theory.html revision 1.3
11.1Schristos<html lang="en"> 21.1Schristos<head> 31.1Schristos <title>Theory and pragmatics of the tz code and data</title> 41.1Schristos <meta charset="UTF-8"> 51.1Schristos</head> 61.1Schristos 71.1Schristos<body> 81.3Schristos<h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1> 91.1Schristos <h3>Outline</h3> 101.1Schristos <nav> 111.1Schristos <ul> 121.3Schristos <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code> 131.3Schristos database</a></li> 141.3Schristos <li><a href="#naming">Names of time zone rulesets</a></li> 151.1Schristos <li><a href="#abbreviations">Time zone abbreviations</a></li> 161.3Schristos <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code> 171.3Schristos database</a></li> 181.1Schristos <li><a href="#functions">Time and date functions</a></li> 191.1Schristos <li><a href="#stability">Interface stability</a></li> 201.1Schristos <li><a href="#calendar">Calendrical issues</a></li> 211.1Schristos <li><a href="#planets">Time and time zones on other planets</a></li> 221.1Schristos </ul> 231.1Schristos </nav> 241.1Schristos 251.3Schristos<section> 261.3Schristos <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2> 271.1Schristos<p> 281.3SchristosThe <a 291.3Schristoshref="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code> 301.3Schristosdatabase</a> attempts to record the history and predicted future of 311.3Schristosall computer-based clocks that track civil time. 321.3SchristosIt organizes <a href="tz-link.html">time zone and daylight saving time 331.3Schristosdata</a> by partitioning the world into <a 341.3Schristoshref="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones">regions</a> 351.3Schristoswhose clocks all agree about timestamps that occur after the <a 361.3Schristoshref="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> 371.3Schristos(1970-01-01 00:00:00 <a 381.3Schristoshref="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr 391.3Schristostitle="Coordinated Universal Time">UTC</abbr></a>). 401.3SchristosThe database labels each such region with a notable location and 411.3Schristosrecords all known clock transitions for that location. 421.3SchristosAlthough 1970 is a somewhat-arbitrary cutoff, there are significant 431.3Schristoschallenges to moving the cutoff earlier even by a decade or two, due 441.3Schristosto the wide variety of local practices before computer timekeeping 451.3Schristosbecame prevalent. 461.1Schristos</p> 471.1Schristos 481.1Schristos<p> 491.1SchristosClock transitions before 1970 are recorded for each such location, 501.1Schristosbecause most systems support timestamps before 1970 and could 511.1Schristosmisbehave if data entries were omitted for pre-1970 transitions. 521.1SchristosHowever, the database is not designed for and does not suffice for 531.1Schristosapplications requiring accurate handling of all past times everywhere, 541.1Schristosas it would take far too much effort and guesswork to record all 551.1Schristosdetails of pre-1970 civil timekeeping. 561.3SchristosAlthough some information outside the scope of the database is 571.2Schristoscollected in a file <code>backzone</code> that is distributed along 581.2Schristoswith the database proper, this file is less reliable and does not 591.2Schristosnecessarily follow database guidelines. 601.1Schristos</p> 611.1Schristos 621.1Schristos<p> 631.3SchristosAs described below, reference source code for using the 641.3Schristos<code><abbr>tz</abbr></code> database is also available. 651.3SchristosThe <code><abbr>tz</abbr></code> code is upwards compatible with <a 661.3Schristoshref="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international 671.3Schristosstandard for <a 681.3Schristoshref="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems. 691.3SchristosAs of this writing, the current edition of POSIX is: <a 701.3Schristoshref="http://pubs.opengroup.org/onlinepubs/9699919799/"> The Open 711.3SchristosGroup Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018 721.3SchristosEdition. 731.3SchristosBecause the database's scope encompasses real-world changes to civil 741.3Schristostimekeeping, its model for describing time is more complex than the 751.3Schristosstandard and daylight saving times supported by POSIX. 761.3SchristosA <code><abbr>tz</abbr></code> region corresponds to a ruleset that can 771.3Schristoshave more than two changes per year, these changes need not merely 781.3Schristosflip back and forth between two alternatives, and the rules themselves 791.3Schristoscan change at times. 801.3SchristosWhether and when a <code><abbr>tz</abbr></code> region changes its 811.3Schristosclock, and even the region's notional base offset from UTC, are variable. 821.3SchristosIt does not always make sense to talk about a region's 831.3Schristos"base offset", since it is not necessarily a single number. 841.1Schristos</p> 851.1Schristos 861.3Schristos</section> 871.1Schristos 881.3Schristos<section> 891.3Schristos <h2 id="naming">Names of time zone rulesets</h2> 901.1Schristos<p> 911.3SchristosEach <code><abbr>tz</abbr></code> region has a unique name that 921.3Schristoscorresponds to a set of time zone rules. 931.1SchristosInexperienced users are not expected to select these names unaided. 941.1SchristosDistributors should provide documentation and/or a simple selection 951.3Schristosinterface that explains the names; for one example, see the 961.3Schristos<code>tzselect</code> program in the <code><abbr>tz</abbr></code> code. 971.3SchristosThe <a href="http://cldr.unicode.org/">Unicode Common Locale Data 981.3SchristosRepository</a> contains data that may be useful for other selection 991.3Schristosinterfaces. 1001.1Schristos</p> 1011.1Schristos 1021.1Schristos<p> 1031.3SchristosThe naming conventions attempt to strike a balance 1041.1Schristosamong the following goals: 1051.1Schristos</p> 1061.3Schristos 1071.1Schristos<ul> 1081.1Schristos <li> 1091.3Schristos Uniquely identify every region where clocks have agreed since 1970. 1101.3Schristos This is essential for the intended use: static clocks keeping local 1111.3Schristos civil time. 1121.1Schristos </li> 1131.1Schristos <li> 1141.3Schristos Indicate to experts where that region is. 1151.1Schristos </li> 1161.1Schristos <li> 1171.3Schristos Be robust in the presence of political changes. 1181.3Schristos For example, names of countries are ordinarily not used, to avoid 1191.3Schristos incompatibilities when countries change their name (e.g., 1201.3Schristos Zaire→Congo) or when locations change countries (e.g., Hong 1211.3Schristos Kong from UK colony to China). 1221.1Schristos </li> 1231.1Schristos <li> 1241.3Schristos Be portable to a wide variety of implementations. 1251.1Schristos </li> 1261.1Schristos <li> 1271.3Schristos Use a consistent naming conventions over the entire world. 1281.1Schristos </li> 1291.1Schristos</ul> 1301.3Schristos 1311.1Schristos<p> 1321.3SchristosNames normally have the form 1331.3Schristos<var>AREA</var><code>/</code><var>LOCATION</var>, where 1341.3Schristos<var>AREA</var> is the name of a continent or ocean, and 1351.3Schristos<var>LOCATION</var> is the name of a specific location within that 1361.3Schristosregion. 1371.3SchristosNorth and South America share the same area, '<code>America</code>'. 1381.3SchristosTypical names are '<code>Africa/Cairo</code>', 1391.3Schristos'<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'. 1401.3SchristosSome names are further qualified to help avoid confusion; for example, 1411.3Schristos'<code>America/Indiana/Petersburg</code>' distinguishes Petersburg, 1421.3SchristosIndiana from other Petersburgs in America. 1431.1Schristos</p> 1441.1Schristos 1451.1Schristos<p> 1461.3SchristosHere are the general guidelines used for 1471.3Schristoschoosing <code><abbr>tz</abbr></code> region names, 1481.1Schristosin decreasing order of importance: 1491.1Schristos</p> 1501.3Schristos 1511.1Schristos<ul> 1521.1Schristos <li> 1531.3Schristos Use only valid POSIX file name components (i.e., the parts of 1541.3Schristos names other than '<code>/</code>'). 1551.3Schristos Do not use the file name components '<code>.</code>' and 1561.3Schristos '<code>..