and daylight saving times supported by POSIX.
     76  1.3  christos A <code><abbr>tz</abbr></code> region corresponds to a ruleset that can
     77  1.3  christos have more than two changes per year, these changes need not merely
     78  1.3  christos flip back and forth between two alternatives, and the rules themselves
     79  1.3  christos can change at times.
     80  1.3  christos Whether and when a <code><abbr>tz</abbr></code> region changes its
     81  1.3  christos clock, and even the region's notional base offset from UTC, are variable.
     82  1.3  christos It does not always make sense to talk about a region's
     83  1.3  christos "base offset", since it is not necessarily a single number.
     84  1.1  christos </p>
     85  1.1  christos 
     86  1.3  christos </section>
     87  1.1  christos 
     88  1.3  christos <section>
     89  1.3  christos   <h2 id="naming">Names of time zone rulesets</h2>
     90  1.1  christos <p>
     91  1.3  christos Each <code><abbr>tz</abbr></code> region has a unique name that
     92  1.3  christos corresponds to a set of time zone rules.
     93  1.1  christos Inexperienced users are not expected to select these names unaided.
     94  1.1  christos Distributors should provide documentation and/or a simple selection
     95  1.3  christos interface that explains the names; for one example, see the
     96  1.3  christos <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
     97  1.3  christos The <a href="http://cldr.unicode.org/">Unicode Common Locale Data
     98  1.3  christos Repository</a> contains data that may be useful for other selection
     99  1.3  christos interfaces.
    100  1.1  christos </p>
    101  1.1  christos 
    102  1.1  christos <p>
    103  1.3  christos The naming conventions attempt to strike a balance
    104  1.1  christos among the following goals:
    105  1.1  christos </p>
    106  1.3  christos 
    107  1.1  christos <ul>
    108  1.1  christos   <li>
    109  1.3  christos     Uniquely identify every region where clocks have agreed since 1970.
    110  1.3  christos     This is essential for the intended use: static clocks keeping local
    111  1.3  christos     civil time.
    112  1.1  christos   </li>
    113  1.1  christos   <li>
    114  1.3  christos     Indicate to experts where that region is.
    115  1.1  christos   </li>
    116  1.1  christos   <li>
    117  1.3  christos     Be robust in the presence of political changes.
    118  1.3  christos     For example, names of countries are ordinarily not used, to avoid
    119  1.3  christos     incompatibilities when countries change their name (e.g.,
    120  1.3  christos     Zaire&rarr;Congo) or when locations change countries (e.g., Hong
    121  1.3  christos     Kong from UK colony to China).
    122  1.1  christos   </li>
    123  1.1  christos   <li>
    124  1.3  christos     Be portable to a wide variety of implementations.
    125  1.1  christos   </li>
    126  1.1  christos   <li>
    127  1.3  christos     Use a consistent naming conventions over the entire world.
    128  1.1  christos   </li>
    129  1.1  christos </ul>
    130  1.3  christos 
    131  1.1  christos <p>
    132  1.3  christos Names normally have the form
    133  1.3  christos <var>AREA</var><code>/</code><var>LOCATION</var>, where
    134  1.3  christos <var>AREA</var> is the name of a continent or ocean, and
    135  1.3  christos <var>LOCATION</var> is the name of a specific location within that
    136  1.3  christos region.
    137  1.3  christos North and South America share the same area, '<code>America</code>'.
    138  1.3  christos Typical names are '<code>Africa/Cairo</code>',
    139  1.3  christos '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
    140  1.3  christos Some names are further qualified to help avoid confusion; for example,
    141  1.3  christos '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
    142  1.3  christos Indiana from other Petersburgs in America.
    143  1.1  christos </p>
    144  1.1  christos 
    145  1.1  christos <p>
    146  1.3  christos Here are the general guidelines used for
    147  1.3  christos choosing <code><abbr>tz</abbr></code> region names,
    148  1.1  christos in decreasing order of importance:
    149  1.1  christos </p>
    150  1.3  christos 
    151  1.1  christos <ul>
    152  1.1  christos   <li>
    153  1.3  christos     Use only valid POSIX file name components (i.e., the parts of
    154  1.3  christos     names other than '<code>/</code>').
    155  1.3  christos     Do not use the file name components '<code>.</code>' and
    156  1.3  christos     '<code>..</code>'.
    157  1.3  christos     Within a file name component, use only <a
    158  1.3  christos     href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
    159  1.3  christos     '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
    160  1.3  christos     Do not use digits, as that might create an ambiguity with <a
    161  1.3  christos     href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
    162  1.3  christos     <code>TZ</code> strings</a>.
    163  1.3  christos     A file name component must not exceed 14 characters or start with
    164  1.3  christos     '<code>-</code>'.
    165  1.3  christos     E.g., prefer <code>Asia/Brunei</code> to
    166  1.3  christos     <code>Asia/Bandar_Seri_Begawan</code>.
    167  1.3  christos     Exceptions: see the discussion of legacy names below.
    168  1.1  christos   </li>
    169  1.1  christos   <li>
    170  1.3  christos     A name must not be empty, or contain '<code>//</code>', or
    171  1.3  christos     start or end with '<code>/</code>'.
    172  1.1  christos   </li>
    173  1.1  christos   <li>
    174  1.3  christos     Do not use names that differ only in case.
    175  1.3  christos     Although the reference implementation is case-sensitive, some
    176  1.3  christos     other implementations are not, and they would mishandle names
    177  1.3  christos     differing only in case.
    178  1.1  christos   </li>
    179  1.1  christos   <li>
    180  1.3  christos     If one name <var>A</var> is an initial prefix of another
    181  1.3  christos     name <var>AB</var> (ignoring case), then <var>B</var> must not
    182  1.3  christos     start with '<code>/</code>', as a regular file cannot have the
    183  1.3  christos     same name as a directory in POSIX.
    184  1.3  christos     For example, <code>America/New_York</code> precludes
    185  1.3  christos     <code>America/New_York/Bronx</code>.
    186  1.1  christos   </li>
    187  1.1  christos   <li>
    188  1.3  christos     Uninhabited regions like the North Pole and Bouvet Island
    189  1.3  christos     do not need locations, since local time is not defined there.
    190  1.1  christos   </li>
    191  1.1  christos   <li>
    192  1.3  christos     There should typically be at least one name for each <a
    193  1.3  christos     href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
    194  1.3  christos     title="International Organization for Standardization">ISO</abbr>
    195  1.3  christos     3166-1</a> officially assigned two-letter code for an inhabited
    196  1.3  christos     country or territory.
    197  1.1  christos   </li>
    198  1.1  christos   <li>
    199  1.3  christos     If all the clocks in a region have agreed since 1970,
    200  1.3  christos     do not bother to include more than one location
    201  1.3  christos     even if subregions' clocks disagreed before 1970.
    202  1.3  christos     Otherwise these tables would become annoyingly large.
    203  1.1  christos   </li>
    204  1.1  christos   <li>
    205  1.3  christos     If a name is ambiguous, use a less ambiguous alternative;
    206  1.3  christos     e.g., many cities are named San Jos and Georgetown, so
    207  1.3  christos     prefer <code>America/Costa_Rica</code> to
    208  1.3  christos     <code>America/San_Jose</code> and <code>America/Guyana</code>
    209  1.3  christos     to <code>America/Georgetown</code>.
    210  1.1  christos   </li>
    211  1.1  christos   <li>
    212  1.3  christos     Keep locations compact.
    213  1.3  christos     Use cities or small islands, not countries or regions, so that any
    214  1.3  christos     future changes do not split individual locations into different
    215  1.3  christos     <code><abbr>tz</abbr></code> regions.
    216  1.3  christos     E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
    217  1.3  christos     since
    218  1.3  christos     <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
    219  1.3  christos     has had multiple time zones</a>.
    220  1.1  christos   </li>
    221  1.1  christos   <li>
    222  1.3  christos     Use mainstream English spelling, e.g., prefer
    223  1.3  christos     <code>Europe/Rome</code> to <code>Europe/Roma</code>, and
    224  1.3  christos     prefer <code>Europe/Athens</code> to the Greek
    225  1.3  christos     <code>Europe/</code> or the Romanized
    226  1.3  christos     <code>Europe/Athna</code>.
    227  1.3  christos     The POSIX file name restrictions encourage this guideline.
    228  1.1  christos   </li>
    229  1.1  christos   <li>
    230  1.3  christos     Use the most populous among locations in a region,
    231  1.3  christos     e.g., prefer <code>Asia/Shanghai</code> to
    232  1.3  christos     <code>Asia/Beijing</code>.
    233  1.3  christos     Among locations with similar populations, pick the best-known
    234  1.3  christos     location, e.g., prefer <code>Europe/Rome</code> to
    235  1.3  christos     <code>Europe/Milan</code>.
