theory.html revision 1.4
11.4Schristos<!DOCTYPE html> 21.1Schristos<html lang="en"> 31.1Schristos<head> 41.1Schristos <title>Theory and pragmatics of the tz code and data</title> 51.1Schristos <meta charset="UTF-8"> 61.4Schristos <style> 71.4Schristos pre {margin-left: 2em; white-space: pre-wrap;} 81.4Schristos </style> 91.1Schristos</head> 101.1Schristos 111.1Schristos<body> 121.3Schristos<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1> 131.1Schristos <h3>Outline</h3> 141.1Schristos <nav> 151.1Schristos <ul> 161.3Schristos <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code> 171.3Schristos database</a></li> 181.4Schristos <li><a href="#naming">Names of timezones</a></li> 191.1Schristos <li><a href="#abbreviations">Time zone abbreviations</a></li> 201.3Schristos <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code> 211.3Schristos database</a></li> 221.1Schristos <li><a href="#functions">Time and date functions</a></li> 231.1Schristos <li><a href="#stability">Interface stability</a></li> 241.1Schristos <li><a href="#calendar">Calendrical issues</a></li> 251.1Schristos <li><a href="#planets">Time and time zones on other planets</a></li> 261.1Schristos </ul> 271.1Schristos </nav> 281.1Schristos 291.3Schristos<section> 301.3Schristos <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2> 311.1Schristos<p> 321.3SchristosThe <a 331.3Schristoshref="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code> 341.3Schristosdatabase</a> attempts to record the history and predicted future of 351.3Schristosall computer-based clocks that track civil time. 361.3SchristosIt organizes <a href="tz-link.html">time zone and daylight saving time 371.3Schristosdata</a> by partitioning the world into <a 381.4Schristoshref="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a> 391.3Schristoswhose clocks all agree about timestamps that occur after the <a 401.3Schristoshref="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> 411.3Schristos(1970-01-01 00:00:00 <a 421.3Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr 431.3Schristostitle="Coordinated Universal Time">UTC</abbr></a>). 441.4SchristosThe database labels each timezone with a notable location and 451.3Schristosrecords all known clock transitions for that location. 461.3SchristosAlthough 1970 is a somewhat-arbitrary cutoff, there are significant 471.3Schristoschallenges to moving the cutoff earlier even by a decade or two, due 481.3Schristosto the wide variety of local practices before computer timekeeping 491.3Schristosbecame prevalent. 501.1Schristos</p> 511.1Schristos 521.1Schristos<p> 531.4SchristosEach timezone typically corresponds to a geographical region that is 541.4Schristossmaller than a traditional time zone, because clocks in a timezone 551.4Schristosall agree after 1970 whereas a traditional time zone merely 561.4Schristosspecifies current standard time. For example, applications that deal 571.4Schristoswith current and future timestamps in the traditional North 581.4SchristosAmerican mountain time zone can choose from the timezones 591.4Schristos<code>America/Denver</code> which observes US-style daylight saving 601.4Schristostime, <code>America/Mazatlan</code> which observes Mexican-style DST, 611.4Schristosand <code>America/Phoenix</code> which does not observe DST. 621.4SchristosApplications that also deal with past timestamps in the mountain time 631.4Schristoszone can choose from over a dozen timezones, such as 641.4Schristos<code>America/Boise</code>, <code>America/Edmonton</code>, and 651.4Schristos<code>America/Hermosillo</code>, each of which currently uses mountain 661.4Schristostime but differs from other timezones for some timestamps after 1970. 671.4Schristos</p> 681.4Schristos 691.4Schristos<p> 701.4SchristosClock transitions before 1970 are recorded for each timezone, 711.1Schristosbecause most systems support timestamps before 1970 and could 721.1Schristosmisbehave if data entries were omitted for pre-1970 transitions. 731.1SchristosHowever, the database is not designed for and does not suffice for 741.1Schristosapplications requiring accurate handling of all past times everywhere, 751.1Schristosas it would take far too much effort and guesswork to record all 761.1Schristosdetails of pre-1970 civil timekeeping. 771.3SchristosAlthough some information outside the scope of the database is 781.2Schristoscollected in a file <code>backzone</code> that is distributed along 791.2Schristoswith the database proper, this file is less reliable and does not 801.2Schristosnecessarily follow database guidelines. 811.1Schristos</p> 821.1Schristos 831.1Schristos<p> 841.3SchristosAs described below, reference source code for using the 851.3Schristos<code><abbr>tz</abbr></code> database is also available. 861.3SchristosThe <code><abbr>tz</abbr></code> code is upwards compatible with <a 871.3Schristoshref="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international 881.3Schristosstandard for <a 891.3Schristoshref="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems. 901.3SchristosAs of this writing, the current edition of POSIX is: <a 911.3Schristoshref="http://pubs.opengroup.org/onlinepubs/9699919799/"> The Open 921.3SchristosGroup Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018 931.3SchristosEdition. 941.3SchristosBecause the database's scope encompasses real-world changes to civil 951.3Schristostimekeeping, its model for describing time is more complex than the 961.3Schristosstandard and daylight saving times supported by POSIX. 971.4SchristosA <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can 981.3Schristoshave more than two changes per year, these changes need not merely 991.3Schristosflip back and forth between two alternatives, and the rules themselves 1001.3Schristoscan change at times. 1011.4SchristosWhether and when a timezone changes its 1021.4Schristosclock, and even the timezone's notional base offset from UTC, are variable. 1031.4SchristosIt does not always make sense to talk about a timezone's 1041.4Schristos"base offset", which is not necessarily a single number. 1051.1Schristos</p> 1061.1Schristos 1071.3Schristos</section> 1081.1Schristos 1091.3Schristos<section> 1101.4Schristos <h2 id="naming">Names of timezones</h2> 1111.1Schristos<p> 1121.4SchristosEach timezone has a unique name. 1131.1SchristosInexperienced users are not expected to select these names unaided. 1141.1SchristosDistributors should provide documentation and/or a simple selection 1151.4Schristosinterface that explains each name via a map or via descriptive text like 1161.4Schristos"Ruthenia" instead of the timezone name "<code>Europe/Uzhgorod</code>". 1171.4SchristosIf geolocation information is available, a selection interface can 1181.4Schristoslocate the user on a timezone map or prioritize names that are 1191.4Schristosgeographically close. For an example selection interface, see the 1201.3Schristos<code>tzselect</code> program in the <code><abbr>tz</abbr></code> code. 1211.3SchristosThe <a href="http://cldr.unicode.org/">Unicode Common Locale Data 1221.3SchristosRepository</a> contains data that may be useful for other selection 1231.4Schristosinterfaces; it maps timezone names like <code>Europe/Uzhgorod</code> 1241.4Schristosto CLDR names like <code>uauzh</code> which are in turn mapped to 1251.4Schristoslocale-dependent strings like "Uzhhorod", "Ungvár", "Ужгород", and 1261.4Schristos"乌日哥罗德". 1271.1Schristos</p> 1281.1Schristos 1291.1Schristos<p> 1301.3SchristosThe naming conventions attempt to strike a balance 1311.1Schristosamong the following goals: 1321.1Schristos</p> 1331.3Schristos 1341.1Schristos<ul> 1351.1Schristos <li> 1361.4Schristos Uniquely identify every timezone where clocks have agreed since 1970. 1371.3Schristos This is essential for the intended use: static clocks keeping local 1381.3Schristos civil time. 1391.1Schristos </li> 1401.1Schristos <li> 1411.4Schristos Indicate to experts where the timezone's clocks typically are. 1421.1Schristos </li> 1431.1Schristos <li> 1441.3Schristos Be robust in the presence of political changes. 1451.3Schristos For example, names of countries are ordinarily not used, to avoid 1461.3Schristos incompatibilities when countries change their name (e.g., 1471.3Schristos Zaire→Congo) or when locations change countries (e.g., Hong 1481.3Schristos Kong from UK colony to China). 1491.1Schristos </li> 1501.1Schristos <li> 1511.3Schristos Be portable to a wide variety of implementations. 1521.1Schristos </li> 1531.1Schristos <li> 1541.3Schristos Use a consistent naming conventions over the entire world. 1551.1Schristos </li> 1561.1Schristos</ul> 1571.3Schristos 1581.1Schristos<p> 1591.3SchristosNames normally have the form 1601.3Schristos<var>AREA</var><code>/</code><var>LOCATION</var>, where 1611.4Schristos<var>AREA</var> is a continent or ocean, and 1621.4Schristos<var>LOCATION</var> is a specific location within the area. 1631.3SchristosNorth and South America share the same area, '<code>America</code>'. 