</code>'. 1571.3Schristos Within a file name component, use only <a 1581.3Schristos href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, 1591.3Schristos '<code>.</code>', '<code>-</code>' and '<code>_</code>'. 1601.3Schristos Do not use digits, as that might create an ambiguity with <a 1611.3Schristos href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX 1621.3Schristos <code>TZ</code> strings</a>. 1631.3Schristos A file name component must not exceed 14 characters or start with 1641.3Schristos '<code>-</code>'. 1651.3Schristos E.g., prefer <code>Asia/Brunei</code> to 1661.3Schristos <code>Asia/Bandar_Seri_Begawan</code>. 1671.3Schristos Exceptions: see the discussion of legacy names below. 1681.1Schristos </li> 1691.1Schristos <li> 1701.3Schristos A name must not be empty, or contain '<code>//</code>', or 1711.3Schristos start or end with '<code>/</code>'. 1721.1Schristos </li> 1731.1Schristos <li> 1741.3Schristos Do not use names that differ only in case. 1751.3Schristos Although the reference implementation is case-sensitive, some 1761.3Schristos other implementations are not, and they would mishandle names 1771.3Schristos differing only in case. 1781.1Schristos </li> 1791.1Schristos <li> 1801.3Schristos If one name <var>A</var> is an initial prefix of another 1811.3Schristos name <var>AB</var> (ignoring case), then <var>B</var> must not 1821.3Schristos start with '<code>/</code>', as a regular file cannot have the 1831.3Schristos same name as a directory in POSIX. 1841.3Schristos For example, <code>America/New_York</code> precludes 1851.3Schristos <code>America/New_York/Bronx</code>. 1861.1Schristos </li> 1871.1Schristos <li> 1881.3Schristos Uninhabited regions like the North Pole and Bouvet Island 1891.3Schristos do not need locations, since local time is not defined there. 1901.1Schristos </li> 1911.1Schristos <li> 1921.3Schristos There should typically be at least one name for each <a 1931.3Schristos href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr 1941.3Schristos title="International Organization for Standardization">ISO</abbr> 1951.3Schristos 3166-1</a> officially assigned two-letter code for an inhabited 1961.3Schristos country or territory. 1971.1Schristos </li> 1981.1Schristos <li> 1991.3Schristos If all the clocks in a region have agreed since 1970, 2001.3Schristos do not bother to include more than one location 2011.3Schristos even if subregions' clocks disagreed before 1970. 2021.3Schristos Otherwise these tables would become annoyingly large. 2031.1Schristos </li> 2041.1Schristos <li> 2051.3Schristos If a name is ambiguous, use a less ambiguous alternative; 2061.3Schristos e.g., many cities are named San José and Georgetown, so 2071.3Schristos prefer <code>America/Costa_Rica</code> to 2081.3Schristos <code>America/San_Jose</code> and <code>America/Guyana</code> 2091.3Schristos to <code>America/Georgetown</code>. 2101.1Schristos </li> 2111.1Schristos <li> 2121.3Schristos Keep locations compact. 2131.3Schristos Use cities or small islands, not countries or regions, so that any 2141.3Schristos future changes do not split individual locations into different 2151.3Schristos <code><abbr>tz</abbr></code> regions. 2161.3Schristos E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>, 2171.3Schristos since 2181.3Schristos <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France 2191.3Schristos has had multiple time zones</a>. 2201.1Schristos </li> 2211.1Schristos <li> 2221.3Schristos Use mainstream English spelling, e.g., prefer 2231.3Schristos <code>Europe/Rome</code> to <code>Europe/Roma</code>, and 2241.3Schristos prefer <code>Europe/Athens</code> to the Greek 2251.3Schristos <code>Europe/Αθήνα</code> or the Romanized 2261.3Schristos <code>Europe/Athína</code>. 2271.3Schristos The POSIX file name restrictions encourage this guideline. 2281.1Schristos </li> 2291.1Schristos <li> 2301.3Schristos Use the most populous among locations in a region, 2311.3Schristos e.g., prefer <code>Asia/Shanghai</code> to 2321.3Schristos <code>Asia/Beijing</code>. 2331.3Schristos Among locations with similar populations, pick the best-known 2341.3Schristos location, e.g., prefer <code>Europe/Rome</code> to 2351.3Schristos <code>Europe/Milan</code>. 2361.1Schristos </li> 2371.1Schristos <li> 2381.3Schristos Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to 2391.3Schristos <code>Atlantic/Canaries</code>. 2401.1Schristos </li> 2411.1Schristos <li> 2421.3Schristos Omit common suffixes like '<code>_Islands</code>' and 2431.3Schristos '<code>_City</code>', unless that would lead to ambiguity. 2441.3Schristos E.g., prefer <code>America/Cayman</code> to 2451.3Schristos <code>America/Cayman_Islands</code> and 2461.3Schristos <code>America/Guatemala</code> to 2471.3Schristos <code>America/Guatemala_City</code>, but prefer 2481.3Schristos <code>America/Mexico_City</code> to 2491.3Schristos <code>America/Mexico</code> 2501.3Schristos because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the 2511.3Schristos country of Mexico has several time zones</a>. 2521.1Schristos </li> 2531.1Schristos <li> 2541.3Schristos Use '<code>_</code>' to represent a space. 2551.1Schristos </li> 2561.1Schristos <li> 2571.3Schristos Omit '<code>.</code>' from abbreviations in names. 2581.3Schristos E.g., prefer <code>Atlantic/St_Helena</code> to 2591.3Schristos <code>Atlantic/St._Helena</code>. 2601.1Schristos </li> 2611.1Schristos <li> 2621.3Schristos Do not change established names if they only marginally violate 2631.3Schristos the above guidelines. 2641.3Schristos For example, do not change the existing name <code>Europe/Rome</code> to 2651.3Schristos <code>Europe/Milan</code> merely because Milan's population has grown 2661.3Schristos to be somewhat greater than Rome's. 2671.1Schristos </li> 2681.1Schristos <li> 2691.3Schristos If a name is changed, put its old spelling in the 2701.3Schristos '<code>backward</code>' file. 2711.3Schristos This means old spellings will continue to work. 2721.1Schristos </li> 2731.1Schristos</ul> 2741.1Schristos 2751.1Schristos<p> 2761.1SchristosThe file '<code>zone1970.tab</code>' lists geographical locations used 2771.3Schristosto name <code><abbr>tz</abbr></code> regions. 2781.3SchristosIt is intended to be an exhaustive list of names for geographic 2791.3Schristosregions as described above; this is a subset of the names in the data. 2801.3SchristosAlthough a '<code>zone1970.tab</code>' location's 2811.3Schristos<a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> 2821.3Schristoscorresponds to 2831.3Schristosits <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean 2841.3Schristostime (<abbr>LMT</abbr>)</a> offset with one hour for every 15° 2851.3Schristoseast longitude, this relationship is not exact. 2861.1Schristos</p> 2871.1Schristos 2881.1Schristos<p> 2891.1SchristosOlder versions of this package used a different naming scheme, 2901.1Schristosand these older names are still supported. 2911.1SchristosSee the file '<code>backward</code>' for most of these older names 2921.1Schristos(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). 2931.1SchristosThe other old-fashioned names still supported are 2941.3Schristos'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and 2951.3Schristos'<code>EET</code>' (see the file '<code>europe</code>'). 2961.1Schristos</p> 2971.1Schristos 2981.1Schristos<p> 2991.1SchristosOlder versions of this package defined legacy names that are 3001.3Schristosincompatible with the first guideline of location names, but which are 3011.3Schristosstill supported. 3021.3SchristosThese legacy names are mostly defined in the file 3031.3Schristos'<code>etcetera</code>'. 3041.3SchristosAlso, the file '<code>backward</code>' defines the legacy names 3051.3Schristos'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', 3061.3Schristosand the file '<code>northamerica</code>' defines the legacy names 3071.3Schristos'<code>EST5EDT</code>', '<code>CST6CDT</code>', 3081.3Schristos'<code>MST7MDT</code>', and '<code>PST8PDT</code>'. 