    236  1.1  christos   </li>
    237  1.1  christos   <li>
    238  1.3  christos     Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
    239  1.3  christos     <code>Atlantic/Canaries</code>.
    240  1.1  christos   </li>
    241  1.1  christos   <li>
    242  1.3  christos     Omit common suffixes like '<code>_Islands</code>' and
    243  1.3  christos     '<code>_City</code>', unless that would lead to ambiguity.
    244  1.3  christos     E.g., prefer <code>America/Cayman</code> to
    245  1.3  christos     <code>America/Cayman_Islands</code> and
    246  1.3  christos     <code>America/Guatemala</code> to
    247  1.3  christos     <code>America/Guatemala_City</code>, but prefer
    248  1.3  christos     <code>America/Mexico_City</code> to
    249  1.3  christos     <code>America/Mexico</code>
    250  1.3  christos     because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
    251  1.3  christos     country of Mexico has several time zones</a>.
    252  1.1  christos   </li>
    253  1.1  christos   <li>
    254  1.3  christos     Use '<code>_</code>' to represent a space.
    255  1.1  christos   </li>
    256  1.1  christos   <li>
    257  1.3  christos     Omit '<code>.</code>' from abbreviations in names.
    258  1.3  christos     E.g., prefer <code>Atlantic/St_Helena</code> to
    259  1.3  christos     <code>Atlantic/St._Helena</code>.
    260  1.1  christos   </li>
    261  1.1  christos   <li>
    262  1.3  christos     Do not change established names if they only marginally violate
    263  1.3  christos     the above guidelines.
    264  1.3  christos     For example, do not change the existing name <code>Europe/Rome</code> to
    265  1.3  christos     <code>Europe/Milan</code> merely because Milan's population has grown
    266  1.3  christos     to be somewhat greater than Rome's.
    267  1.1  christos   </li>
    268  1.1  christos   <li>
    269  1.3  christos     If a name is changed, put its old spelling in the
    270  1.3  christos     '<code>backward</code>' file.
    271  1.3  christos     This means old spellings will continue to work.
    272  1.1  christos   </li>
    273  1.1  christos </ul>
    274  1.1  christos 
    275  1.1  christos <p>
    276  1.1  christos The file '<code>zone1970.tab</code>' lists geographical locations used
    277  1.3  christos to name <code><abbr>tz</abbr></code> regions.
    278  1.3  christos It is intended to be an exhaustive list of names for geographic
    279  1.3  christos regions as described above; this is a subset of the names in the data.
    280  1.3  christos Although a '<code>zone1970.tab</code>' location's
    281  1.3  christos <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
    282  1.3  christos corresponds to
    283  1.3  christos its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
    284  1.3  christos time (<abbr>LMT</abbr>)</a> offset with one hour for every 15&deg;
    285  1.3  christos east longitude, this relationship is not exact.
    286  1.1  christos </p>
    287  1.1  christos 
    288  1.1  christos <p>
    289  1.1  christos Older versions of this package used a different naming scheme,
    290  1.1  christos and these older names are still supported.
    291  1.1  christos See the file '<code>backward</code>' for most of these older names
    292  1.1  christos (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
    293  1.1  christos The other old-fashioned names still supported are
    294  1.3  christos '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
    295  1.3  christos '<code>EET</code>' (see the file '<code>europe</code>').
    296  1.1  christos </p>
    297  1.1  christos 
    298  1.1  christos <p>
    299  1.1  christos Older versions of this package defined legacy names that are
    300  1.3  christos incompatible with the first guideline of location names, but which are
    301  1.3  christos still supported.
    302  1.3  christos These legacy names are mostly defined in the file
    303  1.3  christos '<code>etcetera</code>'.
    304  1.3  christos Also, the file '<code>backward</code>' defines the legacy names
    305  1.3  christos '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
    306  1.3  christos and the file '<code>northamerica</code>' defines the legacy names
    307  1.3  christos '<code>EST5EDT</code>', '<code>CST6CDT</code>',
    308  1.3  christos '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
    309  1.1  christos </p>
    310  1.1  christos 
    311  1.1  christos <p>
    312  1.3  christos Excluding '<code>backward</code>' should not affect the other data.
    313  1.3  christos If '<code>backward</code>' is excluded, excluding
    314  1.3  christos '<code>etcetera</code>' should not affect the remaining data.
    315  1.1  christos </p>
    316  1.3  christos </section>
    317  1.1  christos 
    318  1.3  christos <section>
    319  1.3  christos   <h2 id="abbreviations">Time zone abbreviations</h2>
    320  1.1  christos <p>
    321  1.1  christos When this package is installed, it generates time zone abbreviations
    322  1.1  christos like '<code>EST</code>' to be compatible with human tradition and POSIX.
    323  1.3  christos Here are the general guidelines used for choosing time zone abbreviations,
    324  1.1  christos in decreasing order of importance:
    325  1.3  christos </p>
    326  1.3  christos 
    327  1.1  christos <ul>
    328  1.1  christos   <li>
    329  1.3  christos     Use three to six characters that are ASCII alphanumerics or
    330  1.3  christos     '<code>+</code>' or '<code>-</code>'.
    331  1.3  christos     Previous editions of this database also used characters like
    332  1.3  christos     space and '<code>?</code>', but these characters have a
    333  1.3  christos     special meaning to the
    334  1.3  christos     <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
    335  1.3  christos     and cause commands like
    336  1.3  christos     '<code><a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
    337  1.3  christos     `<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
    338  1.3  christos     to have unexpected effects.
    339  1.3  christos     Previous editions of this guideline required upper-case letters, but the
    340  1.3  christos     Congressman who introduced
    341  1.3  christos     <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
    342  1.3  christos     Standard Time</a> preferred "ChST", so lower-case letters are now
    343  1.3  christos     allowed.
    344  1.3  christos     Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
    345  1.3  christos     '<code>+</code>', and alphanumeric characters from the portable
    346  1.3  christos     character set in the current locale.
    347  1.3  christos     In practice ASCII alphanumerics and '<code>+</code>' and
    348  1.3  christos     '<code>-</code>' are safe in all locales.
    349  1.3  christos 
    350  1.3  christos     <p>
    351  1.3  christos     In other words, in the C locale the POSIX extended regular
    352  1.3  christos     expression <code>[-+[:alnum:]]{3,6}</code> should match the
    353  1.3  christos     abbreviation.
    354  1.3  christos     This guarantees that all abbreviations could have been specified by a
    355  1.3  christos     POSIX <code>TZ</code> string.
    356  1.3  christos     </p>
    357  1.3  christos   </li>
    358  1.3  christos   <li>
    359  1.3  christos     Use abbreviations that are in common use among English-speakers,
    360  1.3  christos     e.g., 'EST' for Eastern Standard Time in North America.
    361  1.3  christos     We assume that applications translate them to other languages
    362  1.3  christos     as part of the normal localization process; for example,
    363  1.3  christos     a French application might translate 'EST' to 'HNE'.
    364  1.3  christos 
    365  1.3  christos     <p>
    366  1.3  christos     <small>These abbreviations (for standard/daylight/etc. time) are:
    367  1.3  christos       ACST/ACDT Australian Central,
    368  1.3  christos       AST/ADT/APT/AWT/ADDT Atlantic,
    369  1.3  christos       AEST/AEDT Australian Eastern,
    370  1.3  christos       AHST/AHDT Alaska-Hawaii,
    371  1.3  christos       AKST/AKDT Alaska,
    372  1.3  christos       AWST/AWDT Australian Western,
    373  1.3  christos       BST/BDT Bering,
    374  1.3  christos       CAT/CAST Central Africa,
    375  1.3  christos       CET/CEST/CEMT Central European,
    376  1.3  christos       ChST Chamorro,
    377  1.3  christos       CST/CDT/CWT/CPT/CDDT Central [North America],
    378  1.3  christos       CST/CDT China,
    379  1.3  christos       GMT/BST/IST/BDST Greenwich,
    380  1.3  christos       EAT East Africa,
    381  1.3  christos       EST/EDT/EWT/EPT/EDDT Eastern [North America],
    382  1.3  christos       EET/EEST Eastern European,
    383  1.3  christos       GST Guam,
    384  1.3  christos       HST/HDT Hawaii,
    385  1.3  christos       HKT/HKST Hong Kong,
    386  1.3  christos       IST India,
    387  1.3  christos       IST/GMT Irish,
    388  1.3  christos       IST/IDT/IDDT Israel,
    389  1.3  christos       JST/JDT Japan,
    390  1.3  christos       KST/KDT Korea,
    391  1.3  christos       MET/MEST Middle European (a backward-compatibility alias for
    392  1.3  christos 	Central European),
    393  1.3  christos       MSK/MSD Moscow,
    394  1.3  christos       MST/MDT/MWT/MPT/MDDT Mountain,
    395  1.3  christos       NST/NDT/NWT/NPT/NDDT Newfoundland,
    396  1.3  christos       NST/NDT/NWT/NPT Nome,
    397  1.3  christos       NZMT/NZST New Zealand through 1945,
    398  1.3  christos       NZST/NZDT New Zealand 1946&ndash;present,
    399  1.3  christos       PKT/PKST Pakistan,
    400  1.3  christos       PST/PDT/PWT/PPT/PDDT Pacific,
    401  1.3  christos       SAST South Africa,
    402  1.3  christos       SST Samoa,
    403  1.3  christos       WAT/WAST West Africa,
    404  1.3  christos       WET/WEST/WEMT Western European,
    405  1.3  christos       WIB Waktu Indonesia Barat,
    406  1.3  christos       WIT Waktu Indonesia Timur,
    407  1.3  christos       WITA Waktu Indonesia Tengah,
    408  1.3  christos       YST/YDT/YWT/YPT/YDDT Yukon</small>.