1641.3SchristosTypical names are '<code>Africa/Cairo</code>', 1651.3Schristos'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'. 1661.3SchristosSome names are further qualified to help avoid confusion; for example, 1671.3Schristos'<code>America/Indiana/Petersburg</code>' distinguishes Petersburg, 1681.3SchristosIndiana from other Petersburgs in America. 1691.1Schristos</p> 1701.1Schristos 1711.1Schristos<p> 1721.3SchristosHere are the general guidelines used for 1731.4Schristoschoosing timezone names, 1741.1Schristosin decreasing order of importance: 1751.1Schristos</p> 1761.3Schristos 1771.1Schristos<ul> 1781.1Schristos <li> 1791.3Schristos Use only valid POSIX file name components (i.e., the parts of 1801.3Schristos names other than '<code>/</code>'). 1811.3Schristos Do not use the file name components '<code>.</code>' and 1821.3Schristos '<code>..</code>'. 1831.3Schristos Within a file name component, use only <a 1841.3Schristos href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, 1851.3Schristos '<code>.</code>', '<code>-</code>' and '<code>_</code>'. 1861.3Schristos Do not use digits, as that might create an ambiguity with <a 1871.3Schristos href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX 1881.3Schristos <code>TZ</code> strings</a>. 1891.3Schristos A file name component must not exceed 14 characters or start with 1901.3Schristos '<code>-</code>'. 1911.3Schristos E.g., prefer <code>Asia/Brunei</code> to 1921.3Schristos <code>Asia/Bandar_Seri_Begawan</code>. 1931.3Schristos Exceptions: see the discussion of legacy names below. 1941.1Schristos </li> 1951.1Schristos <li> 1961.3Schristos A name must not be empty, or contain '<code>//</code>', or 1971.3Schristos start or end with '<code>/</code>'. 1981.1Schristos </li> 1991.1Schristos <li> 2001.3Schristos Do not use names that differ only in case. 2011.3Schristos Although the reference implementation is case-sensitive, some 2021.3Schristos other implementations are not, and they would mishandle names 2031.3Schristos differing only in case. 2041.1Schristos </li> 2051.1Schristos <li> 2061.3Schristos If one name <var>A</var> is an initial prefix of another 2071.3Schristos name <var>AB</var> (ignoring case), then <var>B</var> must not 2081.3Schristos start with '<code>/</code>', as a regular file cannot have the 2091.3Schristos same name as a directory in POSIX. 2101.3Schristos For example, <code>America/New_York</code> precludes 2111.3Schristos <code>America/New_York/Bronx</code>. 2121.1Schristos </li> 2131.1Schristos <li> 2141.3Schristos Uninhabited regions like the North Pole and Bouvet Island 2151.3Schristos do not need locations, since local time is not defined there. 2161.1Schristos </li> 2171.1Schristos <li> 2181.3Schristos There should typically be at least one name for each <a 2191.3Schristos href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr 2201.3Schristos title="International Organization for Standardization">ISO</abbr> 2211.3Schristos 3166-1</a> officially assigned two-letter code for an inhabited 2221.3Schristos country or territory. 2231.1Schristos </li> 2241.1Schristos <li> 2251.4Schristos If all the clocks in a timezone have agreed since 1970, 2261.4Schristos do not bother to include more than one timezone 2271.4Schristos even if some of the clocks disagreed before 1970. 2281.3Schristos Otherwise these tables would become annoyingly large. 2291.1Schristos </li> 2301.1Schristos <li> 2311.3Schristos If a name is ambiguous, use a less ambiguous alternative; 2321.3Schristos e.g., many cities are named San José and Georgetown, so 2331.3Schristos prefer <code>America/Costa_Rica</code> to 2341.3Schristos <code>America/San_Jose</code> and <code>America/Guyana</code> 2351.3Schristos to <code>America/Georgetown</code>. 2361.1Schristos </li> 2371.1Schristos <li> 2381.3Schristos Keep locations compact. 2391.3Schristos Use cities or small islands, not countries or regions, so that any 2401.3Schristos future changes do not split individual locations into different 2411.4Schristos timezones. 2421.3Schristos E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>, 2431.3Schristos since 2441.3Schristos <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France 2451.3Schristos has had multiple time zones</a>. 2461.1Schristos </li> 2471.1Schristos <li> 2481.3Schristos Use mainstream English spelling, e.g., prefer 2491.4Schristos <code>Europe/Rome</code> to <code>Europa/Roma</code>, and 2501.3Schristos prefer <code>Europe/Athens</code> to the Greek 2511.4Schristos <code>Ευρώπη/Αθήνα</code> or the Romanized 2521.4Schristos <code>Evrópi/Athína</code>. 2531.3Schristos The POSIX file name restrictions encourage this guideline. 2541.1Schristos </li> 2551.1Schristos <li> 2561.3Schristos Use the most populous among locations in a region, 2571.3Schristos e.g., prefer <code>Asia/Shanghai</code> to 2581.3Schristos <code>Asia/Beijing</code>. 2591.3Schristos Among locations with similar populations, pick the best-known 2601.3Schristos location, e.g., prefer <code>Europe/Rome</code> to 2611.3Schristos <code>Europe/Milan</code>. 2621.1Schristos </li> 2631.1Schristos <li> 2641.3Schristos Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to 2651.3Schristos <code>Atlantic/Canaries</code>. 2661.1Schristos </li> 2671.1Schristos <li> 2681.3Schristos Omit common suffixes like '<code>_Islands</code>' and 2691.3Schristos '<code>_City</code>', unless that would lead to ambiguity. 2701.3Schristos E.g., prefer <code>America/Cayman</code> to 2711.3Schristos <code>America/Cayman_Islands</code> and 2721.3Schristos <code>America/Guatemala</code> to 2731.3Schristos <code>America/Guatemala_City</code>, but prefer 2741.3Schristos <code>America/Mexico_City</code> to 2751.3Schristos <code>America/Mexico</code> 2761.3Schristos because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the 2771.3Schristos country of Mexico has several time zones</a>. 2781.1Schristos </li> 2791.1Schristos <li> 2801.3Schristos Use '<code>_</code>' to represent a space. 2811.1Schristos </li> 2821.1Schristos <li> 2831.3Schristos Omit '<code>.</code>' from abbreviations in names. 2841.3Schristos E.g., prefer <code>Atlantic/St_Helena</code> to 2851.3Schristos <code>Atlantic/St._Helena</code>. 2861.1Schristos </li> 2871.1Schristos <li> 2881.3Schristos Do not change established names if they only marginally violate 2891.3Schristos the above guidelines. 2901.3Schristos For example, do not change the existing name <code>Europe/Rome</code> to 2911.3Schristos <code>Europe/Milan</code> merely because Milan's population has grown 2921.3Schristos to be somewhat greater than Rome's. 2931.1Schristos </li> 2941.1Schristos <li> 2951.3Schristos If a name is changed, put its old spelling in the 2961.3Schristos '<code>backward</code>' file. 2971.3Schristos This means old spellings will continue to work. 2981.1Schristos </li> 2991.1Schristos</ul> 3001.1Schristos 3011.1Schristos<p> 3021.1SchristosThe file '<code>zone1970.tab</code>' lists geographical locations used 3031.4Schristosto name timezones. 3041.3SchristosIt is intended to be an exhaustive list of names for geographic 3051.4Schristosregions as described above; this is a subset of the timezones in the data. 3061.3SchristosAlthough a '<code>zone1970.tab</code>' location's 3071.3Schristos<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> 3081.3Schristoscorresponds to 3091.3Schristosits <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean 3101.3Schristostime (<abbr>LMT</abbr>)</a> offset with one hour for every 15° 3111.3Schristoseast longitude, this relationship is not exact. 3121.1Schristos</p> 3131.1Schristos 3141.1Schristos<p> 3151.1SchristosOlder versions of this package used a different naming scheme, 3161.1Schristosand these older names are still supported. 3171.1SchristosSee the file '<code>backward</code>' for most of these older names 3181.1Schristos(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). 3191.1SchristosThe other old-fashioned names still supported are 3201.3Schristos'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and 3211.3Schristos'<code>EET</code>' (see the file '<code>europe</code>'). 3221.1Schristos</p> 3231.1Schristos 3241.1Schristos<p> 3251.1SchristosOlder versions of this package defined legacy names that are 3261.3Schristosincompatible with the first guideline of location names, but which are 3271.3Schristosstill supported. 3281.3SchristosThese legacy names are mostly defined in the file 3291.3Schristos'<code>etcetera</code>'. 3301.3SchristosAlso, the file '<code>backward</code>' defines the legacy names 3311.3Schristos'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', 3321.3Schristosand the file '<code>northamerica</code>' defines the legacy names 3331.3Schristos'<code>EST5EDT</code>', '<code>CST6CDT</code>', 3341.3Schristos'<code>MST7MDT</code>', and '<code>PST8PDT</code>'. 