3091.1Schristos</p> 3101.1Schristos 3111.1Schristos<p> 3121.3SchristosExcluding '<code>backward</code>' should not affect the other data. 3131.3SchristosIf '<code>backward</code>' is excluded, excluding 3141.3Schristos'<code>etcetera</code>' should not affect the remaining data. 3151.1Schristos</p> 3161.3Schristos</section> 3171.1Schristos 3181.3Schristos<section> 3191.3Schristos <h2 id="abbreviations">Time zone abbreviations</h2> 3201.1Schristos<p> 3211.1SchristosWhen this package is installed, it generates time zone abbreviations 3221.1Schristoslike '<code>EST</code>' to be compatible with human tradition and POSIX. 3231.3SchristosHere are the general guidelines used for choosing time zone abbreviations, 3241.1Schristosin decreasing order of importance: 3251.3Schristos</p> 3261.3Schristos 3271.1Schristos<ul> 3281.1Schristos <li> 3291.3Schristos Use three to six characters that are ASCII alphanumerics or 3301.3Schristos '<code>+</code>' or '<code>-</code>'. 3311.3Schristos Previous editions of this database also used characters like 3321.3Schristos space and '<code>?</code>', but these characters have a 3331.3Schristos special meaning to the 3341.3Schristos <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a> 3351.3Schristos and cause commands like 3361.3Schristos '<code><a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a> 3371.3Schristos `<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>' 3381.3Schristos to have unexpected effects. 3391.3Schristos Previous editions of this guideline required upper-case letters, but the 3401.3Schristos Congressman who introduced 3411.3Schristos <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro 3421.3Schristos Standard Time</a> preferred "ChST", so lower-case letters are now 3431.3Schristos allowed. 3441.3Schristos Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>', 3451.3Schristos '<code>+</code>', and alphanumeric characters from the portable 3461.3Schristos character set in the current locale. 3471.3Schristos In practice ASCII alphanumerics and '<code>+</code>' and 3481.3Schristos '<code>-</code>' are safe in all locales. 3491.3Schristos 3501.3Schristos <p> 3511.3Schristos In other words, in the C locale the POSIX extended regular 3521.3Schristos expression <code>[-+[:alnum:]]{3,6}</code> should match the 3531.3Schristos abbreviation. 3541.3Schristos This guarantees that all abbreviations could have been specified by a 3551.3Schristos POSIX <code>TZ</code> string. 3561.3Schristos </p> 3571.3Schristos </li> 3581.3Schristos <li> 3591.3Schristos Use abbreviations that are in common use among English-speakers, 3601.3Schristos e.g., 'EST' for Eastern Standard Time in North America. 3611.3Schristos We assume that applications translate them to other languages 3621.3Schristos as part of the normal localization process; for example, 3631.3Schristos a French application might translate 'EST' to 'HNE'. 3641.3Schristos 3651.3Schristos <p> 3661.3Schristos <small>These abbreviations (for standard/daylight/etc. time) are: 3671.3Schristos ACST/ACDT Australian Central, 3681.3Schristos AST/ADT/APT/AWT/ADDT Atlantic, 3691.3Schristos AEST/AEDT Australian Eastern, 3701.3Schristos AHST/AHDT Alaska-Hawaii, 3711.3Schristos AKST/AKDT Alaska, 3721.3Schristos AWST/AWDT Australian Western, 3731.3Schristos BST/BDT Bering, 3741.3Schristos CAT/CAST Central Africa, 3751.3Schristos CET/CEST/CEMT Central European, 3761.3Schristos ChST Chamorro, 3771.3Schristos CST/CDT/CWT/CPT/CDDT Central [North America], 3781.3Schristos CST/CDT China, 3791.3Schristos GMT/BST/IST/BDST Greenwich, 3801.3Schristos EAT East Africa, 3811.3Schristos EST/EDT/EWT/EPT/EDDT Eastern [North America], 3821.3Schristos EET/EEST Eastern European, 3831.3Schristos GST Guam, 3841.3Schristos HST/HDT Hawaii, 3851.3Schristos HKT/HKST Hong Kong, 3861.3Schristos IST India, 3871.3Schristos IST/GMT Irish, 3881.3Schristos IST/IDT/IDDT Israel, 3891.3Schristos JST/JDT Japan, 3901.3Schristos KST/KDT Korea, 3911.3Schristos MET/MEST Middle European (a backward-compatibility alias for 3921.3Schristos Central European), 3931.3Schristos MSK/MSD Moscow, 3941.3Schristos MST/MDT/MWT/MPT/MDDT Mountain, 3951.3Schristos NST/NDT/NWT/NPT/NDDT Newfoundland, 3961.3Schristos NST/NDT/NWT/NPT Nome, 3971.3Schristos NZMT/NZST New Zealand through 1945, 3981.3Schristos NZST/NZDT New Zealand 1946–present, 3991.3Schristos PKT/PKST Pakistan, 4001.3Schristos PST/PDT/PWT/PPT/PDDT Pacific, 4011.3Schristos SAST South Africa, 4021.3Schristos SST Samoa, 4031.3Schristos WAT/WAST West Africa, 4041.3Schristos WET/WEST/WEMT Western European, 4051.3Schristos WIB Waktu Indonesia Barat, 4061.3Schristos WIT Waktu Indonesia Timur, 4071.3Schristos WITA Waktu Indonesia Tengah, 4081.3Schristos YST/YDT/YWT/YPT/YDDT Yukon</small>. 4091.3Schristos </p> 4101.3Schristos </li> 4111.3Schristos <li> 4121.3Schristos <p> 4131.3Schristos For times taken from a city's longitude, use the 4141.3Schristos traditional <var>x</var>MT notation. 4151.3Schristos The only abbreviation like this in current use is '<abbr>GMT</abbr>'. 4161.3Schristos The others are for timestamps before 1960, 4171.3Schristos except that Monrovia Mean Time persisted until 1972. 4181.3Schristos Typically, numeric abbreviations (e.g., '<code>-</code>004430' for 4191.3Schristos MMT) would cause trouble here, as the numeric strings would exceed 4201.3Schristos the POSIX length limit. 4211.3Schristos </p> 4221.3Schristos 4231.3Schristos <p> 4241.3Schristos <small>These abbreviations are: 4251.3Schristos AMT Amsterdam, Asunción, Athens; 4261.3Schristos BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels, 4271.3Schristos Bucharest; 4281.3Schristos CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; 4291.3Schristos DMT Dublin/Dunsink; 4301.3Schristos EMT Easter; 4311.3Schristos FFMT Fort-de-France; 4321.3Schristos FMT Funchal; 4331.3Schristos GMT Greenwich; 4341.3Schristos HMT Havana, Helsinki, Horta, Howrah; 4351.3Schristos IMT Irkutsk, Istanbul; 4361.3Schristos JMT Jerusalem; 4371.3Schristos KMT Kaunas, Kiev, Kingston; 4381.3Schristos LMT Lima, Lisbon, local, Luanda; 4391.3Schristos MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, 4401.3Schristos Moratuwa, Moscow; 4411.3Schristos PLMT Phù Liễn; 4421.3Schristos PMT Paramaribo, Paris, Perm, Pontianak, Prague; 4431.3Schristos PMMT Port Moresby; 4441.3Schristos QMT Quito; 4451.3Schristos RMT Rangoon, Riga, Rome; 4461.3Schristos SDMT Santo Domingo; 4471.3Schristos SJMT San José; 4481.3Schristos SMT Santiago, Simferopol, Singapore, Stanley; 4491.3Schristos TBMT Tbilisi; 4501.3Schristos TMT Tallinn, Tehran; 4511.3Schristos WMT Warsaw</small>. 4521.3Schristos </p> 4531.3Schristos 4541.3Schristos <p> 4551.3Schristos <small>A few abbreviations also follow the pattern that 4561.3Schristos <abbr>GMT<abbr>/<abbr>BST</abbr> established for time in the UK. 4571.3Schristos They are: 4581.3Schristos CMT/BST for Calamarca Mean Time and Bolivian Summer Time 4591.3Schristos 1890–1932, 4601.3Schristos DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time 4611.3Schristos 1880–1916, 4621.3Schristos MMT/MST/MDST for Moscow 1880–1919, and 4631.3Schristos RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926. 4641.3Schristos An extra-special case is SET for Swedish Time (<em>svensk 4651.3Schristos normaltid</em>) 1879–1899, 3° west of the Stockholm 4661.3Schristos Observatory.</small> 4671.3Schristos </p> 4681.3Schristos </li> 4691.3Schristos <li> 4701.3Schristos Use '<abbr>LMT</abbr>' for local mean time of locations before the 4711.3Schristos introduction of standard time; see "<a href="#scope">Scope of the 4721.3Schristos <code><abbr>tz</abbr></code> database</a>". 4731.3Schristos </li> 4741.3Schristos <li> 4751.3Schristos If there is no common English abbreviation, use numeric offsets like 4761.3Schristos <code>-</code>05 and <code>+</code>0830 that are generated 4771.3Schristos by <code>zic</code>'s <code>%z</code> notation. 4781.3Schristos </li> 4791.3Schristos <li> 4801.3Schristos Use current abbreviations for older timestamps to avoid confusion. 4811.3Schristos For example, in 1910 a common English abbreviation for time 4821.3Schristos in central Europe was 'MEZ' (short for both "Middle European 4831.3Schristos Zone" and for "Mitteleuropäische Zeit" in German). 