    409  1.3  christos     </p>
    410  1.3  christos   </li>
    411  1.3  christos   <li>
    412  1.3  christos     <p>
    413  1.3  christos     For times taken from a city's longitude, use the
    414  1.3  christos     traditional <var>x</var>MT notation.
    415  1.3  christos     The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
    416  1.3  christos     The others are for timestamps before 1960,
    417  1.3  christos     except that Monrovia Mean Time persisted until 1972.
    418  1.3  christos     Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
    419  1.3  christos     MMT) would cause trouble here, as the numeric strings would exceed
    420  1.3  christos     the POSIX length limit.
    421  1.3  christos     </p>
    422  1.3  christos 
    423  1.3  christos     <p>
    424  1.3  christos     <small>These abbreviations are:
    425  1.3  christos       AMT Amsterdam, Asuncin, Athens;
    426  1.3  christos       BMT Baghdad, Bangkok, Batavia, Bern, Bogot, Bridgetown, Brussels,
    427  1.3  christos 	Bucharest;
    428  1.3  christos       CMT Calamarca, Caracas, Chisinau, Coln, Copenhagen, Crdoba;
    429  1.3  christos       DMT Dublin/Dunsink;
    430  1.3  christos       EMT Easter;
    431  1.3  christos       FFMT Fort-de-France;
    432  1.3  christos       FMT Funchal;
    433  1.3  christos       GMT Greenwich;
    434  1.3  christos       HMT Havana, Helsinki, Horta, Howrah;
    435  1.3  christos       IMT Irkutsk, Istanbul;
    436  1.3  christos       JMT Jerusalem;
    437  1.3  christos       KMT Kaunas, Kiev, Kingston;
    438  1.3  christos       LMT Lima, Lisbon, local, Luanda;
    439  1.3  christos       MMT Macassar, Madras, Mal, Managua, Minsk, Monrovia, Montevideo,
    440  1.3  christos 	Moratuwa, Moscow;
    441  1.3  christos       PLMT Ph Lin;
    442  1.3  christos       PMT Paramaribo, Paris, Perm, Pontianak, Prague;
    443  1.3  christos       PMMT Port Moresby;
    444  1.3  christos       QMT Quito;
    445  1.3  christos       RMT Rangoon, Riga, Rome;
    446  1.3  christos       SDMT Santo Domingo;
    447  1.3  christos       SJMT San Jos;
    448  1.3  christos       SMT Santiago, Simferopol, Singapore, Stanley;
    449  1.3  christos       TBMT Tbilisi;
    450  1.3  christos       TMT Tallinn, Tehran;
    451  1.3  christos       WMT Warsaw</small>.
    452  1.3  christos     </p>
    453  1.3  christos 
    454  1.3  christos     <p>
    455  1.3  christos     <small>A few abbreviations also follow the pattern that
    456  1.3  christos     <abbr>GMT<abbr>/<abbr>BST</abbr> established for time in the UK.
    457  1.3  christos     They are:
    458  1.3  christos       CMT/BST for Calamarca Mean Time and Bolivian Summer Time
    459  1.3  christos 	1890&ndash;1932,
    460  1.3  christos       DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
    461  1.3  christos 	1880&ndash;1916,
    462  1.3  christos       MMT/MST/MDST for Moscow 1880&ndash;1919, and
    463  1.3  christos       RMT/LST for Riga Mean Time and Latvian Summer time 1880&ndash;1926.
    464  1.3  christos     An extra-special case is SET for Swedish Time (<em>svensk
    465  1.3  christos     normaltid</em>) 1879&ndash;1899, 3&deg; west of the Stockholm
    466  1.3  christos     Observatory.</small>
    467  1.3  christos     </p>
    468  1.3  christos   </li>
    469  1.3  christos   <li>
    470  1.3  christos     Use '<abbr>LMT</abbr>' for local mean time of locations before the
    471  1.3  christos     introduction of standard time; see "<a href="#scope">Scope of the
    472  1.3  christos     <code><abbr>tz</abbr></code> database</a>".
    473  1.3  christos   </li>
    474  1.3  christos   <li>
    475  1.3  christos     If there is no common English abbreviation, use numeric offsets like
    476  1.3  christos     <code>-</code>05 and <code>+</code>0830 that are generated
    477  1.3  christos     by <code>zic</code>'s <code>%z</code> notation.
    478  1.3  christos   </li>
    479  1.3  christos   <li>
    480  1.3  christos     Use current abbreviations for older timestamps to avoid confusion.
    481  1.3  christos     For example, in 1910 a common English abbreviation for time
    482  1.3  christos     in central Europe was 'MEZ' (short for both "Middle European
    483  1.3  christos     Zone" and for "Mitteleuropische Zeit" in German).
    484  1.3  christos     Nowadays 'CET' ("Central European Time") is more common in
    485  1.3  christos     English, and the database uses 'CET' even for circa-1910
    486  1.3  christos     timestamps as this is less confusing for modern users and avoids
    487  1.3  christos     the need for determining when 'CET' supplanted 'MEZ' in common
    488  1.3  christos     usage.
    489  1.3  christos   </li>
    490  1.3  christos   <li>
    491  1.3  christos     Use a consistent style in a <code><abbr>tz</abbr></code> region's history.
    492  1.3  christos     For example, if history tends to use numeric
    493  1.3  christos     abbreviations and a particular entry could go either way, use a
    494  1.3  christos     numeric abbreviation.
    495  1.3  christos   </li>
    496  1.3  christos   <li>
    497  1.3  christos     Use
    498  1.3  christos     <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
    499  1.3  christos     (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
    500  1.3  christos     locations while uninhabited.
    501  1.3  christos     The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
    502  1.3  christos     some sense undefined; this notation is derived
    503  1.3  christos     from <a href="https://tools.ietf.org/html/rfc3339">Internet
    504  1.3  christos     <abbr title="Request For Comments">RFC 3339</a>.
    505  1.1  christos   </li>
    506  1.1  christos </ul>
    507  1.3  christos 
    508  1.1  christos <p>
    509  1.1  christos Application writers should note that these abbreviations are ambiguous
    510  1.2  christos in practice: e.g., 'CST' means one thing in China and something else
    511  1.2  christos in North America, and 'IST' can refer to time in India, Ireland or
    512  1.3  christos Israel.
    513  1.3  christos To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
    514  1.2  christos '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
    515  1.1  christos </p>
    516  1.3  christos </section>
    517  1.1  christos 
    518  1.3  christos <section>
    519  1.3  christos   <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
    520  1.1  christos <p>
    521  1.3  christos The <code><abbr>tz</abbr></code> database is not authoritative, and it
    522  1.3  christos surely has errors.
    523  1.2  christos Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
    524  1.1  christos Users requiring authoritative data should consult national standards
    525  1.1  christos bodies and the references cited in the database's comments.
    526  1.1  christos </p>
    527  1.1  christos 
    528  1.1  christos <p>
    529  1.3  christos Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
    530  1.1  christos </p>
    531  1.3  christos 
    532  1.1  christos <ul>
    533  1.1  christos   <li>
    534  1.3  christos     The <code><abbr>tz</abbr></code> database predicts future
    535  1.3  christos     timestamps, and current predictions
    536  1.3  christos     will be incorrect after future governments change the rules.
    537  1.3  christos     For example, if today someone schedules a meeting for 13:00 next
    538  1.3  christos     October 1, Casablanca time, and tomorrow Morocco changes its
    539  1.3  christos     daylight saving rules, software can mess up after the rule change
    540  1.3  christos     if it blithely relies on conversions made before the change.
    541  1.3  christos   </li>
    542  1.3  christos   <li>
    543  1.3  christos     The pre-1970 entries in this database cover only a tiny sliver of how
    544  1.3  christos     clocks actually behaved; the vast majority of the necessary
    545  1.3  christos     information was lost or never recorded.