3351.1Schristos</p> 3361.1Schristos 3371.1Schristos<p> 3381.3SchristosExcluding '<code>backward</code>' should not affect the other data. 3391.3SchristosIf '<code>backward</code>' is excluded, excluding 3401.3Schristos'<code>etcetera</code>' should not affect the remaining data. 3411.1Schristos</p> 3421.3Schristos</section> 3431.1Schristos 3441.3Schristos<section> 3451.3Schristos <h2 id="abbreviations">Time zone abbreviations</h2> 3461.1Schristos<p> 3471.1SchristosWhen this package is installed, it generates time zone abbreviations 3481.1Schristoslike '<code>EST</code>' to be compatible with human tradition and POSIX. 3491.3SchristosHere are the general guidelines used for choosing time zone abbreviations, 3501.1Schristosin decreasing order of importance: 3511.3Schristos</p> 3521.3Schristos 3531.1Schristos<ul> 3541.1Schristos <li> 3551.3Schristos Use three to six characters that are ASCII alphanumerics or 3561.3Schristos '<code>+</code>' or '<code>-</code>'. 3571.3Schristos Previous editions of this database also used characters like 3581.3Schristos space and '<code>?</code>', but these characters have a 3591.3Schristos special meaning to the 3601.3Schristos <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a> 3611.3Schristos and cause commands like 3621.3Schristos '<code><a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a> 3631.3Schristos `<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>' 3641.3Schristos to have unexpected effects. 3651.3Schristos Previous editions of this guideline required upper-case letters, but the 3661.3Schristos Congressman who introduced 3671.3Schristos <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro 3681.3Schristos Standard Time</a> preferred "ChST", so lower-case letters are now 3691.3Schristos allowed. 3701.3Schristos Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>', 3711.3Schristos '<code>+</code>', and alphanumeric characters from the portable 3721.3Schristos character set in the current locale. 3731.3Schristos In practice ASCII alphanumerics and '<code>+</code>' and 3741.3Schristos '<code>-</code>' are safe in all locales. 3751.3Schristos 3761.3Schristos <p> 3771.3Schristos In other words, in the C locale the POSIX extended regular 3781.3Schristos expression <code>[-+[:alnum:]]{3,6}</code> should match the 3791.3Schristos abbreviation. 3801.3Schristos This guarantees that all abbreviations could have been specified by a 3811.3Schristos POSIX <code>TZ</code> string. 3821.3Schristos </p> 3831.3Schristos </li> 3841.3Schristos <li> 3851.3Schristos Use abbreviations that are in common use among English-speakers, 3861.3Schristos e.g., 'EST' for Eastern Standard Time in North America. 3871.3Schristos We assume that applications translate them to other languages 3881.3Schristos as part of the normal localization process; for example, 3891.3Schristos a French application might translate 'EST' to 'HNE'. 3901.3Schristos 3911.3Schristos <p> 3921.3Schristos <small>These abbreviations (for standard/daylight/etc. time) are: 3931.3Schristos ACST/ACDT Australian Central, 3941.3Schristos AST/ADT/APT/AWT/ADDT Atlantic, 3951.3Schristos AEST/AEDT Australian Eastern, 3961.3Schristos AHST/AHDT Alaska-Hawaii, 3971.3Schristos AKST/AKDT Alaska, 3981.3Schristos AWST/AWDT Australian Western, 3991.3Schristos BST/BDT Bering, 4001.3Schristos CAT/CAST Central Africa, 4011.3Schristos CET/CEST/CEMT Central European, 4021.3Schristos ChST Chamorro, 4031.3Schristos CST/CDT/CWT/CPT/CDDT Central [North America], 4041.3Schristos CST/CDT China, 4051.3Schristos GMT/BST/IST/BDST Greenwich, 4061.3Schristos EAT East Africa, 4071.3Schristos EST/EDT/EWT/EPT/EDDT Eastern [North America], 4081.3Schristos EET/EEST Eastern European, 4091.3Schristos GST Guam, 4101.3Schristos HST/HDT Hawaii, 4111.3Schristos HKT/HKST Hong Kong, 4121.3Schristos IST India, 4131.3Schristos IST/GMT Irish, 4141.3Schristos IST/IDT/IDDT Israel, 4151.3Schristos JST/JDT Japan, 4161.3Schristos KST/KDT Korea, 4171.3Schristos MET/MEST Middle European (a backward-compatibility alias for 4181.3Schristos Central European), 4191.3Schristos MSK/MSD Moscow, 4201.3Schristos MST/MDT/MWT/MPT/MDDT Mountain, 4211.3Schristos NST/NDT/NWT/NPT/NDDT Newfoundland, 4221.3Schristos NST/NDT/NWT/NPT Nome, 4231.3Schristos NZMT/NZST New Zealand through 1945, 4241.3Schristos NZST/NZDT New Zealand 1946–present, 4251.3Schristos PKT/PKST Pakistan, 4261.3Schristos PST/PDT/PWT/PPT/PDDT Pacific, 4271.4Schristos PST/PDT Philippine, 4281.3Schristos SAST South Africa, 4291.3Schristos SST Samoa, 4301.3Schristos WAT/WAST West Africa, 4311.3Schristos WET/WEST/WEMT Western European, 4321.3Schristos WIB Waktu Indonesia Barat, 4331.3Schristos WIT Waktu Indonesia Timur, 4341.3Schristos WITA Waktu Indonesia Tengah, 4351.3Schristos YST/YDT/YWT/YPT/YDDT Yukon</small>. 4361.3Schristos </p> 4371.3Schristos </li> 4381.3Schristos <li> 4391.3Schristos <p> 4401.3Schristos For times taken from a city's longitude, use the 4411.3Schristos traditional <var>x</var>MT notation. 4421.3Schristos The only abbreviation like this in current use is '<abbr>GMT</abbr>'. 4431.3Schristos The others are for timestamps before 1960, 4441.3Schristos except that Monrovia Mean Time persisted until 1972. 4451.3Schristos Typically, numeric abbreviations (e.g., '<code>-</code>004430' for 4461.3Schristos MMT) would cause trouble here, as the numeric strings would exceed 4471.3Schristos the POSIX length limit. 4481.3Schristos </p> 4491.3Schristos 4501.3Schristos <p> 4511.3Schristos <small>These abbreviations are: 4521.3Schristos AMT Amsterdam, Asunción, Athens; 4531.3Schristos BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels, 4541.3Schristos Bucharest; 4551.3Schristos CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; 4561.3Schristos DMT Dublin/Dunsink; 4571.3Schristos EMT Easter; 4581.3Schristos FFMT Fort-de-France; 4591.3Schristos FMT Funchal; 4601.3Schristos GMT Greenwich; 4611.3Schristos HMT Havana, Helsinki, Horta, Howrah; 4621.3Schristos IMT Irkutsk, Istanbul; 4631.3Schristos JMT Jerusalem; 4641.3Schristos KMT Kaunas, Kiev, Kingston; 4651.3Schristos LMT Lima, Lisbon, local, Luanda; 4661.3Schristos MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, 4671.3Schristos Moratuwa, Moscow; 4681.3Schristos PLMT Phù Liễn; 4691.3Schristos PMT Paramaribo, Paris, Perm, Pontianak, Prague; 4701.3Schristos PMMT Port Moresby; 4711.3Schristos QMT Quito; 4721.3Schristos RMT Rangoon, Riga, Rome; 4731.3Schristos SDMT Santo Domingo; 4741.3Schristos SJMT San José; 4751.3Schristos SMT Santiago, Simferopol, Singapore, Stanley; 4761.3Schristos TBMT Tbilisi; 4771.3Schristos TMT Tallinn, Tehran; 4781.3Schristos WMT Warsaw</small>. 4791.3Schristos </p> 4801.3Schristos 4811.3Schristos <p> 4821.3Schristos <small>A few abbreviations also follow the pattern that 4831.4Schristos <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK. 4841.3Schristos They are: 4851.3Schristos CMT/BST for Calamarca Mean Time and Bolivian Summer Time 4861.3Schristos 1890–1932, 4871.3Schristos DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time 4881.3Schristos 1880–1916, 4891.3Schristos MMT/MST/MDST for Moscow 1880–1919, and 4901.3Schristos RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926. 4911.3Schristos An extra-special case is SET for Swedish Time (<em>svensk 4921.3Schristos normaltid</em>) 1879–1899, 3° west of the Stockholm 4931.3Schristos Observatory.</small> 4941.3Schristos </p> 4951.3Schristos </li> 4961.3Schristos <li> 4971.3Schristos Use '<abbr>LMT</abbr>' for local mean time of locations before the 4981.3Schristos introduction of standard time; see "<a href="#scope">Scope of the 4991.3Schristos <code><abbr>tz</abbr></code> database</a>". 5001.3Schristos </li> 5011.3Schristos <li> 5021.3Schristos If there is no common English abbreviation, use numeric offsets like 5031.4Schristos <code>-</code>05 and <code>+</code>0530 that are generated 5041.3Schristos by <code>zic</code>'s <code>%z</code> notation. 5051.3Schristos </li> 5061.3Schristos <li> 5071.3Schristos Use current abbreviations for older timestamps to avoid confusion. 5081.3Schristos For example, in 1910 a common English abbreviation for time 5091.3Schristos in central Europe was 'MEZ' (short for both "Middle European 5101.3Schristos Zone" and for "Mitteleuropäische Zeit" in German). 5111.3Schristos Nowadays 'CET' ("Central European Time") is more common in 5121.3Schristos English, and the database uses 'CET' even for circa-1910 5131.3Schristos timestamps as this is less confusing for modern users and avoids 5141.3Schristos the need for determining when 'CET' supplanted 'MEZ' in common 5151.3Schristos usage. 5161.3Schristos </li> 5171.3Schristos <li> 5181.4Schristos Use a consistent style in a timezone's history. 5191.4Schristos For example, if a history tends to use numeric 5201.3Schristos abbreviations and a particular entry could go either way, use a 5211.3Schristos numeric abbreviation. 5221.3Schristos </li> 5231.3Schristos <li> 5241.3Schristos Use 5251.3Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> 5261.3Schristos (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for 5271.3Schristos locations while uninhabited. 