4841.3Schristos Nowadays 'CET' ("Central European Time") is more common in 4851.3Schristos English, and the database uses 'CET' even for circa-1910 4861.3Schristos timestamps as this is less confusing for modern users and avoids 4871.3Schristos the need for determining when 'CET' supplanted 'MEZ' in common 4881.3Schristos usage. 4891.3Schristos </li> 4901.3Schristos <li> 4911.3Schristos Use a consistent style in a <code><abbr>tz</abbr></code> region's history. 4921.3Schristos For example, if history tends to use numeric 4931.3Schristos abbreviations and a particular entry could go either way, use a 4941.3Schristos numeric abbreviation. 4951.3Schristos </li> 4961.3Schristos <li> 4971.3Schristos Use 4981.3Schristos <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> 4991.3Schristos (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for 5001.3Schristos locations while uninhabited. 5011.3Schristos The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in 5021.3Schristos some sense undefined; this notation is derived 5031.3Schristos from <a href="https://tools.ietf.org/html/rfc3339">Internet 5041.3Schristos <abbr title="Request For Comments">RFC 3339</a>. 5051.1Schristos </li> 5061.1Schristos</ul> 5071.3Schristos 5081.1Schristos<p> 5091.1SchristosApplication writers should note that these abbreviations are ambiguous 5101.2Schristosin practice: e.g., 'CST' means one thing in China and something else 5111.2Schristosin North America, and 'IST' can refer to time in India, Ireland or 5121.3SchristosIsrael. 5131.3SchristosTo avoid ambiguity, use numeric <abbr>UT</abbr> offsets like 5141.2Schristos'<code>-</code>0600' instead of time zone abbreviations like 'CST'. 5151.1Schristos</p> 5161.3Schristos</section> 5171.1Schristos 5181.3Schristos<section> 5191.3Schristos <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2> 5201.1Schristos<p> 5211.3SchristosThe <code><abbr>tz</abbr></code> database is not authoritative, and it 5221.3Schristossurely has errors. 5231.2SchristosCorrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>. 5241.1SchristosUsers requiring authoritative data should consult national standards 5251.1Schristosbodies and the references cited in the database's comments. 5261.1Schristos</p> 5271.1Schristos 5281.1Schristos<p> 5291.3SchristosErrors in the <code><abbr>tz</abbr></code> database arise from many sources: 5301.1Schristos</p> 5311.3Schristos 5321.1Schristos<ul> 5331.1Schristos <li> 5341.3Schristos The <code><abbr>tz</abbr></code> database predicts future 5351.3Schristos timestamps, and current predictions 5361.3Schristos will be incorrect after future governments change the rules. 5371.3Schristos For example, if today someone schedules a meeting for 13:00 next 5381.3Schristos October 1, Casablanca time, and tomorrow Morocco changes its 5391.3Schristos daylight saving rules, software can mess up after the rule change 5401.3Schristos if it blithely relies on conversions made before the change. 5411.3Schristos </li> 5421.3Schristos <li> 5431.3Schristos The pre-1970 entries in this database cover only a tiny sliver of how 5441.3Schristos clocks actually behaved; the vast majority of the necessary 5451.3Schristos information was lost or never recorded. 5461.3Schristos Thousands more <code><abbr>tz</abbr></code> regions would be needed if 5471.3Schristos the <code><abbr>tz</abbr></code> database's scope were extended to 5481.3Schristos cover even just the known or guessed history of standard time; for 5491.3Schristos example, the current single entry for France would need to split 5501.3Schristos into dozens of entries, perhaps hundreds. 5511.3Schristos And in most of the world even this approach would be misleading 5521.3Schristos due to widespread disagreement or indifference about what times 5531.3Schristos should be observed. 5541.3Schristos In her 2015 book 5551.3Schristos <cite><a 5561.3Schristos href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The 5571.3Schristos Global Transformation of Time, 1870–1950</a></cite>, 5581.3Schristos Vanessa Ogle writes 5591.3Schristos "Outside of Europe and North America there was no system of time 5601.3Schristos zones at all, often not even a stable landscape of mean times, 5611.3Schristos prior to the middle decades of the twentieth century". 5621.3Schristos See: Timothy Shenk, <a 5631.3Schristoshref="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: 5641.3Schristos A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. 5651.3Schristos </li> 5661.3Schristos <li> 5671.3Schristos Most of the pre-1970 data entries come from unreliable sources, often 5681.3Schristos astrology books that lack citations and whose compilers evidently 5691.3Schristos invented entries when the true facts were unknown, without 5701.3Schristos reporting which entries were known and which were invented. 5711.3Schristos These books often contradict each other or give implausible entries, 5721.3Schristos and on the rare occasions when they are checked they are 5731.3Schristos typically found to be incorrect. 5741.3Schristos </li> 5751.3Schristos <li> 5761.3Schristos For the UK the <code><abbr>tz</abbr></code> database relies on 5771.3Schristos years of first-class work done by 5781.3Schristos Joseph Myers and others; see 5791.3Schristos "<a href="https://www.polyomino.org.uk/british-time/">History of 5801.3Schristos legal time in Britain</a>". 5811.3Schristos Other countries are not done nearly as well. 5821.3Schristos </li> 5831.3Schristos <li> 5841.3Schristos Sometimes, different people in the same city maintain clocks 5851.3Schristos that differ significantly. 5861.3Schristos Historically, railway time was used by railroad companies (which 5871.3Schristos did not always 5881.3Schristos agree with each other), church-clock time was used for birth 5891.3Schristos certificates, etc. 5901.3Schristos More recently, competing political groups might disagree about 5911.3Schristos clock settings. Often this is merely common practice, but 5921.3Schristos sometimes it is set by law. 5931.3Schristos For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France 5941.3Schristos was legally <abbr>UT</abbr> +00:09:21 outside train stations and 5951.3Schristos <abbr>UT</abbr> +00:04:21 inside. Other examples include 5961.3Schristos Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and 5971.3Schristos Ürümqi to this day. 5981.3Schristos </li> 5991.3Schristos <li> 6001.3Schristos Although a named location in the <code><abbr>tz</abbr></code> 6011.3Schristos database stands for the containing region, its pre-1970 data 6021.3Schristos entries are often accurate for only a small subset of that region. 6031.3Schristos For example, <code>Europe/London</code> stands for the United 6041.3Schristos Kingdom, but its pre-1847 times are valid only for locations that 6051.3Schristos have London's exact meridian, and its 1847 transition 6061.3Schristos to <abbr>GMT</abbr> is known to be valid only for the L&NW and 6071.3Schristos the Caledonian railways. 6081.3Schristos </li> 6091.3Schristos <li> 6101.3Schristos The <code><abbr>tz</abbr></code> database does not record the 6111.3Schristos earliest time for which a <code><abbr>tz</abbr></code> region's 6121.3Schristos data entries are thereafter valid for every location in the region. 6131.3Schristos For example, <code>Europe/London</code> is valid for all locations 6141.3Schristos in its region after <abbr>GMT</abbr> was made the standard time, 6151.3Schristos but the date of standardization (1880-08-02) is not in the 6161.3Schristos <code><abbr>tz</abbr></code> database, other than in commentary. 6171.3Schristos For many <code><abbr>tz</abbr></code> regions the earliest time of 6181.3Schristos validity is unknown. 6191.3Schristos </li> 6201.3Schristos <li> 6211.3Schristos The <code><abbr>tz</abbr></code> database does not record a 6221.3Schristos region's boundaries, and in many cases the boundaries are not known. 6231.3Schristos For example, the <code><abbr>tz</abbr></code> region 6241.3Schristos <code>America/Kentucky/Louisville</code> represents a region 6251.3Schristos around the city of Louisville, the boundaries of which are 6261.3Schristos unclear. 6271.3Schristos </li> 6281.3Schristos <li> 6291.3Schristos Changes that are modeled as instantaneous transitions in the 6301.3Schristos <code><abbr>tz</abbr></code> 6311.3Schristos database were often spread out over hours, days, or even decades. 