    546  1.3  christos     Thousands more <code><abbr>tz</abbr></code> regions would be needed if
    547  1.3  christos     the <code><abbr>tz</abbr></code> database's scope were extended to
    548  1.3  christos     cover even just the known or guessed history of standard time; for
    549  1.3  christos     example, the current single entry for France would need to split
    550  1.3  christos     into dozens of entries, perhaps hundreds.
    551  1.3  christos     And in most of the world even this approach would be misleading
    552  1.3  christos     due to widespread disagreement or indifference about what times
    553  1.3  christos     should be observed.
    554  1.3  christos     In her 2015 book
    555  1.3  christos     <cite><a
    556  1.3  christos     href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
    557  1.3  christos     Global Transformation of Time, 1870&ndash;1950</a></cite>,
    558  1.3  christos     Vanessa Ogle writes
    559  1.3  christos     "Outside of Europe and North America there was no system of time
    560  1.3  christos     zones at all, often not even a stable landscape of mean times,
    561  1.3  christos     prior to the middle decades of the twentieth century".
    562  1.3  christos     See: Timothy Shenk, <a
    563  1.3  christos href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
    564  1.3  christos       A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
    565  1.3  christos   </li>
    566  1.3  christos   <li>
    567  1.3  christos     Most of the pre-1970 data entries come from unreliable sources, often
    568  1.3  christos     astrology books that lack citations and whose compilers evidently
    569  1.3  christos     invented entries when the true facts were unknown, without
    570  1.3  christos     reporting which entries were known and which were invented.
    571  1.3  christos     These books often contradict each other or give implausible entries,
    572  1.3  christos     and on the rare occasions when they are checked they are
    573  1.3  christos     typically found to be incorrect.
    574  1.3  christos   </li>
    575  1.3  christos   <li>
    576  1.3  christos     For the UK the <code><abbr>tz</abbr></code> database relies on
    577  1.3  christos     years of first-class work done by
    578  1.3  christos     Joseph Myers and others; see
    579  1.3  christos     "<a href="https://www.polyomino.org.uk/british-time/">History of
    580  1.3  christos     legal time in Britain</a>".
    581  1.3  christos     Other countries are not done nearly as well.
    582  1.3  christos   </li>
    583  1.3  christos   <li>
    584  1.3  christos     Sometimes, different people in the same city maintain clocks
    585  1.3  christos     that differ significantly.
    586  1.3  christos     Historically, railway time was used by railroad companies (which
    587  1.3  christos     did not always
    588  1.3  christos     agree with each other), church-clock time was used for birth
    589  1.3  christos     certificates, etc.
    590  1.3  christos     More recently, competing political groups might disagree about
    591  1.3  christos     clock settings. Often this is merely common practice, but
    592  1.3  christos     sometimes it is set by law.
    593  1.3  christos     For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
    594  1.3  christos     was legally <abbr>UT</abbr> +00:09:21 outside train stations and
    595  1.3  christos     <abbr>UT</abbr> +00:04:21 inside. Other examples include
    596  1.3  christos     Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
    597  1.3  christos     rmqi to this day.
    598  1.3  christos   </li>
    599  1.3  christos   <li>
    600  1.3  christos     Although a named location in the <code><abbr>tz</abbr></code>
    601  1.3  christos     database stands for the containing region, its pre-1970 data
    602  1.3  christos     entries are often accurate for only a small subset of that region.
    603  1.3  christos     For example, <code>Europe/London</code> stands for the United
    604  1.3  christos     Kingdom, but its pre-1847 times are valid only for locations that
    605  1.3  christos     have London's exact meridian, and its 1847 transition
    606  1.3  christos     to <abbr>GMT</abbr> is known to be valid only for the L&amp;NW and
    607  1.3  christos     the Caledonian railways.
    608  1.3  christos   </li>
    609  1.3  christos   <li>
    610  1.3  christos     The <code><abbr>tz</abbr></code> database does not record the
    611  1.3  christos     earliest time for which a <code><abbr>tz</abbr></code> region's
    612  1.3  christos     data entries are thereafter valid for every location in the region.
    613  1.3  christos     For example, <code>Europe/London</code> is valid for all locations
    614  1.3  christos     in its region after <abbr>GMT</abbr> was made the standard time,
    615  1.3  christos     but the date of standardization (1880-08-02) is not in the
    616  1.3  christos     <code><abbr>tz</abbr></code> database, other than in commentary.
    617  1.3  christos     For many <code><abbr>tz</abbr></code> regions the earliest time of
    618  1.3  christos     validity is unknown.
    619  1.3  christos   </li>
    620  1.3  christos   <li>
    621  1.3  christos     The <code><abbr>tz</abbr></code> database does not record a
    622  1.3  christos     region's boundaries, and in many cases the boundaries are not known.
    623  1.3  christos     For example, the <code><abbr>tz</abbr></code> region
    624  1.3  christos     <code>America/Kentucky/Louisville</code> represents a region
    625  1.3  christos     around the city of Louisville, the boundaries of which are
    626  1.3  christos     unclear.
    627  1.3  christos   </li>
    628  1.3  christos   <li>
    629  1.3  christos     Changes that are modeled as instantaneous transitions in the
    630  1.3  christos     <code><abbr>tz</abbr></code>
    631  1.3  christos     database were often spread out over hours, days, or even decades.
    632  1.3  christos   </li>
    633  1.3  christos   <li>
    634  1.3  christos     Even if the time is specified by law, locations sometimes
    635  1.3  christos     deliberately flout the law.
    636  1.3  christos   </li>
    637  1.3  christos   <li>
    638  1.3  christos     Early timekeeping practices, even assuming perfect clocks, were
    639  1.3  christos     often not specified to the accuracy that the
    640  1.3  christos     <code><abbr>tz</abbr></code> database requires.
    641  1.3  christos   </li>
    642  1.3  christos   <li>
    643  1.3  christos     Sometimes historical timekeeping was specified more precisely
    644  1.3  christos     than what the <code><abbr>tz</abbr></code> code can handle.
    645  1.3  christos     For example, from 1909 to 1937 <a
    646  1.3  christos     href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
    647  1.3  christos     hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean
    648  1.3  christos     Time (estimated to be <abbr>UT</abbr>
    649  1.3  christos     +00:19:32.13), but the <code><abbr>tz</abbr></code>
    650  1.3  christos     code cannot represent the fractional second.
    651  1.3  christos     In practice these old specifications were rarely if ever
    652  1.3  christos     implemented to subsecond precision.
    653  1.3  christos   </li>
    654  1.3  christos   <li>
    655  1.3  christos     Even when all the timestamp transitions recorded by the
    656  1.3  christos     <code><abbr>tz</abbr></code> database are correct, the
    657  1.3  christos     <code><abbr>tz</abbr></code> rules that generate them may not
    658  1.3  christos     faithfully reflect the historical rules.
    659  1.3  christos     For example, from 1922 until World War II the UK moved clocks
    660  1.3  christos     forward the day following the third Saturday in April unless that
    661  1.3  christos     was Easter, in which case it moved clocks forward the previous
    662  1.3  christos     Sunday.
    663  1.3  christos     Because the <code><abbr>tz</abbr></code> database has no
    664  1.3  christos     way to specify Easter, these exceptional years are entered as
    665  1.3  christos     separate <code><abbr>tz</abbr> Rule</code> lines, even though the
    666  1.3  christos     legal rules did not change.
    667  1.3  christos   </li>
    668  1.3  christos   <li>
    669  1.3  christos     The <code><abbr>tz</abbr></code> database models pre-standard time
    670  1.3  christos     using the <a
    671  1.3  christos     href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
    672  1.3  christos     Gregorian calendar</a> and local mean time, but many people used
    673  1.3  christos     other calendars and other timescales.
    674  1.3  christos     For example, the Roman Empire used
    675  1.3  christos     the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
    676  1.3  christos     calendar</a>,
    677  1.3  christos     and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
    678  1.3  christos     timekeeping</a> had twelve varying-length daytime hours with a
    679  1.3  christos     non-hour-based system at night.
    680  1.3  christos   </li>
    681  1.3  christos   <li>
    682  1.3  christos     Early clocks were less reliable, and data entries do not represent
    683  1.3  christos     clock error.
    684  1.3  christos   </li>
    685  1.3  christos   <li>
    686  1.3  christos     The <code><abbr>tz</abbr></code> database assumes Universal Time
    687  1.3  christos     (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
    688  1.3  christos     standardized for older timestamps.
    689  1.3  christos     In the <code><abbr>tz</abbr></code> database commentary,
    690  1.3  christos     <abbr>UT</abbr> denotes a family of time standards that includes
    691  1.3  christos     Coordinated Universal Time (<abbr>UTC</abbr>) along with other
    692  1.3  christos     variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
    693  1.3  christos     with days starting at midnight.