5281.3Schristos The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in 5291.3Schristos some sense undefined; this notation is derived 5301.3Schristos from <a href="https://tools.ietf.org/html/rfc3339">Internet 5311.4Schristos <abbr title="Request For Comments">RFC</abbr> 3339</a>. 5321.1Schristos </li> 5331.1Schristos</ul> 5341.3Schristos 5351.1Schristos<p> 5361.1SchristosApplication writers should note that these abbreviations are ambiguous 5371.2Schristosin practice: e.g., 'CST' means one thing in China and something else 5381.2Schristosin North America, and 'IST' can refer to time in India, Ireland or 5391.3SchristosIsrael. 5401.3SchristosTo avoid ambiguity, use numeric <abbr>UT</abbr> offsets like 5411.2Schristos'<code>-</code>0600' instead of time zone abbreviations like 'CST'. 5421.1Schristos</p> 5431.3Schristos</section> 5441.1Schristos 5451.3Schristos<section> 5461.3Schristos <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2> 5471.1Schristos<p> 5481.3SchristosThe <code><abbr>tz</abbr></code> database is not authoritative, and it 5491.3Schristossurely has errors. 5501.2SchristosCorrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>. 5511.1SchristosUsers requiring authoritative data should consult national standards 5521.1Schristosbodies and the references cited in the database's comments. 5531.1Schristos</p> 5541.1Schristos 5551.1Schristos<p> 5561.3SchristosErrors in the <code><abbr>tz</abbr></code> database arise from many sources: 5571.1Schristos</p> 5581.3Schristos 5591.1Schristos<ul> 5601.1Schristos <li> 5611.3Schristos The <code><abbr>tz</abbr></code> database predicts future 5621.3Schristos timestamps, and current predictions 5631.3Schristos will be incorrect after future governments change the rules. 5641.3Schristos For example, if today someone schedules a meeting for 13:00 next 5651.3Schristos October 1, Casablanca time, and tomorrow Morocco changes its 5661.3Schristos daylight saving rules, software can mess up after the rule change 5671.3Schristos if it blithely relies on conversions made before the change. 5681.3Schristos </li> 5691.3Schristos <li> 5701.3Schristos The pre-1970 entries in this database cover only a tiny sliver of how 5711.3Schristos clocks actually behaved; the vast majority of the necessary 5721.3Schristos information was lost or never recorded. 5731.4Schristos Thousands more timezones would be needed if 5741.3Schristos the <code><abbr>tz</abbr></code> database's scope were extended to 5751.3Schristos cover even just the known or guessed history of standard time; for 5761.3Schristos example, the current single entry for France would need to split 5771.3Schristos into dozens of entries, perhaps hundreds. 5781.3Schristos And in most of the world even this approach would be misleading 5791.3Schristos due to widespread disagreement or indifference about what times 5801.3Schristos should be observed. 5811.3Schristos In her 2015 book 5821.3Schristos <cite><a 5831.3Schristos href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The 5841.3Schristos Global Transformation of Time, 1870–1950</a></cite>, 5851.3Schristos Vanessa Ogle writes 5861.3Schristos "Outside of Europe and North America there was no system of time 5871.3Schristos zones at all, often not even a stable landscape of mean times, 5881.3Schristos prior to the middle decades of the twentieth century". 5891.3Schristos See: Timothy Shenk, <a 5901.3Schristoshref="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: 5911.3Schristos A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. 5921.3Schristos </li> 5931.3Schristos <li> 5941.3Schristos Most of the pre-1970 data entries come from unreliable sources, often 5951.3Schristos astrology books that lack citations and whose compilers evidently 5961.3Schristos invented entries when the true facts were unknown, without 5971.3Schristos reporting which entries were known and which were invented. 5981.3Schristos These books often contradict each other or give implausible entries, 5991.3Schristos and on the rare occasions when they are checked they are 6001.3Schristos typically found to be incorrect. 6011.3Schristos </li> 6021.3Schristos <li> 6031.3Schristos For the UK the <code><abbr>tz</abbr></code> database relies on 6041.3Schristos years of first-class work done by 6051.3Schristos Joseph Myers and others; see 6061.3Schristos "<a href="https://www.polyomino.org.uk/british-time/">History of 6071.3Schristos legal time in Britain</a>". 6081.3Schristos Other countries are not done nearly as well. 6091.3Schristos </li> 6101.3Schristos <li> 6111.3Schristos Sometimes, different people in the same city maintain clocks 6121.3Schristos that differ significantly. 6131.3Schristos Historically, railway time was used by railroad companies (which 6141.3Schristos did not always 6151.3Schristos agree with each other), church-clock time was used for birth 6161.3Schristos certificates, etc. 6171.3Schristos More recently, competing political groups might disagree about 6181.3Schristos clock settings. Often this is merely common practice, but 6191.3Schristos sometimes it is set by law. 6201.3Schristos For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France 6211.3Schristos was legally <abbr>UT</abbr> +00:09:21 outside train stations and 6221.3Schristos <abbr>UT</abbr> +00:04:21 inside. Other examples include 6231.3Schristos Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and 6241.3Schristos Ürümqi to this day. 6251.3Schristos </li> 6261.3Schristos <li> 6271.3Schristos Although a named location in the <code><abbr>tz</abbr></code> 6281.3Schristos database stands for the containing region, its pre-1970 data 6291.3Schristos entries are often accurate for only a small subset of that region. 6301.3Schristos For example, <code>Europe/London</code> stands for the United 6311.3Schristos Kingdom, but its pre-1847 times are valid only for locations that 6321.3Schristos have London's exact meridian, and its 1847 transition 6331.3Schristos to <abbr>GMT</abbr> is known to be valid only for the L&NW and 6341.3Schristos the Caledonian railways. 6351.3Schristos </li> 6361.3Schristos <li> 6371.3Schristos The <code><abbr>tz</abbr></code> database does not record the 6381.4Schristos earliest time for which a timezone's 6391.3Schristos data entries are thereafter valid for every location in the region. 6401.3Schristos For example, <code>Europe/London</code> is valid for all locations 6411.3Schristos in its region after <abbr>GMT</abbr> was made the standard time, 6421.3Schristos but the date of standardization (1880-08-02) is not in the 6431.3Schristos <code><abbr>tz</abbr></code> database, other than in commentary. 6441.4Schristos For many timezones the earliest time of 6451.3Schristos validity is unknown. 6461.3Schristos </li> 6471.3Schristos <li> 6481.3Schristos The <code><abbr>tz</abbr></code> database does not record a 6491.3Schristos region's boundaries, and in many cases the boundaries are not known. 6501.4Schristos For example, the timezone 6511.3Schristos <code>America/Kentucky/Louisville</code> represents a region 6521.3Schristos around the city of Louisville, the boundaries of which are 6531.3Schristos unclear. 6541.3Schristos </li> 6551.3Schristos <li> 6561.3Schristos Changes that are modeled as instantaneous transitions in the 6571.3Schristos <code><abbr>tz</abbr></code> 6581.3Schristos database were often spread out over hours, days, or even decades. 6591.3Schristos </li> 6601.3Schristos <li> 6611.3Schristos Even if the time is specified by law, locations sometimes 6621.3Schristos deliberately flout the law. 6631.3Schristos </li> 6641.3Schristos <li> 6651.3Schristos Early timekeeping practices, even assuming perfect clocks, were 6661.3Schristos often not specified to the accuracy that the 6671.3Schristos <code><abbr>tz</abbr></code> database requires. 6681.3Schristos </li> 6691.3Schristos <li> 6701.3Schristos Sometimes historical timekeeping was specified more precisely 6711.3Schristos than what the <code><abbr>tz</abbr></code> code can handle. 6721.3Schristos For example, from 1909 to 1937 <a 6731.3Schristos href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" 6741.3Schristos hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean 6751.3Schristos Time (estimated to be <abbr>UT</abbr> 6761.3Schristos +00:19:32.13), but the <code><abbr>tz</abbr></code> 6771.3Schristos code cannot represent the fractional second. 6781.3Schristos In practice these old specifications were rarely if ever 6791.3Schristos implemented to subsecond precision. 6801.3Schristos </li> 6811.3Schristos <li> 6821.3Schristos Even when all the timestamp transitions recorded by the 6831.3Schristos <code><abbr>tz</abbr></code> database are correct, the 6841.3Schristos <code><abbr>tz</abbr></code> rules that generate them may not 6851.3Schristos faithfully reflect the historical rules. 6861.3Schristos For example, from 1922 until World War II the UK moved clocks 6871.3Schristos forward the day following the third Saturday in April unless that 6881.3Schristos was Easter, in which case it moved clocks forward the previous 6891.3Schristos Sunday. 