6321.3Schristos </li> 6331.3Schristos <li> 6341.3Schristos Even if the time is specified by law, locations sometimes 6351.3Schristos deliberately flout the law. 6361.3Schristos </li> 6371.3Schristos <li> 6381.3Schristos Early timekeeping practices, even assuming perfect clocks, were 6391.3Schristos often not specified to the accuracy that the 6401.3Schristos <code><abbr>tz</abbr></code> database requires. 6411.3Schristos </li> 6421.3Schristos <li> 6431.3Schristos Sometimes historical timekeeping was specified more precisely 6441.3Schristos than what the <code><abbr>tz</abbr></code> code can handle. 6451.3Schristos For example, from 1909 to 1937 <a 6461.3Schristos href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" 6471.3Schristos hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean 6481.3Schristos Time (estimated to be <abbr>UT</abbr> 6491.3Schristos +00:19:32.13), but the <code><abbr>tz</abbr></code> 6501.3Schristos code cannot represent the fractional second. 6511.3Schristos In practice these old specifications were rarely if ever 6521.3Schristos implemented to subsecond precision. 6531.3Schristos </li> 6541.3Schristos <li> 6551.3Schristos Even when all the timestamp transitions recorded by the 6561.3Schristos <code><abbr>tz</abbr></code> database are correct, the 6571.3Schristos <code><abbr>tz</abbr></code> rules that generate them may not 6581.3Schristos faithfully reflect the historical rules. 6591.3Schristos For example, from 1922 until World War II the UK moved clocks 6601.3Schristos forward the day following the third Saturday in April unless that 6611.3Schristos was Easter, in which case it moved clocks forward the previous 6621.3Schristos Sunday. 6631.3Schristos Because the <code><abbr>tz</abbr></code> database has no 6641.3Schristos way to specify Easter, these exceptional years are entered as 6651.3Schristos separate <code><abbr>tz</abbr> Rule</code> lines, even though the 6661.3Schristos legal rules did not change. 6671.3Schristos </li> 6681.3Schristos <li> 6691.3Schristos The <code><abbr>tz</abbr></code> database models pre-standard time 6701.3Schristos using the <a 6711.3Schristos href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic 6721.3Schristos Gregorian calendar</a> and local mean time, but many people used 6731.3Schristos other calendars and other timescales. 6741.3Schristos For example, the Roman Empire used 6751.3Schristos the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian 6761.3Schristos calendar</a>, 6771.3Schristos and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman 6781.3Schristos timekeeping</a> had twelve varying-length daytime hours with a 6791.3Schristos non-hour-based system at night. 6801.3Schristos </li> 6811.3Schristos <li> 6821.3Schristos Early clocks were less reliable, and data entries do not represent 6831.3Schristos clock error. 6841.3Schristos </li> 6851.3Schristos <li> 6861.3Schristos The <code><abbr>tz</abbr></code> database assumes Universal Time 6871.3Schristos (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not 6881.3Schristos standardized for older timestamps. 6891.3Schristos In the <code><abbr>tz</abbr></code> database commentary, 6901.3Schristos <abbr>UT</abbr> denotes a family of time standards that includes 6911.3Schristos Coordinated Universal Time (<abbr>UTC</abbr>) along with other 6921.3Schristos variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>, 6931.3Schristos with days starting at midnight. 6941.3Schristos Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern 6951.3Schristos timestamps, <abbr>UTC</abbr> was not defined until 1960, so 6961.3Schristos commentary uses the more-general abbreviation <abbr>UT</abbr> for 6971.3Schristos timestamps that might predate 1960. 6981.3Schristos Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly, 6991.3Schristos and since pre-1972 <abbr>UTC</abbr> seconds varied in length, 7001.3Schristos interpretation of older timestamps can be problematic when 7011.3Schristos subsecond accuracy is needed. 7021.3Schristos </li> 7031.3Schristos <li> 7041.3Schristos Civil time was not based on atomic time before 1972, and we do not 7051.3Schristos know the history of 7061.3Schristos <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's 7071.3Schristos rotation</a> accurately enough to map <a 7081.3Schristos href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr 7091.3Schristos title="International System of Units">SI</abbr></a> seconds to 7101.3Schristos historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a> 7111.3Schristos to more than about one-hour accuracy. 7121.3Schristos See: Stephenson FR, Morrison LV, Hohenkerk CY. 7131.3Schristos <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement of 7141.3Schristos the Earth's rotation: 720 BC to AD 2015</a>. 7151.3Schristos <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. 7161.3Schristos Also see: Espenak F. <a 7171.3Schristos href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty 7181.3Schristos in Delta T (ΔT)</a>. 7191.3Schristos </li> 7201.3Schristos <li> 7211.3Schristos The relationship between POSIX time (that is, <abbr>UTC</abbr> but 7221.3Schristos ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap 7231.3Schristos seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972. 7241.3Schristos Although the POSIX 7251.3Schristos clock officially stops during an inserted leap second, at least one 7261.3Schristos proposed standard has it jumping back a second instead; and in 7271.3Schristos practice POSIX clocks more typically either progress glacially during 7281.3Schristos a leap second, or are slightly slowed while near a leap second. 7291.3Schristos </li> 7301.3Schristos <li> 7311.3Schristos The <code><abbr>tz</abbr></code> database does not represent how 7321.3Schristos uncertain its information is. 7331.3Schristos Ideally it would contain information about when data entries are 7341.3Schristos incomplete or dicey. 7351.3Schristos Partial temporal knowledge is a field of active research, though, 7361.3Schristos and it is not clear how to apply it here. 7371.1Schristos </li> 7381.1Schristos</ul> 7391.1Schristos 7401.1Schristos<p> 7411.3SchristosIn short, many, perhaps most, of the <code><abbr>tz</abbr></code> 7421.3Schristosdatabase's pre-1970 and future timestamps are either wrong or 7431.3Schristosmisleading. 7441.3SchristosAny attempt to pass the 7451.3Schristos<code><abbr>tz</abbr></code> database off as the definition of time 7461.3Schristosshould be unacceptable to anybody who cares about the facts. 7471.3SchristosIn particular, the <code><abbr>tz</abbr></code> database's 7481.3Schristos<abbr>LMT</abbr> offsets should not be considered meaningful, and 7491.3Schristosshould not prompt creation of <code><abbr>tz</abbr></code> regions 7501.3Schristosmerely because two locations 7511.3Schristosdiffer in <abbr>LMT</abbr> or transitioned to standard time at 7521.3Schristosdifferent dates. 7531.3Schristos</p> 7541.3Schristos</section> 7551.3Schristos 7561.3Schristos<section> 7571.3Schristos <h2 id="functions">Time and date functions</h2> 7581.3Schristos<p> 7591.3SchristosThe <code><abbr>tz</abbr></code> code contains time and date functions 7601.3Schristosthat are upwards compatible with those of POSIX. 7611.3SchristosCode compatible with this package is already 7621.3Schristos<a href="tz-link.html#tzdb">part of many platforms</a>, where the 7631.3Schristosprimary use of this package is to update obsolete time-related files. 7641.3SchristosTo do this, you may need to compile the time zone compiler 7651.3Schristos'<code>zic</code>' supplied with this package instead of using the 7661.3Schristossystem '<code>zic</code>', since the format of <code>zic</code>'s 7671.3Schristosinput is occasionally extended, and a platform may still be shipping 7681.3Schristosan older <code>zic</code>. 7691.1Schristos</p> 7701.1Schristos 7711.3Schristos<h3 id="POSIX">POSIX properties and limitations</h3> 7721.1Schristos<ul> 7731.1Schristos <li> 7741.1Schristos <p> 7751.3Schristos In POSIX, time display in a process is controlled by the 7761.3Schristos environment variable <code>TZ</code>. 