    694  1.3  christos     Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
    695  1.3  christos     timestamps, <abbr>UTC</abbr> was not defined until 1960, so
    696  1.3  christos     commentary uses the more-general abbreviation <abbr>UT</abbr> for
    697  1.3  christos     timestamps that might predate 1960.
    698  1.3  christos     Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
    699  1.3  christos     and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
    700  1.3  christos     interpretation of older timestamps can be problematic when
    701  1.3  christos     subsecond accuracy is needed.
    702  1.3  christos   </li>
    703  1.3  christos   <li>
    704  1.3  christos     Civil time was not based on atomic time before 1972, and we do not
    705  1.3  christos     know the history of
    706  1.3  christos     <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
    707  1.3  christos     rotation</a> accurately enough to map <a
    708  1.3  christos     href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
    709  1.3  christos     title="International System of Units">SI</abbr></a> seconds to
    710  1.3  christos     historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
    711  1.3  christos     to more than about one-hour accuracy.
    712  1.3  christos     See: Stephenson FR, Morrison LV, Hohenkerk CY.
    713  1.3  christos     <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
    714  1.3  christos     the Earth's rotation: 720 BC to AD 2015</a>.
    715  1.3  christos     <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
    716  1.3  christos     Also see: Espenak F. <a
    717  1.3  christos     href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
    718  1.3  christos     in Delta T (T)</a>.
    719  1.3  christos   </li>
    720  1.3  christos   <li>
    721  1.3  christos     The relationship between POSIX time (that is, <abbr>UTC</abbr> but
    722  1.3  christos     ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
    723  1.3  christos     seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
    724  1.3  christos     Although the POSIX
    725  1.3  christos     clock officially stops during an inserted leap second, at least one
    726  1.3  christos     proposed standard has it jumping back a second instead; and in
    727  1.3  christos     practice POSIX clocks more typically either progress glacially during
    728  1.3  christos     a leap second, or are slightly slowed while near a leap second.
    729  1.3  christos   </li>
    730  1.3  christos   <li>
    731  1.3  christos     The <code><abbr>tz</abbr></code> database does not represent how
    732  1.3  christos     uncertain its information is.
    733  1.3  christos     Ideally it would contain information about when data entries are
    734  1.3  christos     incomplete or dicey.
    735  1.3  christos     Partial temporal knowledge is a field of active research, though,
    736  1.3  christos     and it is not clear how to apply it here.
    737  1.1  christos   </li>
    738  1.1  christos </ul>
    739  1.1  christos 
    740  1.1  christos <p>
    741  1.3  christos In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
    742  1.3  christos database's pre-1970 and future timestamps are either wrong or
    743  1.3  christos misleading.
    744  1.3  christos Any attempt to pass the
    745  1.3  christos <code><abbr>tz</abbr></code> database off as the definition of time
    746  1.3  christos should be unacceptable to anybody who cares about the facts.
    747  1.3  christos In particular, the <code><abbr>tz</abbr></code> database's
    748  1.3  christos <abbr>LMT</abbr> offsets should not be considered meaningful, and
    749  1.3  christos should not prompt creation of <code><abbr>tz</abbr></code> regions
    750  1.3  christos merely because two locations
    751  1.3  christos differ in <abbr>LMT</abbr> or transitioned to standard time at
    752  1.3  christos different dates.
    753  1.3  christos </p>
    754  1.3  christos </section>
    755  1.3  christos 
    756  1.3  christos <section>
    757  1.3  christos   <h2 id="functions">Time and date functions</h2>
    758  1.3  christos <p>
    759  1.3  christos The <code><abbr>tz</abbr></code> code contains time and date functions
    760  1.3  christos that are upwards compatible with those of POSIX.
    761  1.3  christos Code compatible with this package is already
    762  1.3  christos <a href="tz-link.html#tzdb">part of many platforms</a>, where the
    763  1.3  christos primary use of this package is to update obsolete time-related files.
    764  1.3  christos To do this, you may need to compile the time zone compiler
    765  1.3  christos '<code>zic</code>' supplied with this package instead of using the
    766  1.3  christos system '<code>zic</code>', since the format of <code>zic</code>'s
    767  1.3  christos input is occasionally extended, and a platform may still be shipping
    768  1.3  christos an older <code>zic</code>.
    769  1.1  christos </p>
    770  1.1  christos 
    771  1.3  christos <h3 id="POSIX">POSIX properties and limitations</h3>
    772  1.1  christos <ul>
    773  1.1  christos   <li>
    774  1.1  christos     <p>
    775  1.3  christos     In POSIX, time display in a process is controlled by the
    776  1.3  christos     environment variable <code>TZ</code>.
    777  1.3  christos     Unfortunately, the POSIX
    778  1.3  christos     <code>TZ</code> string takes a form that is hard to describe and
    779  1.3  christos     is error-prone in practice.
    780  1.3  christos     Also, POSIX <code>TZ</code> strings cannot deal with daylight
    781  1.3  christos     saving time rules not based on the Gregorian calendar (as in
    782  1.3  christos     Iran), or with situations where more than two time zone
    783  1.3  christos     abbreviations or <abbr>UT</abbr> offsets are used in an area.
    784  1.1  christos     </p>
    785  1.3  christos 
    786  1.1  christos     <p>
    787  1.3  christos     The POSIX <code>TZ</code> string takes the following form:
    788  1.1  christos     </p>
    789  1.3  christos 
    790  1.1  christos     <p>
    791  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>]]]
    792  1.1  christos     </p>
    793  1.3  christos 
    794  1.1  christos     <p>
    795  1.3  christos     where:
    796  1.3  christos     </p>
    797  1.3  christos 
    798  1.1  christos     <dl>
    799  1.1  christos       <dt><var>std</var> and <var>dst</var></dt><dd>
    800  1.3  christos 	are 3 or more characters specifying the standard
    801  1.3  christos 	and daylight saving time (<abbr>DST</abbr>) zone names.
    802  1.3  christos 	Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
    803  1.3  christos 	may also be in a quoted form like '<code>&lt;+09&gt;</code>';
    804  1.3  christos 	this allows "<code>+</code>" and "<code>-</code>" in the names.
    805  1.1  christos       </dd>
    806  1.1  christos       <dt><var>offset</var></dt><dd>
    807  1.3  christos 	is of the form
    808  1.3  christos 	'<code>[&plusmn;]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
    809  1.3  christos 	and specifies the offset west of <abbr>UT</abbr>.
    810  1.3  christos 	'<var>hh</var>' may be a single digit;
    811  1.3  christos 	0&le;<var>hh</var>&le;24.
    812  1.3  christos 	The default <abbr>DST</abbr> offset is one hour ahead of
    813  1.3  christos 	standard time.
    814  1.1  christos       </dd>
    815  1.1  christos       <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
    816  1.3  christos 	specifies the beginning and end of <abbr>DST</abbr>.
    817  1.3  christos 	If this is absent, the system supplies its own ruleset
    818  1.3  christos 	for <abbr>DST</abbr>, and its rules can differ from year to year;
    819  1.3  christos 	typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
    820  1.1  christos       </dd>
    821  1.1  christos       <dt><var>time</var></dt><dd>
    822  1.3  christos 	takes the form
    823  1.3  christos 	'<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
    824  1.3  christos 	and defaults to 02:00.
    825  1.3  christos 	This is the same format as the offset, except that a
    826  1.3  christos 	leading '<code>+</code>' or '<code>-</code>' is not allowed.
    827  1.1  christos       </dd>
    828  1.1  christos       <dt><var>date</var></dt><dd>
    829  1.3  christos 	takes one of the following forms:
    830  1.1  christos 	<dl>
    831  1.1  christos 	  <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
    832  1.3  christos 	    origin-1 day number not counting February 29
    833  1.3  christos 	  </dd>
    834  1.1  christos 	  <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
    835  1.3  christos 	    origin-0 day number counting February 29 if present
    836  1.3  christos 	  </dd>
    837  1.3  christos 	  <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
    838  1.3  christos 	    (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5,
    839  1.3  christos 	    1&le;<var>m</var>&le;12)</dt><dd>
    840  1.3  christos 	    for the <var>d</var>th day of week <var>n</var> of
    841  1.3  christos 	    month <var>m</var> of the year, where week 1 is the first
    842  1.3  christos 	    week in which day <var>d</var> appears, and
    843  1.3  christos 	    '<code>5</code>' stands for the last week in which
    844  1.3  christos 	    day <var>d</var> appears (which may be either the 4th or
    845  1.3  christos 	    5th week).
    846  1.3  christos 	    Typically, this is the only useful form; the <var>n</var>
    847  1.3  christos 	    and <code>J</code><var>n</var> forms are rarely used.