6901.3Schristos Because the <code><abbr>tz</abbr></code> database has no 6911.3Schristos way to specify Easter, these exceptional years are entered as 6921.3Schristos separate <code><abbr>tz</abbr> Rule</code> lines, even though the 6931.3Schristos legal rules did not change. 6941.4Schristos When transitions are known but the historical rules behind them are not, 6951.4Schristos the database contains <code>Zone</code> and <code>Rule</code> 6961.4Schristos entries that are intended to represent only the generated 6971.4Schristos transitions, not any underlying historical rules; however, this 6981.4Schristos intent is recorded at best only in commentary. 6991.3Schristos </li> 7001.3Schristos <li> 7011.4Schristos The <code><abbr>tz</abbr></code> database models time 7021.3Schristos using the <a 7031.3Schristos href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic 7041.4Schristos Gregorian calendar</a> with days containing 24 equal-length hours 7051.4Schristos numbered 00 through 23, except when clock transitions occur. 7061.4Schristos Pre-standard time is modeled as local mean time. 7071.4Schristos However, historically many people used other calendars and other timescales. 7081.3Schristos For example, the Roman Empire used 7091.3Schristos the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian 7101.3Schristos calendar</a>, 7111.3Schristos and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman 7121.3Schristos timekeeping</a> had twelve varying-length daytime hours with a 7131.3Schristos non-hour-based system at night. 7141.4Schristos And even today, some local practices diverge from the Gregorian 7151.4Schristos calendar with 24-hour days. These divergences range from 7161.4Schristos relatively minor, such as Japanese bars giving times like "24:30" for the 7171.4Schristos wee hours of the morning, to more-significant differences such as <a 7181.4Schristos href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the 7191.4Schristos east African practice of starting the day at dawn</a>, renumbering 7201.4Schristos the Western 06:00 to be 12:00. These practices are largely outside 7211.4Schristos the scope of the <code><abbr>tz</abbr></code> code and data, which 7221.4Schristos provide only limited support for date and time localization 7231.4Schristos such as that required by POSIX. If DST is not used a different time zone 7241.4Schristos can often do the trick; for example, in Kenya a <code>TZ</code> setting 7251.4Schristos like <code><-03>3</code> or <code>America/Cayenne</code> starts 7261.4Schristos the day six hours later than <code>Africa/Nairobi</code> does. 7271.3Schristos </li> 7281.3Schristos <li> 7291.3Schristos Early clocks were less reliable, and data entries do not represent 7301.3Schristos clock error. 7311.3Schristos </li> 7321.3Schristos <li> 7331.3Schristos The <code><abbr>tz</abbr></code> database assumes Universal Time 7341.3Schristos (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not 7351.3Schristos standardized for older timestamps. 7361.3Schristos In the <code><abbr>tz</abbr></code> database commentary, 7371.3Schristos <abbr>UT</abbr> denotes a family of time standards that includes 7381.3Schristos Coordinated Universal Time (<abbr>UTC</abbr>) along with other 7391.3Schristos variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>, 7401.3Schristos with days starting at midnight. 7411.3Schristos Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern 7421.3Schristos timestamps, <abbr>UTC</abbr> was not defined until 1960, so 7431.3Schristos commentary uses the more-general abbreviation <abbr>UT</abbr> for 7441.3Schristos timestamps that might predate 1960. 7451.3Schristos Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly, 7461.3Schristos and since pre-1972 <abbr>UTC</abbr> seconds varied in length, 7471.3Schristos interpretation of older timestamps can be problematic when 7481.3Schristos subsecond accuracy is needed. 7491.3Schristos </li> 7501.3Schristos <li> 7511.3Schristos Civil time was not based on atomic time before 1972, and we do not 7521.3Schristos know the history of 7531.3Schristos <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's 7541.3Schristos rotation</a> accurately enough to map <a 7551.3Schristos href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr 7561.3Schristos title="International System of Units">SI</abbr></a> seconds to 7571.3Schristos historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a> 7581.3Schristos to more than about one-hour accuracy. 7591.3Schristos See: Stephenson FR, Morrison LV, Hohenkerk CY. 7601.4Schristos <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of 7611.3Schristos the Earth's rotation: 720 BC to AD 2015</a>. 7621.3Schristos <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. 7631.3Schristos Also see: Espenak F. <a 7641.3Schristos href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty 7651.3Schristos in Delta T (ΔT)</a>. 7661.3Schristos </li> 7671.3Schristos <li> 7681.3Schristos The relationship between POSIX time (that is, <abbr>UTC</abbr> but 7691.3Schristos ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap 7701.3Schristos seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972. 7711.3Schristos Although the POSIX 7721.3Schristos clock officially stops during an inserted leap second, at least one 7731.3Schristos proposed standard has it jumping back a second instead; and in 7741.3Schristos practice POSIX clocks more typically either progress glacially during 7751.3Schristos a leap second, or are slightly slowed while near a leap second. 7761.3Schristos </li> 7771.3Schristos <li> 7781.3Schristos The <code><abbr>tz</abbr></code> database does not represent how 7791.3Schristos uncertain its information is. 7801.3Schristos Ideally it would contain information about when data entries are 7811.3Schristos incomplete or dicey. 7821.3Schristos Partial temporal knowledge is a field of active research, though, 7831.3Schristos and it is not clear how to apply it here. 7841.1Schristos </li> 7851.1Schristos</ul> 7861.1Schristos 7871.1Schristos<p> 7881.3SchristosIn short, many, perhaps most, of the <code><abbr>tz</abbr></code> 7891.3Schristosdatabase's pre-1970 and future timestamps are either wrong or 7901.3Schristosmisleading. 7911.3SchristosAny attempt to pass the 7921.3Schristos<code><abbr>tz</abbr></code> database off as the definition of time 7931.3Schristosshould be unacceptable to anybody who cares about the facts. 7941.3SchristosIn particular, the <code><abbr>tz</abbr></code> database's 7951.3Schristos<abbr>LMT</abbr> offsets should not be considered meaningful, and 7961.4Schristosshould not prompt creation of timezones 7971.3Schristosmerely because two locations 7981.3Schristosdiffer in <abbr>LMT</abbr> or transitioned to standard time at 7991.3Schristosdifferent dates. 8001.3Schristos</p> 8011.3Schristos</section> 8021.3Schristos 8031.3Schristos<section> 8041.3Schristos <h2 id="functions">Time and date functions</h2> 8051.3Schristos<p> 8061.3SchristosThe <code><abbr>tz</abbr></code> code contains time and date functions 8071.3Schristosthat are upwards compatible with those of POSIX. 8081.3SchristosCode compatible with this package is already 8091.3Schristos<a href="tz-link.html#tzdb">part of many platforms</a>, where the 8101.3Schristosprimary use of this package is to update obsolete time-related files. 8111.3SchristosTo do this, you may need to compile the time zone compiler 8121.3Schristos'<code>zic</code>' supplied with this package instead of using the 8131.3Schristossystem '<code>zic</code>', since the format of <code>zic</code>'s 8141.3Schristosinput is occasionally extended, and a platform may still be shipping 8151.3Schristosan older <code>zic</code>. 8161.1Schristos</p> 8171.1Schristos 8181.3Schristos<h3 id="POSIX">POSIX properties and limitations</h3> 8191.1Schristos<ul> 8201.1Schristos <li> 8211.1Schristos <p> 8221.3Schristos In POSIX, time display in a process is controlled by the 8231.3Schristos environment variable <code>TZ</code>. 8241.3Schristos Unfortunately, the POSIX 8251.3Schristos <code>TZ</code> string takes a form that is hard to describe and 8261.3Schristos is error-prone in practice. 8271.3Schristos Also, POSIX <code>TZ</code> strings cannot deal with daylight 8281.3Schristos saving time rules not based on the Gregorian calendar (as in 8291.3Schristos Iran), or with situations where more than two time zone 8301.3Schristos abbreviations or <abbr>UT</abbr> offsets are used in an area. 8311.1Schristos </p> 8321.3Schristos 8331.1Schristos <p> 8341.3Schristos The POSIX <code>TZ</code> string takes the following form: 8351.1Schristos </p> 8361.3Schristos 8371.1Schristos <p> 8381.3Schristos <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>]]] 8391.1Schristos </p> 8401.3Schristos 8411.1Schristos <p> 8421.3Schristos where: 8431.3Schristos </p> 8441.3Schristos 8451.1Schristos <dl> 8461.1Schristos <dt><var>std</var> and <var>dst</var></dt><dd> 8471.3Schristos are 3 or more characters specifying the standard 8481.4Schristos and daylight saving time (<abbr>DST</abbr>) zone abbreviations. 