7771.3Schristos Unfortunately, the POSIX 7781.3Schristos <code>TZ</code> string takes a form that is hard to describe and 7791.3Schristos is error-prone in practice. 7801.3Schristos Also, POSIX <code>TZ</code> strings cannot deal with daylight 7811.3Schristos saving time rules not based on the Gregorian calendar (as in 7821.3Schristos Iran), or with situations where more than two time zone 7831.3Schristos abbreviations or <abbr>UT</abbr> offsets are used in an area. 7841.1Schristos </p> 7851.3Schristos 7861.1Schristos <p> 7871.3Schristos The POSIX <code>TZ</code> string takes the following form: 7881.1Schristos </p> 7891.3Schristos 7901.1Schristos <p> 7911.3Schristos <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>]]] 7921.1Schristos </p> 7931.3Schristos 7941.1Schristos <p> 7951.3Schristos where: 7961.3Schristos </p> 7971.3Schristos 7981.1Schristos <dl> 7991.1Schristos <dt><var>std</var> and <var>dst</var></dt><dd> 8001.3Schristos are 3 or more characters specifying the standard 8011.3Schristos and daylight saving time (<abbr>DST</abbr>) zone names. 8021.3Schristos Starting with POSIX.1-2001, <var>std</var> and <var>dst</var> 8031.3Schristos may also be in a quoted form like '<code><+09></code>'; 8041.3Schristos this allows "<code>+</code>" and "<code>-</code>" in the names. 8051.1Schristos </dd> 8061.1Schristos <dt><var>offset</var></dt><dd> 8071.3Schristos is of the form 8081.3Schristos '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' 8091.3Schristos and specifies the offset west of <abbr>UT</abbr>. 8101.3Schristos '<var>hh</var>' may be a single digit; 8111.3Schristos 0≤<var>hh</var>≤24. 8121.3Schristos The default <abbr>DST</abbr> offset is one hour ahead of 8131.3Schristos standard time. 8141.1Schristos </dd> 8151.1Schristos <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> 8161.3Schristos specifies the beginning and end of <abbr>DST</abbr>. 8171.3Schristos If this is absent, the system supplies its own ruleset 8181.3Schristos for <abbr>DST</abbr>, and its rules can differ from year to year; 8191.3Schristos typically <abbr>US</abbr> <abbr>DST</abbr> rules are used. 8201.1Schristos </dd> 8211.1Schristos <dt><var>time</var></dt><dd> 8221.3Schristos takes the form 8231.3Schristos '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' 8241.3Schristos and defaults to 02:00. 8251.3Schristos This is the same format as the offset, except that a 8261.3Schristos leading '<code>+</code>' or '<code>-</code>' is not allowed. 8271.1Schristos </dd> 8281.1Schristos <dt><var>date</var></dt><dd> 8291.3Schristos takes one of the following forms: 8301.1Schristos <dl> 8311.1Schristos <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> 8321.3Schristos origin-1 day number not counting February 29 8331.3Schristos </dd> 8341.1Schristos <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> 8351.3Schristos origin-0 day number counting February 29 if present 8361.3Schristos </dd> 8371.3Schristos <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> 8381.3Schristos (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 8391.3Schristos 1≤<var>m</var>≤12)</dt><dd> 8401.3Schristos for the <var>d</var>th day of week <var>n</var> of 8411.3Schristos month <var>m</var> of the year, where week 1 is the first 8421.3Schristos week in which day <var>d</var> appears, and 8431.3Schristos '<code>5</code>' stands for the last week in which 8441.3Schristos day <var>d</var> appears (which may be either the 4th or 8451.3Schristos 5th week). 8461.3Schristos Typically, this is the only useful form; the <var>n</var> 8471.3Schristos and <code>J</code><var>n</var> forms are rarely used. 8481.1Schristos </dd> 8491.3Schristos </dl> 8501.3Schristos </dd> 8511.3Schristos </dl> 8521.3Schristos 8531.3Schristos <p> 8541.3Schristos Here is an example POSIX <code>TZ</code> string for New 8551.3Schristos Zealand after 2007. 8561.3Schristos It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead 8571.3Schristos of <abbr>UT</abbr>, and that daylight saving time 8581.3Schristos (<abbr>NZDT</abbr>) is observed from September's last Sunday at 8591.3Schristos 02:00 until April's first Sunday at 03:00: 8601.3Schristos </p> 8611.3Schristos 8621.3Schristos <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> 8631.3Schristos 8641.3Schristos <p> 8651.3Schristos This POSIX <code>TZ</code> string is hard to remember, and 8661.3Schristos mishandles some timestamps before 2008. 8671.3Schristos With this package you can use this instead: 8681.3Schristos </p> 8691.3Schristos 8701.3Schristos <pre><code>TZ='Pacific/Auckland'</code></pre> 8711.3Schristos </li> 8721.3Schristos <li> 8731.3Schristos POSIX does not define the exact meaning of <code>TZ</code> values like 8741.3Schristos "<code>EST5EDT</code>". 8751.3Schristos Typically the current <abbr>US</abbr> <abbr>DST</abbr> rules 8761.3Schristos are used to interpret such values, but this means that the 8771.3Schristos <abbr>US</abbr> <abbr>DST</abbr> rules are compiled into each 8781.3Schristos program that does time conversion. 8791.3Schristos This means that when 8801.3Schristos <abbr>US</abbr> time conversion rules change (as in the United 8811.3Schristos States in 1987), all programs that do time conversion must be 8821.3Schristos recompiled to ensure proper results. 8831.3Schristos </li> 8841.3Schristos <li> 8851.3Schristos The <code>TZ</code> environment variable is process-global, which 8861.3Schristos makes it hard to write efficient, thread-safe applications that 8871.3Schristos need access to multiple time zone rulesets. 8881.3Schristos </li> 8891.3Schristos <li> 8901.3Schristos In POSIX, there is no tamper-proof way for a process to learn the 8911.3Schristos system's best idea of local wall clock. 8921.3Schristos (This is important for applications that an administrator wants 8931.3Schristos used only at certain times – without regard to whether the 8941.3Schristos user has fiddled the 8951.3Schristos <code>TZ</code> environment variable. 8961.3Schristos While an administrator can "do everything in <abbr>UT</abbr>" to 8971.3Schristos get around the problem, doing so is inconvenient and precludes 8981.3Schristos handling daylight saving time shifts - as might be required to 8991.3Schristos limit phone calls to off-peak hours.) 9001.3Schristos </li> 9011.3Schristos <li> 9021.3Schristos POSIX provides no convenient and efficient way to determine 9031.3Schristos the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary 9041.3Schristos timestamps, particularly for <code><abbr>tz</abbr></code> regions 9051.3Schristos that do not fit into the POSIX model. 9061.3Schristos </li> 9071.3Schristos <li> 9081.3Schristos POSIX requires that systems ignore leap seconds. 9091.3Schristos </li> 9101.3Schristos <li> 9111.3Schristos The <code><abbr>tz</abbr></code> code attempts to support all the 9121.3Schristos <code>time_t</code> implementations allowed by POSIX. 9131.3Schristos The <code>time_t</code> type represents a nonnegative count of seconds 9141.3Schristos since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds. 9151.3Schristos In practice, <code>time_t</code> is usually a signed 64- or 32-bit 9161.3Schristos integer; 32-bit signed <code>time_t</code> values stop working after 9171.3Schristos 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these 9181.3Schristos days typically use a signed 64-bit integer. 9191.3Schristos Unsigned 32-bit integers are used on one or two platforms, and 36-bit 9201.3Schristos and 40-bit integers are also used occasionally. 9211.3Schristos Although earlier POSIX versions allowed <code>time_t</code> to be a 9221.3Schristos floating-point type, this was not supported by any practical systems, 9231.3Schristos and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both 9241.3Schristos require <code>time_t</code> to be an integer type. 9251.1Schristos </li> 9261.1Schristos</ul> 9271.3Schristos 9281.3Schristos<h3 id="POSIX-extensions">Extensions to POSIX in the 9291.3Schristos<code><abbr>tz</abbr></code> code</h3> 9301.1Schristos<ul> 9311.1Schristos <li> 9321.1Schristos <p> 9331.3Schristos The <code>TZ</code> environment variable is used in generating 9341.3Schristos the name of a binary file from which time-related information is read 9351.3Schristos (or is interpreted à la POSIX); <code>TZ</code> is no longer 9361.3Schristos constrained to be a three-letter time zone 9371.3Schristos abbreviation followed by a number of hours and an optional three-letter 9381.3Schristos daylight time zone abbreviation. 