    848  1.1  christos 	  </dd>
    849  1.3  christos 	</dl>
    850  1.3  christos       </dd>
    851  1.3  christos     </dl>
    852  1.3  christos 
    853  1.3  christos     <p>
    854  1.3  christos     Here is an example POSIX <code>TZ</code> string for New
    855  1.3  christos     Zealand after 2007.
    856  1.3  christos     It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
    857  1.3  christos     of <abbr>UT</abbr>, and that daylight saving time
    858  1.3  christos     (<abbr>NZDT</abbr>) is observed from September's last Sunday at
    859  1.3  christos     02:00 until April's first Sunday at 03:00:
    860  1.3  christos     </p>
    861  1.3  christos 
    862  1.3  christos     <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
    863  1.3  christos 
    864  1.3  christos     <p>
    865  1.3  christos     This POSIX <code>TZ</code> string is hard to remember, and
    866  1.3  christos     mishandles some timestamps before 2008.
    867  1.3  christos     With this package you can use this instead:
    868  1.3  christos     </p>
    869  1.3  christos 
    870  1.3  christos     <pre><code>TZ='Pacific/Auckland'</code></pre>
    871  1.3  christos   </li>
    872  1.3  christos   <li>
    873  1.3  christos     POSIX does not define the exact meaning of <code>TZ</code> values like
    874  1.3  christos     "<code>EST5EDT</code>".
    875  1.3  christos     Typically the current <abbr>US</abbr> <abbr>DST</abbr> rules
    876  1.3  christos     are used to interpret such values, but this means that the
    877  1.3  christos     <abbr>US</abbr> <abbr>DST</abbr> rules are compiled into each
    878  1.3  christos     program that does time conversion.
    879  1.3  christos     This means that when
    880  1.3  christos     <abbr>US</abbr> time conversion rules change (as in the United
    881  1.3  christos     States in 1987), all programs that do time conversion must be
    882  1.3  christos     recompiled to ensure proper results.
    883  1.3  christos   </li>
    884  1.3  christos   <li>
    885  1.3  christos     The <code>TZ</code> environment variable is process-global, which
    886  1.3  christos     makes it hard to write efficient, thread-safe applications that
    887  1.3  christos     need access to multiple time zone rulesets.
    888  1.3  christos   </li>
    889  1.3  christos   <li>
    890  1.3  christos     In POSIX, there is no tamper-proof way for a process to learn the
    891  1.3  christos     system's best idea of local wall clock.
    892  1.3  christos     (This is important for applications that an administrator wants
    893  1.3  christos     used only at certain times &ndash; without regard to whether the
    894  1.3  christos     user has fiddled the
    895  1.3  christos     <code>TZ</code> environment variable.
    896  1.3  christos     While an administrator can "do everything in <abbr>UT</abbr>" to
    897  1.3  christos     get around the problem, doing so is inconvenient and precludes
    898  1.3  christos     handling daylight saving time shifts - as might be required to
    899  1.3  christos     limit phone calls to off-peak hours.)
    900  1.3  christos   </li>
    901  1.3  christos   <li>
    902  1.3  christos     POSIX provides no convenient and efficient way to determine
    903  1.3  christos     the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
    904  1.3  christos     timestamps, particularly for <code><abbr>tz</abbr></code> regions
    905  1.3  christos     that do not fit into the POSIX model.
    906  1.3  christos   </li>
    907  1.3  christos   <li>
    908  1.3  christos     POSIX requires that systems ignore leap seconds.
    909  1.3  christos   </li>
    910  1.3  christos   <li>
    911  1.3  christos     The <code><abbr>tz</abbr></code> code attempts to support all the
    912  1.3  christos     <code>time_t</code> implementations allowed by POSIX.
    913  1.3  christos     The <code>time_t</code> type represents a nonnegative count of seconds
    914  1.3  christos     since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
    915  1.3  christos     In practice, <code>time_t</code> is usually a signed 64- or 32-bit
    916  1.3  christos     integer; 32-bit signed <code>time_t</code> values stop working after
    917  1.3  christos     2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
    918  1.3  christos     days typically use a signed 64-bit integer.
    919  1.3  christos     Unsigned 32-bit integers are used on one or two platforms, and 36-bit
    920  1.3  christos     and 40-bit integers are also used occasionally.
    921  1.3  christos     Although earlier POSIX versions allowed <code>time_t</code> to be a
    922  1.3  christos     floating-point type, this was not supported by any practical systems,
    923  1.3  christos     and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
    924  1.3  christos     require <code>time_t</code> to be an integer type.
    925  1.1  christos   </li>
    926  1.1  christos </ul>
    927  1.3  christos 
    928  1.3  christos <h3 id="POSIX-extensions">Extensions to POSIX in the
    929  1.3  christos <code><abbr>tz</abbr></code> code</h3>
    930  1.1  christos <ul>
    931  1.1  christos   <li>
    932  1.1  christos     <p>
    933  1.3  christos     The <code>TZ</code> environment variable is used in generating
    934  1.3  christos     the name of a binary file from which time-related information is read
    935  1.3  christos     (or is interpreted  la POSIX); <code>TZ</code> is no longer
    936  1.3  christos     constrained to be a three-letter time zone
    937  1.3  christos     abbreviation followed by a number of hours and an optional three-letter
    938  1.3  christos     daylight time zone abbreviation.
    939  1.3  christos     The daylight saving time rules to be used for a
    940  1.3  christos     particular <code><abbr>tz</abbr></code> region are encoded in the
    941  1.3  christos     binary file; the format of the file
    942  1.3  christos     allows U.S., Australian, and other rules to be encoded, and
    943  1.3  christos     allows for situations where more than two time zone
    944  1.3  christos     abbreviations are used.
    945  1.1  christos     </p>
    946  1.1  christos     <p>
    947  1.3  christos     It was recognized that allowing the <code>TZ</code> environment
    948  1.3  christos     variable to take on values such as '<code>America/New_York</code>'
    949  1.3  christos     might cause "old" programs (that expect <code>TZ</code> to have a
    950  1.3  christos     certain form) to operate incorrectly; consideration was given to using
    951  1.3  christos     some other environment variable (for example, <code>TIMEZONE</code>)
    952  1.3  christos     to hold the string used to generate the binary file's name.
    953  1.3  christos     In the end, however, it was decided to continue using
    954  1.3  christos     <code>TZ</code>: it is widely used for time zone purposes;
    955  1.3  christos     separately maintaining both <code>TZ</code>
    956  1.3  christos     and <code>TIMEZONE</code> seemed a nuisance; and systems where
    957  1.3  christos     "new" forms of <code>TZ</code> might cause problems can simply
    958  1.3  christos     use <code>TZ</code> values such as "<code>EST5EDT</code>" which
    959  1.3  christos     can be used both by "new" programs ( la POSIX) and "old"
    960  1.3  christos     programs (as zone names and offsets).
    961  1.1  christos     </p>
    962  1.3  christos   </li>
    963  1.3  christos   <li>
    964  1.3  christos     The code supports platforms with a <abbr>UT</abbr> offset member
    965  1.3  christos     in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
    966  1.3  christos   </li>
    967  1.3  christos   <li>
    968  1.3  christos     The code supports platforms with a time zone abbreviation member in
    969  1.3  christos     <code>struct tm</code>, e.g., <code>tm_zone</code>.
    970  1.3  christos   </li>
    971  1.3  christos   <li>
    972  1.3  christos     Functions <code>tzalloc</code>, <code>tzfree</code>,
    973  1.3  christos     <code>localtime_rz</code>, and <code>mktime_z</code> for
    974  1.3  christos     more-efficient thread-safe applications that need to use multiple
    975  1.3  christos     time zone rulesets.
    976  1.3  christos     The <code>tzalloc</code> and <code>tzfree</code> functions
    977  1.3  christos     allocate and free objects of type <code>timezone_t</code>,
    978  1.3  christos     and <code>localtime_rz</code> and <code>mktime_z</code> are
    979  1.3  christos     like <code>localtime_r</code> and <code>mktime</code> with an
    980  1.3  christos     extra <code>timezone_t</code> argument.
    981  1.3  christos     The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
    982  1.3  christos   </li>
    983  1.3  christos   <li>
    984  1.3  christos     A function <code>tzsetwall</code> has been added to arrange for the
    985  1.3  christos     system's best approximation to local wall clock time to be delivered
    986  1.3  christos     by subsequent calls to <code>localtime</code>.
    987  1.3  christos     Source code for portable applications that "must" run on local wall
    988  1.3  christos     clock time should call <code>tzsetwall</code>;
    989  1.3  christos     if such code is moved to "old" systems that do not
    990  1.3  christos     provide <code>tzsetwall</code>, you will not be able to generate an
    991  1.3  christos     executable program.