8491.3Schristos Starting with POSIX.1-2001, <var>std</var> and <var>dst</var> 8501.3Schristos may also be in a quoted form like '<code><+09></code>'; 8511.3Schristos this allows "<code>+</code>" and "<code>-</code>" in the names. 8521.1Schristos </dd> 8531.1Schristos <dt><var>offset</var></dt><dd> 8541.3Schristos is of the form 8551.3Schristos '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' 8561.3Schristos and specifies the offset west of <abbr>UT</abbr>. 8571.3Schristos '<var>hh</var>' may be a single digit; 8581.3Schristos 0≤<var>hh</var>≤24. 8591.3Schristos The default <abbr>DST</abbr> offset is one hour ahead of 8601.3Schristos standard time. 8611.1Schristos </dd> 8621.1Schristos <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> 8631.3Schristos specifies the beginning and end of <abbr>DST</abbr>. 8641.3Schristos If this is absent, the system supplies its own ruleset 8651.3Schristos for <abbr>DST</abbr>, and its rules can differ from year to year; 8661.3Schristos typically <abbr>US</abbr> <abbr>DST</abbr> rules are used. 8671.1Schristos </dd> 8681.1Schristos <dt><var>time</var></dt><dd> 8691.3Schristos takes the form 8701.3Schristos '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' 8711.3Schristos and defaults to 02:00. 8721.3Schristos This is the same format as the offset, except that a 8731.3Schristos leading '<code>+</code>' or '<code>-</code>' is not allowed. 8741.1Schristos </dd> 8751.1Schristos <dt><var>date</var></dt><dd> 8761.3Schristos takes one of the following forms: 8771.1Schristos <dl> 8781.1Schristos <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> 8791.3Schristos origin-1 day number not counting February 29 8801.3Schristos </dd> 8811.1Schristos <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> 8821.3Schristos origin-0 day number counting February 29 if present 8831.3Schristos </dd> 8841.3Schristos <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> 8851.3Schristos (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 8861.3Schristos 1≤<var>m</var>≤12)</dt><dd> 8871.3Schristos for the <var>d</var>th day of week <var>n</var> of 8881.3Schristos month <var>m</var> of the year, where week 1 is the first 8891.3Schristos week in which day <var>d</var> appears, and 8901.3Schristos '<code>5</code>' stands for the last week in which 8911.3Schristos day <var>d</var> appears (which may be either the 4th or 8921.3Schristos 5th week). 8931.3Schristos Typically, this is the only useful form; the <var>n</var> 8941.3Schristos and <code>J</code><var>n</var> forms are rarely used. 8951.1Schristos </dd> 8961.3Schristos </dl> 8971.3Schristos </dd> 8981.3Schristos </dl> 8991.3Schristos 9001.3Schristos <p> 9011.3Schristos Here is an example POSIX <code>TZ</code> string for New 9021.3Schristos Zealand after 2007. 9031.3Schristos It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead 9041.3Schristos of <abbr>UT</abbr>, and that daylight saving time 9051.3Schristos (<abbr>NZDT</abbr>) is observed from September's last Sunday at 9061.3Schristos 02:00 until April's first Sunday at 03:00: 9071.3Schristos </p> 9081.3Schristos 9091.3Schristos <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> 9101.3Schristos 9111.3Schristos <p> 9121.3Schristos This POSIX <code>TZ</code> string is hard to remember, and 9131.3Schristos mishandles some timestamps before 2008. 9141.3Schristos With this package you can use this instead: 9151.3Schristos </p> 9161.3Schristos 9171.3Schristos <pre><code>TZ='Pacific/Auckland'</code></pre> 9181.3Schristos </li> 9191.3Schristos <li> 9201.4Schristos POSIX does not define the <abbr>DST</abbr> transitions 9211.4Schristos for <code>TZ</code> values like 9221.3Schristos "<code>EST5EDT</code>". 9231.4Schristos Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules 9241.4Schristos were used to interpret such values, but this meant that the 9251.4Schristos <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each 9261.4Schristos program that did time conversion. This meant that when 9271.4Schristos <abbr>US</abbr> time conversion rules changed (as in the United 9281.4Schristos States in 1987), all programs that did time conversion had to be 9291.3Schristos recompiled to ensure proper results. 9301.3Schristos </li> 9311.3Schristos <li> 9321.3Schristos The <code>TZ</code> environment variable is process-global, which 9331.3Schristos makes it hard to write efficient, thread-safe applications that 9341.4Schristos need access to multiple timezones. 9351.3Schristos </li> 9361.3Schristos <li> 9371.3Schristos In POSIX, there is no tamper-proof way for a process to learn the 9381.3Schristos system's best idea of local wall clock. 9391.4Schristos This is important for applications that an administrator wants 9401.3Schristos used only at certain times – without regard to whether the 9411.3Schristos user has fiddled the 9421.3Schristos <code>TZ</code> environment variable. 9431.3Schristos While an administrator can "do everything in <abbr>UT</abbr>" to 9441.3Schristos get around the problem, doing so is inconvenient and precludes 9451.4Schristos handling daylight saving time shifts – as might be required to 9461.4Schristos limit phone calls to off-peak hours. 9471.3Schristos </li> 9481.3Schristos <li> 9491.3Schristos POSIX provides no convenient and efficient way to determine 9501.3Schristos the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary 9511.4Schristos timestamps, particularly for timezones 9521.3Schristos that do not fit into the POSIX model. 9531.3Schristos </li> 9541.3Schristos <li> 9551.3Schristos POSIX requires that systems ignore leap seconds. 9561.3Schristos </li> 9571.3Schristos <li> 9581.3Schristos The <code><abbr>tz</abbr></code> code attempts to support all the 9591.3Schristos <code>time_t</code> implementations allowed by POSIX. 9601.3Schristos The <code>time_t</code> type represents a nonnegative count of seconds 9611.3Schristos since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds. 9621.3Schristos In practice, <code>time_t</code> is usually a signed 64- or 32-bit 9631.3Schristos integer; 32-bit signed <code>time_t</code> values stop working after 9641.3Schristos 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these 9651.3Schristos days typically use a signed 64-bit integer. 9661.3Schristos Unsigned 32-bit integers are used on one or two platforms, and 36-bit 9671.3Schristos and 40-bit integers are also used occasionally. 9681.3Schristos Although earlier POSIX versions allowed <code>time_t</code> to be a 9691.4Schristos floating-point type, this was not supported by any practical system, 9701.3Schristos and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both 9711.3Schristos require <code>time_t</code> to be an integer type. 9721.1Schristos </li> 9731.1Schristos</ul> 9741.3Schristos 9751.3Schristos<h3 id="POSIX-extensions">Extensions to POSIX in the 9761.3Schristos<code><abbr>tz</abbr></code> code</h3> 9771.1Schristos<ul> 9781.1Schristos <li> 9791.1Schristos <p> 9801.3Schristos The <code>TZ</code> environment variable is used in generating 9811.4Schristos the name of a file from which time-related information is read 9821.3Schristos (or is interpreted à la POSIX); <code>TZ</code> is no longer 9831.4Schristos constrained to be a string containing abbreviations 9841.4Schristos and numeric data as described <a href="#POSIX">above</a>. 9851.4Schristos The file's format is <dfn><abbr>TZif</abbr></dfn>, 9861.4Schristos a timezone information format that contains binary data. 9871.3Schristos The daylight saving time rules to be used for a 9881.4Schristos particular timezone are encoded in the 9891.4Schristos <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>, 9901.4Schristos Australian, and other rules to be encoded, and 9911.3Schristos allows for situations where more than two time zone 9921.3Schristos abbreviations are used. 9931.1Schristos </p> 9941.1Schristos <p> 9951.3Schristos It was recognized that allowing the <code>TZ</code> environment 9961.3Schristos variable to take on values such as '<code>America/New_York</code>' 9971.3Schristos might cause "old" programs (that expect <code>TZ</code> to have a 9981.3Schristos certain form) to operate incorrectly; consideration was given to using 9991.3Schristos some other environment variable (for example, <code>TIMEZONE</code>) 10001.4Schristos to hold the string used to generate the <abbr>TZif</abbr> file's name. 10011.3Schristos In the end, however, it was decided to continue using 10021.3Schristos <code>TZ</code>: it is widely used for time zone purposes; 10031.3Schristos separately maintaining both <code>TZ</code> 10041.3Schristos and <code>TIMEZONE</code> seemed a nuisance; and systems where 10051.3Schristos "new" forms of <code>TZ</code> might cause problems can simply 10061.4Schristos use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which 10071.4Schristos can be used by "new" programs as well as by "old" programs that 10081.4Schristos assume pre-POSIX <code>TZ</code> values. 