9391.3Schristos The daylight saving time rules to be used for a 9401.3Schristos particular <code><abbr>tz</abbr></code> region are encoded in the 9411.3Schristos binary file; the format of the file 9421.3Schristos allows U.S., Australian, and other rules to be encoded, and 9431.3Schristos allows for situations where more than two time zone 9441.3Schristos abbreviations are used. 9451.1Schristos </p> 9461.1Schristos <p> 9471.3Schristos It was recognized that allowing the <code>TZ</code> environment 9481.3Schristos variable to take on values such as '<code>America/New_York</code>' 9491.3Schristos might cause "old" programs (that expect <code>TZ</code> to have a 9501.3Schristos certain form) to operate incorrectly; consideration was given to using 9511.3Schristos some other environment variable (for example, <code>TIMEZONE</code>) 9521.3Schristos to hold the string used to generate the binary file's name. 9531.3Schristos In the end, however, it was decided to continue using 9541.3Schristos <code>TZ</code>: it is widely used for time zone purposes; 9551.3Schristos separately maintaining both <code>TZ</code> 9561.3Schristos and <code>TIMEZONE</code> seemed a nuisance; and systems where 9571.3Schristos "new" forms of <code>TZ</code> might cause problems can simply 9581.3Schristos use <code>TZ</code> values such as "<code>EST5EDT</code>" which 9591.3Schristos can be used both by "new" programs (à la POSIX) and "old" 9601.3Schristos programs (as zone names and offsets). 9611.1Schristos </p> 9621.3Schristos </li> 9631.3Schristos <li> 9641.3Schristos The code supports platforms with a <abbr>UT</abbr> offset member 9651.3Schristos in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>. 9661.3Schristos </li> 9671.3Schristos <li> 9681.3Schristos The code supports platforms with a time zone abbreviation member in 9691.3Schristos <code>struct tm</code>, e.g., <code>tm_zone</code>. 9701.3Schristos </li> 9711.3Schristos <li> 9721.3Schristos Functions <code>tzalloc</code>, <code>tzfree</code>, 9731.3Schristos <code>localtime_rz</code>, and <code>mktime_z</code> for 9741.3Schristos more-efficient thread-safe applications that need to use multiple 9751.3Schristos time zone rulesets. 9761.3Schristos The <code>tzalloc</code> and <code>tzfree</code> functions 9771.3Schristos allocate and free objects of type <code>timezone_t</code>, 9781.3Schristos and <code>localtime_rz</code> and <code>mktime_z</code> are 9791.3Schristos like <code>localtime_r</code> and <code>mktime</code> with an 9801.3Schristos extra <code>timezone_t</code> argument. 9811.3Schristos The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>. 9821.3Schristos </li> 9831.3Schristos <li> 9841.3Schristos A function <code>tzsetwall</code> has been added to arrange for the 9851.3Schristos system's best approximation to local wall clock time to be delivered 9861.3Schristos by subsequent calls to <code>localtime</code>. 9871.3Schristos Source code for portable applications that "must" run on local wall 9881.3Schristos clock time should call <code>tzsetwall</code>; 9891.3Schristos if such code is moved to "old" systems that do not 9901.3Schristos provide <code>tzsetwall</code>, you will not be able to generate an 9911.3Schristos executable program. 9921.3Schristos (These functions also arrange for local wall clock time to 9931.3Schristos be used if <code>tzset</code> is called – directly or 9941.3Schristos indirectly – and there is no <code>TZ</code> environment 9951.3Schristos variable; portable applications should not, however, rely on this 9961.3Schristos behavior since it is not the way <a 9971.3Schristos href="https://en.wikipedia.org/wiki/UNIX_System_V#SVR2"><abbr>SVR2</abbr></a> 9981.3Schristos systems behave.) 9991.3Schristos </li> 10001.3Schristos <li> 10011.3Schristos Negative <code>time_t</code> values are supported, on systems 10021.3Schristos where <code>time_t</code> is signed. 10031.3Schristos </li> 10041.3Schristos <li> 10051.3Schristos These functions can account for leap seconds, thanks to Bradley White. 10061.3Schristos </li> 10071.1Schristos</ul> 10081.3Schristos 10091.3Schristos<h3 id="vestigial">POSIX features no longer needed</h3> 10101.1Schristos<p> 10111.3SchristosPOSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a> 10121.3Schristosdefine some <a href="https://en.wikipedia.org/wiki/API"><abbr 10131.3Schristostitle="application programming interface">API</abbr>s</a> that are vestigial: 10141.3Schristosthey are not needed, and are relics of a too-simple model that does 10151.3Schristosnot suffice to handle many real-world timestamps. 10161.3SchristosAlthough the <code><abbr>tz</abbr></code> code supports these 10171.3Schristosvestigial <abbr>API</abbr>s for backwards compatibility, they should 10181.3Schristosbe avoided in portable applications. 10191.3SchristosThe vestigial <abbr>API</abbr>s are: 10201.1Schristos</p> 10211.1Schristos<ul> 10221.1Schristos <li> 10231.3Schristos The POSIX <code>tzname</code> variable does not suffice and is no 10241.3Schristos longer needed. 10251.3Schristos To get a timestamp's time zone abbreviation, consult 10261.3Schristos the <code>tm_zone</code> member if available; otherwise, 10271.3Schristos use <code>strftime</code>'s <code>"%Z"</code> conversion 10281.3Schristos specification. 10291.3Schristos </li> 10301.3Schristos <li> 10311.3Schristos The POSIX <code>daylight</code> and <code>timezone</code> 10321.3Schristos variables do not suffice and are no longer needed. 10331.3Schristos To get a timestamp's <abbr>UT</abbr> offset, consult 10341.3Schristos the <code>tm_gmtoff</code> member if available; otherwise, 10351.3Schristos subtract values returned by <code>localtime</code> 10361.3Schristos and <code>gmtime</code> using the rules of the Gregorian calendar, 10371.3Schristos or use <code>strftime</code>'s <code>"%z"</code> conversion 10381.3Schristos specification if a string like <code>"+0900"</code> suffices. 10391.3Schristos </li> 10401.3Schristos <li> 10411.3Schristos The <code>tm_isdst</code> member is almost never needed and most of 10421.3Schristos its uses should be discouraged in favor of the abovementioned 10431.3Schristos <abbr>API</abbr>s. 10441.3Schristos Although it can still be used in arguments to 10451.3Schristos <code>mktime</code> to disambiguate timestamps near 10461.3Schristos a <abbr>DST</abbr> transition when the clock jumps back, this 10471.3Schristos disambiguation does not work when standard time itself jumps back, 10481.3Schristos which can occur when a location changes to a time zone with a 10491.3Schristos lesser <abbr>UT</abbr> offset. 10501.3Schristos </li> 10511.3Schristos</ul> 10521.3Schristos 10531.3Schristos<h3 id="other-portability">Other portability notes</h3> 10541.3Schristos<ul> 10551.3Schristos <li> 10561.3Schristos The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition 10571.3Schristos UNIX</a> <code>timezone</code> function is not present in this 10581.3Schristos package; it is impossible to reliably map <code>timezone</code>'s 10591.3Schristos arguments (a "minutes west of <abbr>GMT</abbr>" value and a 10601.3Schristos "daylight saving time in effect" flag) to a time zone 10611.3Schristos abbreviation, and we refuse to guess. 10621.3Schristos Programs that in the past used the <code>timezone</code> function 10631.3Schristos may now examine <code>localtime(&clock)->tm_zone</code> 10641.3Schristos (if <code>TM_ZONE</code> is defined) or 10651.3Schristos <code>tzname[localtime(&clock)->tm_isdst]</code> 10661.3Schristos (if <code>HAVE_TZNAME</code> is defined) to learn the correct time 10671.3Schristos zone abbreviation to use. 10681.3Schristos </li> 10691.3Schristos <li> 10701.3Schristos The <a 10711.3Schristos href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a> 10721.3Schristos <code>gettimeofday</code> function is not 10731.3Schristos used in this package. 