    992  1.3  christos     (These functions also arrange for local wall clock time to
    993  1.3  christos     be used if <code>tzset</code> is called &ndash; directly or
    994  1.3  christos     indirectly &ndash; and there is no <code>TZ</code> environment
    995  1.3  christos     variable; portable applications should not, however, rely on this
    996  1.3  christos     behavior since it is not the way <a
    997  1.3  christos     href="https://en.wikipedia.org/wiki/UNIX_System_V#SVR2"><abbr>SVR2</abbr></a>
    998  1.3  christos     systems behave.)
    999  1.3  christos   </li>
   1000  1.3  christos   <li>
   1001  1.3  christos     Negative <code>time_t</code> values are supported, on systems
   1002  1.3  christos     where <code>time_t</code> is signed.
   1003  1.3  christos   </li>
   1004  1.3  christos   <li>
   1005  1.3  christos     These functions can account for leap seconds, thanks to Bradley White.
   1006  1.3  christos   </li>
   1007  1.1  christos </ul>
   1008  1.3  christos 
   1009  1.3  christos <h3 id="vestigial">POSIX features no longer needed</h3>
   1010  1.1  christos <p>
   1011  1.3  christos POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
   1012  1.3  christos define some <a href="https://en.wikipedia.org/wiki/API"><abbr
   1013  1.3  christos title="application programming interface">API</abbr>s</a> that are vestigial:
   1014  1.3  christos they are not needed, and are relics of a too-simple model that does
   1015  1.3  christos not suffice to handle many real-world timestamps.
   1016  1.3  christos Although the <code><abbr>tz</abbr></code> code supports these
   1017  1.3  christos vestigial <abbr>API</abbr>s for backwards compatibility, they should
   1018  1.3  christos be avoided in portable applications.
   1019  1.3  christos The vestigial <abbr>API</abbr>s are:
   1020  1.1  christos </p>
   1021  1.1  christos <ul>
   1022  1.1  christos   <li>
   1023  1.3  christos     The POSIX <code>tzname</code> variable does not suffice and is no
   1024  1.3  christos     longer needed.
   1025  1.3  christos     To get a timestamp's time zone abbreviation, consult
   1026  1.3  christos     the <code>tm_zone</code> member if available; otherwise,
   1027  1.3  christos     use <code>strftime</code>'s <code>"%Z"</code> conversion
   1028  1.3  christos     specification.
   1029  1.3  christos   </li>
   1030  1.3  christos   <li>
   1031  1.3  christos     The POSIX <code>daylight</code> and <code>timezone</code>
   1032  1.3  christos     variables do not suffice and are no longer needed.
   1033  1.3  christos     To get a timestamp's <abbr>UT</abbr> offset, consult
   1034  1.3  christos     the <code>tm_gmtoff</code> member if available; otherwise,
   1035  1.3  christos     subtract values returned by <code>localtime</code>
   1036  1.3  christos     and <code>gmtime</code> using the rules of the Gregorian calendar,
   1037  1.3  christos     or use <code>strftime</code>'s <code>"%z"</code> conversion
   1038  1.3  christos     specification if a string like <code>"+0900"</code> suffices.
   1039  1.3  christos   </li>
   1040  1.3  christos   <li>
   1041  1.3  christos     The <code>tm_isdst</code> member is almost never needed and most of
   1042  1.3  christos     its uses should be discouraged in favor of the abovementioned
   1043  1.3  christos     <abbr>API</abbr>s.
   1044  1.3  christos     Although it can still be used in arguments to
   1045  1.3  christos     <code>mktime</code> to disambiguate timestamps near
   1046  1.3  christos     a <abbr>DST</abbr> transition when the clock jumps back, this
   1047  1.3  christos     disambiguation does not work when standard time itself jumps back,
   1048  1.3  christos     which can occur when a location changes to a time zone with a
   1049  1.3  christos     lesser <abbr>UT</abbr> offset.
   1050  1.3  christos   </li>
   1051  1.3  christos </ul>
   1052  1.3  christos 
   1053  1.3  christos <h3 id="other-portability">Other portability notes</h3>
   1054  1.3  christos <ul>
   1055  1.3  christos   <li>
   1056  1.3  christos     The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
   1057  1.3  christos     UNIX</a> <code>timezone</code> function is not present in this
   1058  1.3  christos     package; it is impossible to reliably map <code>timezone</code>'s
   1059  1.3  christos     arguments (a "minutes west of <abbr>GMT</abbr>" value and a
   1060  1.3  christos     "daylight saving time in effect" flag) to a time zone
   1061  1.3  christos     abbreviation, and we refuse to guess.
   1062  1.3  christos     Programs that in the past used the <code>timezone</code> function
   1063  1.3  christos     may now examine <code>localtime(&amp;clock)-&gt;tm_zone</code>
   1064  1.3  christos     (if <code>TM_ZONE</code> is defined) or
   1065  1.3  christos     <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code>
   1066  1.3  christos     (if <code>HAVE_TZNAME</code> is defined) to learn the correct time
   1067  1.3  christos     zone abbreviation to use.
   1068  1.3  christos   </li>
   1069  1.3  christos   <li>
   1070  1.3  christos     The <a
   1071  1.3  christos     href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
   1072  1.3  christos     <code>gettimeofday</code> function is not
   1073  1.3  christos     used in this package.
   1074  1.3  christos     This formerly let users obtain the current <abbr>UTC</abbr> offset
   1075  1.3  christos     and <abbr>DST</abbr> flag, but this functionality was removed in
   1076  1.3  christos     later versions of <abbr>BSD</abbr>.
   1077  1.3  christos   </li>
   1078  1.3  christos   <li>
   1079  1.3  christos     In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
   1080  1.3  christos     near-maximum <code>time_t</code> values when doing conversions
   1081  1.3  christos     for places that do not use <abbr>UT</abbr>.
   1082  1.3  christos     This package takes care to do these conversions correctly.
   1083  1.3  christos     A comment in the source code tells how to get compatibly wrong
   1084  1.3  christos     results.
   1085  1.3  christos   </li>
   1086  1.3  christos   <li>
   1087  1.3  christos     The functions that are conditionally compiled
   1088  1.3  christos     if <code>STD_INSPIRED</code> is defined should, at this point, be
   1089  1.3  christos     looked on primarily as food for thought.
   1090  1.3  christos     They are not in any sense "standard compatible" &ndash; some are
   1091  1.3  christos     not, in fact, specified in <em>any</em> standard.
   1092  1.3  christos     They do, however, represent responses of various authors to
   1093  1.3  christos     standardization proposals.
   1094  1.3  christos   </li>
   1095  1.3  christos   <li>
   1096  1.3  christos     Other time conversion proposals, in particular the one developed
   1097  1.3  christos     by folks at Hewlett Packard, offer a wider selection of functions
   1098  1.3  christos     that provide capabilities beyond those provided here.
   1099  1.3  christos     The absence of such functions from this package is not meant to
   1100  1.3  christos     discourage the development, standardization, or use of such
   1101  1.3  christos     functions.
   1102  1.3  christos     Rather, their absence reflects the decision to make this package
   1103  1.3  christos     contain valid extensions to POSIX, to ensure its broad
   1104  1.3  christos     acceptability.
   1105  1.3  christos     If more powerful time conversion functions can be standardized, so
   1106  1.3  christos     much the better.
   1107  1.1  christos   </li>
   1108  1.1  christos </ul>
   1109  1.3  christos </section>
   1110  1.1  christos 
   1111  1.3  christos <section>
   1112  1.3  christos   <h2 id="stability">Interface stability</h2>
   1113  1.1  christos <p>
   1114  1.3  christos The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
   1115  1.1  christos </p>
   1116  1.1  christos 
   1117  1.1  christos <ul>
   1118  1.1  christos   <li>
   1119  1.3  christos     A set of <code><abbr>tz</abbr></code> region names as per
   1120  1.3  christos       "<a href="#naming">Names of time zone rulesets</a>" above.
   1121  1.1  christos   </li>
   1122  1.1  christos   <li>
   1123  1.3  christos     Library functions described in "<a href="#functions">Time and date
   1124  1.3  christos       functions</a>" above.
   1125  1.1  christos   </li>
   1126  1.1  christos   <li>
   1127  1.3  christos     The programs <code>tzselect</code>, <code>zdump</code>,
   1128  1.3  christos     and <code>zic</code>, documented in their man pages.
   1129  1.1  christos   </li>
   1130  1.1  christos   <li>
   1131  1.3  christos     The format of <code>zic</code> input files, documented in
   1132  1.3  christos     the <code>zic</code> man page.
   1133  1.1  christos   </li>
   1134  1.1  christos   <li>
   1135  1.3  christos     The format of <code>zic</code> output files, documented in
   1136  1.3  christos     the <code>tzfile</code> man page.
   1137  1.1  christos   </li>
   1138  1.1  christos   <li>
   1139  1.3  christos     The format of zone table files, documented in <code>zone1970.tab</code>.