10091.1Schristos </p> 10101.3Schristos </li> 10111.3Schristos <li> 10121.3Schristos The code supports platforms with a <abbr>UT</abbr> offset member 10131.3Schristos in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>. 10141.3Schristos </li> 10151.3Schristos <li> 10161.3Schristos The code supports platforms with a time zone abbreviation member in 10171.3Schristos <code>struct tm</code>, e.g., <code>tm_zone</code>. 10181.3Schristos </li> 10191.3Schristos <li> 10201.3Schristos Functions <code>tzalloc</code>, <code>tzfree</code>, 10211.3Schristos <code>localtime_rz</code>, and <code>mktime_z</code> for 10221.3Schristos more-efficient thread-safe applications that need to use multiple 10231.4Schristos timezones. 10241.3Schristos The <code>tzalloc</code> and <code>tzfree</code> functions 10251.3Schristos allocate and free objects of type <code>timezone_t</code>, 10261.3Schristos and <code>localtime_rz</code> and <code>mktime_z</code> are 10271.3Schristos like <code>localtime_r</code> and <code>mktime</code> with an 10281.3Schristos extra <code>timezone_t</code> argument. 10291.3Schristos The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>. 10301.3Schristos </li> 10311.3Schristos <li> 10321.3Schristos A function <code>tzsetwall</code> has been added to arrange for the 10331.3Schristos system's best approximation to local wall clock time to be delivered 10341.3Schristos by subsequent calls to <code>localtime</code>. 10351.3Schristos Source code for portable applications that "must" run on local wall 10361.3Schristos clock time should call <code>tzsetwall</code>; 10371.3Schristos if such code is moved to "old" systems that do not 10381.3Schristos provide <code>tzsetwall</code>, you will not be able to generate an 10391.3Schristos executable program. 10401.3Schristos (These functions also arrange for local wall clock time to 10411.3Schristos be used if <code>tzset</code> is called – directly or 10421.3Schristos indirectly – and there is no <code>TZ</code> environment 10431.3Schristos variable; portable applications should not, however, rely on this 10441.3Schristos behavior since it is not the way <a 10451.3Schristos href="https://en.wikipedia.org/wiki/UNIX_System_V#SVR2"><abbr>SVR2</abbr></a> 10461.3Schristos systems behave.) 10471.3Schristos </li> 10481.3Schristos <li> 10491.3Schristos Negative <code>time_t</code> values are supported, on systems 10501.3Schristos where <code>time_t</code> is signed. 10511.3Schristos </li> 10521.3Schristos <li> 10531.3Schristos These functions can account for leap seconds, thanks to Bradley White. 10541.3Schristos </li> 10551.1Schristos</ul> 10561.3Schristos 10571.3Schristos<h3 id="vestigial">POSIX features no longer needed</h3> 10581.1Schristos<p> 10591.3SchristosPOSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a> 10601.3Schristosdefine some <a href="https://en.wikipedia.org/wiki/API"><abbr 10611.3Schristostitle="application programming interface">API</abbr>s</a> that are vestigial: 10621.3Schristosthey are not needed, and are relics of a too-simple model that does 10631.3Schristosnot suffice to handle many real-world timestamps. 10641.3SchristosAlthough the <code><abbr>tz</abbr></code> code supports these 10651.3Schristosvestigial <abbr>API</abbr>s for backwards compatibility, they should 10661.3Schristosbe avoided in portable applications. 10671.3SchristosThe vestigial <abbr>API</abbr>s are: 10681.1Schristos</p> 10691.1Schristos<ul> 10701.1Schristos <li> 10711.3Schristos The POSIX <code>tzname</code> variable does not suffice and is no 10721.3Schristos longer needed. 10731.3Schristos To get a timestamp's time zone abbreviation, consult 10741.3Schristos the <code>tm_zone</code> member if available; otherwise, 10751.3Schristos use <code>strftime</code>'s <code>"%Z"</code> conversion 10761.3Schristos specification. 10771.3Schristos </li> 10781.3Schristos <li> 10791.3Schristos The POSIX <code>daylight</code> and <code>timezone</code> 10801.3Schristos variables do not suffice and are no longer needed. 10811.3Schristos To get a timestamp's <abbr>UT</abbr> offset, consult 10821.3Schristos the <code>tm_gmtoff</code> member if available; otherwise, 10831.3Schristos subtract values returned by <code>localtime</code> 10841.3Schristos and <code>gmtime</code> using the rules of the Gregorian calendar, 10851.3Schristos or use <code>strftime</code>'s <code>"%z"</code> conversion 10861.3Schristos specification if a string like <code>"+0900"</code> suffices. 10871.3Schristos </li> 10881.3Schristos <li> 10891.3Schristos The <code>tm_isdst</code> member is almost never needed and most of 10901.3Schristos its uses should be discouraged in favor of the abovementioned 10911.3Schristos <abbr>API</abbr>s. 10921.3Schristos Although it can still be used in arguments to 10931.3Schristos <code>mktime</code> to disambiguate timestamps near 10941.3Schristos a <abbr>DST</abbr> transition when the clock jumps back, this 10951.3Schristos disambiguation does not work when standard time itself jumps back, 10961.3Schristos which can occur when a location changes to a time zone with a 10971.3Schristos lesser <abbr>UT</abbr> offset. 10981.3Schristos </li> 10991.3Schristos</ul> 11001.3Schristos 11011.3Schristos<h3 id="other-portability">Other portability notes</h3> 11021.3Schristos<ul> 11031.3Schristos <li> 11041.3Schristos The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition 11051.3Schristos UNIX</a> <code>timezone</code> function is not present in this 11061.3Schristos package; it is impossible to reliably map <code>timezone</code>'s 11071.3Schristos arguments (a "minutes west of <abbr>GMT</abbr>" value and a 11081.3Schristos "daylight saving time in effect" flag) to a time zone 11091.3Schristos abbreviation, and we refuse to guess. 11101.3Schristos Programs that in the past used the <code>timezone</code> function 11111.3Schristos may now examine <code>localtime(&clock)->tm_zone</code> 11121.3Schristos (if <code>TM_ZONE</code> is defined) or 11131.3Schristos <code>tzname[localtime(&clock)->tm_isdst]</code> 11141.3Schristos (if <code>HAVE_TZNAME</code> is defined) to learn the correct time 11151.3Schristos zone abbreviation to use. 11161.3Schristos </li> 11171.3Schristos <li> 11181.3Schristos The <a 11191.3Schristos href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a> 11201.3Schristos <code>gettimeofday</code> function is not 11211.3Schristos used in this package. 11221.3Schristos This formerly let users obtain the current <abbr>UTC</abbr> offset 11231.3Schristos and <abbr>DST</abbr> flag, but this functionality was removed in 11241.3Schristos later versions of <abbr>BSD</abbr>. 11251.3Schristos </li> 11261.3Schristos <li> 11271.3Schristos In <abbr>SVR2</abbr>, time conversion fails for near-minimum or 11281.3Schristos near-maximum <code>time_t</code> values when doing conversions 11291.3Schristos for places that do not use <abbr>UT</abbr>. 11301.3Schristos This package takes care to do these conversions correctly. 11311.3Schristos A comment in the source code tells how to get compatibly wrong 11321.3Schristos results. 11331.3Schristos </li> 11341.3Schristos <li> 11351.3Schristos The functions that are conditionally compiled 11361.3Schristos if <code>STD_INSPIRED</code> is defined should, at this point, be 11371.3Schristos looked on primarily as food for thought. 11381.3Schristos They are not in any sense "standard compatible" – some are 11391.3Schristos not, in fact, specified in <em>any</em> standard. 11401.3Schristos They do, however, represent responses of various authors to 11411.3Schristos standardization proposals. 11421.3Schristos </li> 11431.3Schristos <li> 11441.4Schristos Other time conversion proposals, in particular those supported by the 11451.4Schristos <a href="https://howardhinnant.github.io/date/tz.html">Time Zone 11461.4Schristos Database Parser</a>, offer a wider selection of functions 11471.3Schristos that provide capabilities beyond those provided here. 11481.3Schristos The absence of such functions from this package is not meant to 11491.3Schristos discourage the development, standardization, or use of such 11501.3Schristos functions. 11511.3Schristos Rather, their absence reflects the decision to make this package 11521.3Schristos contain valid extensions to POSIX, to ensure its broad 11531.3Schristos acceptability. 11541.3Schristos If more powerful time conversion functions can be standardized, so 11551.3Schristos much the better. 11561.1Schristos </li> 11571.1Schristos</ul> 11581.3Schristos</section> 11591.1Schristos 11601.3Schristos<section> 11611.3Schristos <h2 id="stability">Interface stability</h2> 11621.1Schristos<p> 11631.3SchristosThe <code><abbr>tz</abbr></code> code and data supply the following interfaces: 11641.1Schristos</p> 11651.1Schristos 11661.1Schristos<ul> 11671.1Schristos <li> 11681.4Schristos A set of timezone names as per 11691.4Schristos "<a href="#naming">Names of timezones</a>" above. 11701.1Schristos </li> 11711.1Schristos <li> 11721.3Schristos Library functions described in "<a href="#functions">Time and date 11731.3Schristos functions</a>" above. 