   1140  1.1  christos   </li>
   1141  1.1  christos   <li>
   1142  1.3  christos     The format of the country code file, documented in <code>iso3166.tab</code>.
   1143  1.1  christos   </li>
   1144  1.1  christos   <li>
   1145  1.3  christos     The version number of the code and data, as the first line of
   1146  1.3  christos     the text file '<code>version</code>' in each release.
   1147  1.1  christos   </li>
   1148  1.1  christos </ul>
   1149  1.3  christos 
   1150  1.1  christos <p>
   1151  1.1  christos Interface changes in a release attempt to preserve compatibility with
   1152  1.3  christos recent releases.
   1153  1.3  christos For example, <code><abbr>tz</abbr></code> data files typically do not
   1154  1.3  christos rely on recently-added <code>zic</code> features, so that users can
   1155  1.3  christos run older <code>zic</code> versions to process newer data files.
   1156  1.3  christos <a href="tz-link.html#download">Downloading
   1157  1.3  christos the <code><abbr>tz</abbr></code> database</a> describes how releases
   1158  1.3  christos are tagged and distributed.
   1159  1.1  christos </p>
   1160  1.1  christos 
   1161  1.1  christos <p>
   1162  1.3  christos Interfaces not listed above are less stable.
   1163  1.3  christos For example, users should not rely on particular <abbr>UT</abbr>
   1164  1.3  christos offsets or abbreviations for timestamps, as data entries are often
   1165  1.3  christos based on guesswork and these guesses may be corrected or improved.
   1166  1.1  christos </p>
   1167  1.3  christos </section>
   1168  1.1  christos 
   1169  1.3  christos <section>
   1170  1.3  christos   <h2 id="calendar">Calendrical issues</h2>
   1171  1.1  christos <p>
   1172  1.1  christos Calendrical issues are a bit out of scope for a time zone database,
   1173  1.1  christos but they indicate the sort of problems that we would run into if we
   1174  1.3  christos extended the time zone database further into the past.
   1175  1.3  christos An excellent resource in this area is Edward M. Reingold
   1176  1.3  christos and Nachum Dershowitz, <cite><a
   1177  1.3  christos href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
   1178  1.3  christos Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
   1179  1.3  christos Other information and sources are given in the file '<code>calendars</code>'
   1180  1.3  christos in the <code><abbr>tz</abbr></code> distribution.
   1181  1.3  christos They sometimes disagree.
   1182  1.3  christos </p>
   1183  1.3  christos </section>
   1184  1.3  christos 
   1185  1.3  christos <section>
   1186  1.3  christos   <h2 id="planets">Time and time zones on other planets</h2>
   1187  1.3  christos <p>
   1188  1.3  christos Some people's work schedules
   1189  1.3  christos use <a href="https://en.wikipedia.org/wiki/Timekeeping on Mars">Mars time</a>.
   1190  1.3  christos Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
   1191  1.3  christos and off during the
   1192  1.3  christos <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder#End_of_mission">Mars
   1193  1.3  christos Pathfinder</a> mission.
   1194  1.3  christos Some of their family members also adapted to Mars time.
   1195  1.3  christos Dozens of special Mars watches were built for JPL workers who kept
   1196  1.3  christos Mars time during the Mars Exploration Rovers mission (2004).
   1197  1.3  christos These timepieces look like normal Seikos and Citizens but use Mars
   1198  1.3  christos seconds rather than terrestrial seconds.
   1199  1.1  christos </p>
   1200  1.1  christos 
   1201  1.1  christos <p>
   1202  1.1  christos A Mars solar day is called a "sol" and has a mean period equal to
   1203  1.3  christos about 24 hours 39 minutes 35.244 seconds in terrestrial time.
   1204  1.3  christos It is divided into a conventional 24-hour clock, so each Mars second
   1205  1.3  christos equals about 1.02749125 terrestrial seconds.
   1206  1.1  christos </p>
   1207  1.1  christos 
   1208  1.1  christos <p>
   1209  1.3  christos The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
   1210  1.3  christos meridian</a> of Mars goes through the center of the crater
   1211  1.3  christos <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
   1212  1.3  christos honor of the British astronomer who built the Greenwich telescope that
   1213  1.3  christos defines Earth's prime meridian.
   1214  1.3  christos Mean solar time on the Mars prime meridian is
   1215  1.3  christos called <a href="https://en.wikipedia.org/wiki/Mars_Coordinated_Time">Mars
   1216  1.3  christos Coordinated Time (<abbr>MTC</abbr>)</a>.
   1217  1.1  christos </p>
   1218  1.1  christos 
   1219  1.1  christos <p>
   1220  1.1  christos Each landed mission on Mars has adopted a different reference for
   1221  1.1  christos solar time keeping, so there is no real standard for Mars time zones.
   1222  1.3  christos For example, the
   1223  1.3  christos <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
   1224  1.3  christos Exploration Rover</a> project (2004) defined two time zones "Local
   1225  1.3  christos Solar Time A" and "Local Solar Time B" for its two missions, each zone
   1226  1.3  christos designed so that its time equals local true solar time at
   1227  1.3  christos approximately the middle of the nominal mission.
   1228  1.3  christos Such a "time zone" is not particularly suited for any application
   1229  1.3  christos other than the mission itself.
   1230  1.1  christos </p>
   1231  1.1  christos 
   1232  1.1  christos <p>
   1233  1.1  christos Many calendars have been proposed for Mars, but none have achieved
   1234  1.3  christos wide acceptance.
   1235  1.3  christos Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
   1236  1.1  christos sequential count of Mars solar days elapsed since about 1873-12-29
   1237  1.3  christos 12:00 <abbr>GMT</abbr>.
   1238  1.1  christos </p>
   1239  1.1  christos 
   1240  1.1  christos <p>
   1241  1.1  christos In our solar system, Mars is the planet with time and calendar most
   1242  1.3  christos like Earth's.
   1243  1.3  christos On other planets, Sun-based time and calendars would work quite
   1244  1.3  christos differently.
   1245  1.3  christos For example, although Mercury's
   1246  1.3  christos <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
   1247  1.3  christos rotation period</a> is 58.646 Earth days, Mercury revolves around the
   1248  1.3  christos Sun so rapidly that an observer on Mercury's equator would see a
   1249  1.3  christos sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
   1250  1.3  christos Mercury day.
   1251  1.3  christos Venus is more complicated, partly because its rotation is slightly
   1252  1.3  christos <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
   1253  1.3  christos its year is 1.92 of its days.
   1254  1.3  christos Gas giants like Jupiter are trickier still, as their polar and
   1255  1.3  christos equatorial regions rotate at different rates, so that the length of a
   1256  1.3  christos day depends on latitude.
   1257  1.3  christos This effect is most pronounced on Neptune, where the day is about 12
   1258  1.3  christos hours at the poles and 18 hours at the equator.
   1259  1.1  christos </p>
   1260  1.1  christos 
   1261  1.1  christos <p>
   1262  1.3  christos Although the <code><abbr>tz</abbr></code> database does not support
   1263  1.3  christos time on other planets, it is documented here in the hopes that support
   1264  1.3  christos will be added eventually.
   1265  1.1  christos </p>
   1266  1.1  christos 
   1267  1.1  christos <p>
   1268  1.3  christos Sources for time on other planets:
   1269  1.1  christos </p>
   1270  1.3  christos 
   1271  1.1  christos <ul>
   1272  1.1  christos   <li>
   1273  1.3  christos     Michael Allison and Robert Schmunk,
   1274  1.3  christos     "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
   1275  1.3  christos       Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
   1276  1.3  christos     (2015-06-30).
   1277  1.1  christos   </li>
   1278  1.1  christos   <li>
   1279  1.3  christos     Jia-Rui Chong,
   1280  1.3  christos     "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays
   1281  1.3  christos     Fit for a Martian</a>", <cite>Los Angeles Times</cite>
   1282  1.3  christos     (2004-01-14), pp A1, A20&ndash;A21.
   1283  1.1  christos   </li>
   1284  1.1  christos   <li>
   1285  1.3  christos     Tom Chmielewski,
   1286  1.3  christos     "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
   1287  1.3  christos     Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
   1288  1.1  christos   </li>
   1289  1.1  christos   <li>
   1290  1.3  christos     Matt Williams,
   1291  1.3  christos     "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
   1292  1.3  christos     long is a day on the other planets of the solar system?</a>"
   1293  1.3  christos     (2017-04-27).
   1294  1.1  christos   </li>
   1295  1.1  christos </ul>
   1296  1.3  christos </section>
   1297  1.1  christos 
   1298  1.3  christos <footer>
   1299  1.3  christos   <hr>
   1300  1.3  christos   This file is in the public domain, so clarified as of 2009-05-17 by
   1301  1.3  christos   Arthur David Olson.
   1302  1.3  christos </footer>
   1303  1.1  christos </body>
   1304  1.1  christos </html>
   1305