11741.1Schristos </li> 11751.1Schristos <li> 11761.3Schristos The programs <code>tzselect</code>, <code>zdump</code>, 11771.3Schristos and <code>zic</code>, documented in their man pages. 11781.1Schristos </li> 11791.1Schristos <li> 11801.3Schristos The format of <code>zic</code> input files, documented in 11811.3Schristos the <code>zic</code> man page. 11821.1Schristos </li> 11831.1Schristos <li> 11841.3Schristos The format of <code>zic</code> output files, documented in 11851.3Schristos the <code>tzfile</code> man page. 11861.1Schristos </li> 11871.1Schristos <li> 11881.3Schristos The format of zone table files, documented in <code>zone1970.tab</code>. 11891.1Schristos </li> 11901.1Schristos <li> 11911.3Schristos The format of the country code file, documented in <code>iso3166.tab</code>. 11921.1Schristos </li> 11931.1Schristos <li> 11941.3Schristos The version number of the code and data, as the first line of 11951.3Schristos the text file '<code>version</code>' in each release. 11961.1Schristos </li> 11971.1Schristos</ul> 11981.3Schristos 11991.1Schristos<p> 12001.1SchristosInterface changes in a release attempt to preserve compatibility with 12011.3Schristosrecent releases. 12021.3SchristosFor example, <code><abbr>tz</abbr></code> data files typically do not 12031.3Schristosrely on recently-added <code>zic</code> features, so that users can 12041.3Schristosrun older <code>zic</code> versions to process newer data files. 12051.3Schristos<a href="tz-link.html#download">Downloading 12061.3Schristosthe <code><abbr>tz</abbr></code> database</a> describes how releases 12071.3Schristosare tagged and distributed. 12081.1Schristos</p> 12091.1Schristos 12101.1Schristos<p> 12111.3SchristosInterfaces not listed above are less stable. 12121.3SchristosFor example, users should not rely on particular <abbr>UT</abbr> 12131.3Schristosoffsets or abbreviations for timestamps, as data entries are often 12141.3Schristosbased on guesswork and these guesses may be corrected or improved. 12151.1Schristos</p> 12161.3Schristos</section> 12171.1Schristos 12181.3Schristos<section> 12191.3Schristos <h2 id="calendar">Calendrical issues</h2> 12201.1Schristos<p> 12211.1SchristosCalendrical issues are a bit out of scope for a time zone database, 12221.1Schristosbut they indicate the sort of problems that we would run into if we 12231.3Schristosextended the time zone database further into the past. 12241.3SchristosAn excellent resource in this area is Edward M. Reingold 12251.3Schristosand Nachum Dershowitz, <cite><a 12261.3Schristoshref="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical 12271.3SchristosCalculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018). 12281.3SchristosOther information and sources are given in the file '<code>calendars</code>' 12291.3Schristosin the <code><abbr>tz</abbr></code> distribution. 12301.3SchristosThey sometimes disagree. 12311.3Schristos</p> 12321.3Schristos</section> 12331.3Schristos 12341.3Schristos<section> 12351.3Schristos <h2 id="planets">Time and time zones on other planets</h2> 12361.3Schristos<p> 12371.3SchristosSome people's work schedules 12381.4Schristosuse <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>. 12391.3SchristosJet Propulsion Laboratory (JPL) coordinators kept Mars time on 12401.3Schristosand off during the 12411.3Schristos<a href="https://en.wikipedia.org/wiki/Mars_Pathfinder#End_of_mission">Mars 12421.3SchristosPathfinder</a> mission. 12431.3SchristosSome of their family members also adapted to Mars time. 12441.3SchristosDozens of special Mars watches were built for JPL workers who kept 12451.3SchristosMars time during the Mars Exploration Rovers mission (2004). 12461.3SchristosThese timepieces look like normal Seikos and Citizens but use Mars 12471.3Schristosseconds rather than terrestrial seconds. 12481.1Schristos</p> 12491.1Schristos 12501.1Schristos<p> 12511.1SchristosA Mars solar day is called a "sol" and has a mean period equal to 12521.3Schristosabout 24 hours 39 minutes 35.244 seconds in terrestrial time. 12531.3SchristosIt is divided into a conventional 24-hour clock, so each Mars second 12541.3Schristosequals about 1.02749125 terrestrial seconds. 12551.1Schristos</p> 12561.1Schristos 12571.1Schristos<p> 12581.3SchristosThe <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime 12591.3Schristosmeridian</a> of Mars goes through the center of the crater 12601.3Schristos<a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in 12611.3Schristoshonor of the British astronomer who built the Greenwich telescope that 12621.3Schristosdefines Earth's prime meridian. 12631.3SchristosMean solar time on the Mars prime meridian is 12641.3Schristoscalled <a href="https://en.wikipedia.org/wiki/Mars_Coordinated_Time">Mars 12651.3SchristosCoordinated Time (<abbr>MTC</abbr>)</a>. 12661.1Schristos</p> 12671.1Schristos 12681.1Schristos<p> 12691.1SchristosEach landed mission on Mars has adopted a different reference for 12701.4Schristossolar timekeeping, so there is no real standard for Mars time zones. 12711.3SchristosFor example, the 12721.3Schristos<a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars 12731.3SchristosExploration Rover</a> project (2004) defined two time zones "Local 12741.3SchristosSolar Time A" and "Local Solar Time B" for its two missions, each zone 12751.3Schristosdesigned so that its time equals local true solar time at 12761.3Schristosapproximately the middle of the nominal mission. 12771.3SchristosSuch a "time zone" is not particularly suited for any application 12781.3Schristosother than the mission itself. 12791.1Schristos</p> 12801.1Schristos 12811.1Schristos<p> 12821.1SchristosMany calendars have been proposed for Mars, but none have achieved 12831.3Schristoswide acceptance. 12841.3SchristosAstronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a 12851.1Schristossequential count of Mars solar days elapsed since about 1873-12-29 12861.3Schristos12:00 <abbr>GMT</abbr>. 12871.1Schristos</p> 12881.1Schristos 12891.1Schristos<p> 12901.1SchristosIn our solar system, Mars is the planet with time and calendar most 12911.3Schristoslike Earth's. 12921.3SchristosOn other planets, Sun-based time and calendars would work quite 12931.3Schristosdifferently. 12941.3SchristosFor example, although Mercury's 12951.3Schristos<a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal 12961.3Schristosrotation period</a> is 58.646 Earth days, Mercury revolves around the 12971.3SchristosSun so rapidly that an observer on Mercury's equator would see a 12981.3Schristossunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a 12991.3SchristosMercury day. 13001.3SchristosVenus is more complicated, partly because its rotation is slightly 13011.3Schristos<a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>: 13021.3Schristosits year is 1.92 of its days. 13031.3SchristosGas giants like Jupiter are trickier still, as their polar and 13041.3Schristosequatorial regions rotate at different rates, so that the length of a 13051.3Schristosday depends on latitude. 13061.3SchristosThis effect is most pronounced on Neptune, where the day is about 12 13071.3Schristoshours at the poles and 18 hours at the equator. 13081.1Schristos</p> 13091.1Schristos 13101.1Schristos<p> 13111.3SchristosAlthough the <code><abbr>tz</abbr></code> database does not support 13121.3Schristostime on other planets, it is documented here in the hopes that support 13131.3Schristoswill be added eventually. 13141.1Schristos</p> 13151.1Schristos 13161.1Schristos<p> 13171.3SchristosSources for time on other planets: 13181.1Schristos</p> 13191.3Schristos 13201.1Schristos<ul> 13211.1Schristos <li> 13221.3Schristos Michael Allison and Robert Schmunk, 13231.3Schristos "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical 13241.3Schristos Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" 13251.3Schristos (2015-06-30). 13261.1Schristos </li> 13271.1Schristos <li> 13281.3Schristos Jia-Rui Chong, 13291.3Schristos "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays 13301.3Schristos Fit for a Martian</a>", <cite>Los Angeles Times</cite> 13311.3Schristos (2004-01-14), pp A1, A20–A21. 13321.1Schristos </li> 13331.1Schristos <li> 13341.3Schristos Tom Chmielewski, 13351.3Schristos "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet 13361.3Schristos Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26) 13371.1Schristos </li> 13381.1Schristos <li> 13391.3Schristos Matt Williams, 13401.3Schristos "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How 13411.3Schristos long is a day on the other planets of the solar system?</a>" 13421.4Schristos (2016-01-20). 13431.1Schristos </li> 13441.1Schristos</ul> 13451.3Schristos</section> 13461.1Schristos 13471.3Schristos<footer> 13481.3Schristos <hr> 13491.3Schristos This file is in the public domain, so clarified as of 2009-05-17 by 13501.3Schristos Arthur David Olson. 13511.3Schristos</footer> 13521.1Schristos</body> 13531.1Schristos</html> 1354