10741.3Schristos This formerly let users obtain the current <abbr>UTC</abbr> offset 10751.3Schristos and <abbr>DST</abbr> flag, but this functionality was removed in 10761.3Schristos later versions of <abbr>BSD</abbr>. 10771.3Schristos </li> 10781.3Schristos <li> 10791.3Schristos In <abbr>SVR2</abbr>, time conversion fails for near-minimum or 10801.3Schristos near-maximum <code>time_t</code> values when doing conversions 10811.3Schristos for places that do not use <abbr>UT</abbr>. 10821.3Schristos This package takes care to do these conversions correctly. 10831.3Schristos A comment in the source code tells how to get compatibly wrong 10841.3Schristos results. 10851.3Schristos </li> 10861.3Schristos <li> 10871.3Schristos The functions that are conditionally compiled 10881.3Schristos if <code>STD_INSPIRED</code> is defined should, at this point, be 10891.3Schristos looked on primarily as food for thought. 10901.3Schristos They are not in any sense "standard compatible" – some are 10911.3Schristos not, in fact, specified in <em>any</em> standard. 10921.3Schristos They do, however, represent responses of various authors to 10931.3Schristos standardization proposals. 10941.3Schristos </li> 10951.3Schristos <li> 10961.3Schristos Other time conversion proposals, in particular the one developed 10971.3Schristos by folks at Hewlett Packard, offer a wider selection of functions 10981.3Schristos that provide capabilities beyond those provided here. 10991.3Schristos The absence of such functions from this package is not meant to 11001.3Schristos discourage the development, standardization, or use of such 11011.3Schristos functions. 11021.3Schristos Rather, their absence reflects the decision to make this package 11031.3Schristos contain valid extensions to POSIX, to ensure its broad 11041.3Schristos acceptability. 11051.3Schristos If more powerful time conversion functions can be standardized, so 11061.3Schristos much the better. 11071.1Schristos </li> 11081.1Schristos</ul> 11091.3Schristos</section> 11101.1Schristos 11111.3Schristos<section> 11121.3Schristos <h2 id="stability">Interface stability</h2> 11131.1Schristos<p> 11141.3SchristosThe <code><abbr>tz</abbr></code> code and data supply the following interfaces: 11151.1Schristos</p> 11161.1Schristos 11171.1Schristos<ul> 11181.1Schristos <li> 11191.3Schristos A set of <code><abbr>tz</abbr></code> region names as per 11201.3Schristos "<a href="#naming">Names of time zone rulesets</a>" above. 11211.1Schristos </li> 11221.1Schristos <li> 11231.3Schristos Library functions described in "<a href="#functions">Time and date 11241.3Schristos functions</a>" above. 11251.1Schristos </li> 11261.1Schristos <li> 11271.3Schristos The programs <code>tzselect</code>, <code>zdump</code>, 11281.3Schristos and <code>zic</code>, documented in their man pages. 11291.1Schristos </li> 11301.1Schristos <li> 11311.3Schristos The format of <code>zic</code> input files, documented in 11321.3Schristos the <code>zic</code> man page. 11331.1Schristos </li> 11341.1Schristos <li> 11351.3Schristos The format of <code>zic</code> output files, documented in 11361.3Schristos the <code>tzfile</code> man page. 11371.1Schristos </li> 11381.1Schristos <li> 11391.3Schristos The format of zone table files, documented in <code>zone1970.tab</code>. 11401.1Schristos </li> 11411.1Schristos <li> 11421.3Schristos The format of the country code file, documented in <code>iso3166.tab</code>. 11431.1Schristos </li> 11441.1Schristos <li> 11451.3Schristos The version number of the code and data, as the first line of 11461.3Schristos the text file '<code>version</code>' in each release. 11471.1Schristos </li> 11481.1Schristos</ul> 11491.3Schristos 11501.1Schristos<p> 11511.1SchristosInterface changes in a release attempt to preserve compatibility with 11521.3Schristosrecent releases. 11531.3SchristosFor example, <code><abbr>tz</abbr></code> data files typically do not 11541.3Schristosrely on recently-added <code>zic</code> features, so that users can 11551.3Schristosrun older <code>zic</code> versions to process newer data files. 11561.3Schristos<a href="tz-link.html#download">Downloading 11571.3Schristosthe <code><abbr>tz</abbr></code> database</a> describes how releases 11581.3Schristosare tagged and distributed. 11591.1Schristos</p> 11601.1Schristos 11611.1Schristos<p> 11621.3SchristosInterfaces not listed above are less stable. 11631.3SchristosFor example, users should not rely on particular <abbr>UT</abbr> 11641.3Schristosoffsets or abbreviations for timestamps, as data entries are often 11651.3Schristosbased on guesswork and these guesses may be corrected or improved. 11661.1Schristos</p> 11671.3Schristos</section> 11681.1Schristos 11691.3Schristos<section> 11701.3Schristos <h2 id="calendar">Calendrical issues</h2> 11711.1Schristos<p> 11721.1SchristosCalendrical issues are a bit out of scope for a time zone database, 11731.1Schristosbut they indicate the sort of problems that we would run into if we 11741.3Schristosextended the time zone database further into the past. 11751.3SchristosAn excellent resource in this area is Edward M. Reingold 11761.3Schristosand Nachum Dershowitz, <cite><a 11771.3Schristoshref="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical 11781.3SchristosCalculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018). 11791.3SchristosOther information and sources are given in the file '<code>calendars</code>' 11801.3Schristosin the <code><abbr>tz</abbr></code> distribution. 11811.3SchristosThey sometimes disagree. 11821.3Schristos</p> 11831.3Schristos</section> 11841.3Schristos 11851.3Schristos<section> 11861.3Schristos <h2 id="planets">Time and time zones on other planets</h2> 11871.3Schristos<p> 11881.3SchristosSome people's work schedules 11891.3Schristosuse <a href="https://en.wikipedia.org/wiki/Timekeeping on Mars">Mars time</a>. 11901.3SchristosJet Propulsion Laboratory (JPL) coordinators kept Mars time on 11911.3Schristosand off during the 11921.3Schristos<a href="https://en.wikipedia.org/wiki/Mars_Pathfinder#End_of_mission">Mars 11931.3SchristosPathfinder</a> mission. 11941.3SchristosSome of their family members also adapted to Mars time. 11951.3SchristosDozens of special Mars watches were built for JPL workers who kept 11961.3SchristosMars time during the Mars Exploration Rovers mission (2004). 11971.3SchristosThese timepieces look like normal Seikos and Citizens but use Mars 11981.3Schristosseconds rather than terrestrial seconds. 11991.1Schristos</p> 12001.1Schristos 12011.1Schristos<p> 12021.1SchristosA Mars solar day is called a "sol" and has a mean period equal to 12031.3Schristosabout 24 hours 39 minutes 35.244 seconds in terrestrial time. 12041.3SchristosIt is divided into a conventional 24-hour clock, so each Mars second 12051.3Schristosequals about 1.02749125 terrestrial seconds. 12061.1Schristos</p> 12071.1Schristos 12081.1Schristos<p> 12091.3SchristosThe <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime 12101.3Schristosmeridian</a> of Mars goes through the center of the crater 12111.3Schristos<a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in 12121.3Schristoshonor of the British astronomer who built the Greenwich telescope that 12131.3Schristosdefines Earth's prime meridian. 12141.3SchristosMean solar time on the Mars prime meridian is 12151.3Schristoscalled <a href="https://en.wikipedia.org/wiki/Mars_Coordinated_Time">Mars 12161.3SchristosCoordinated Time (<abbr>MTC</abbr>)</a>. 12171.1Schristos</p> 12181.1Schristos 12191.1Schristos<p> 12201.1SchristosEach landed mission on Mars has adopted a different reference for 12211.1Schristossolar time keeping, so there is no real standard for Mars time zones. 12221.3SchristosFor example, the 12231.3Schristos<a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars 12241.3SchristosExploration Rover</a> project (2004) defined two time zones "Local 12251.3SchristosSolar Time A" and "Local Solar Time B" for its two missions, each zone 12261.3Schristosdesigned so that its time equals local true solar time at 12271.3Schristosapproximately the middle of the nominal mission. 12281.3SchristosSuch a "time zone" is not particularly suited for any application 12291.3Schristosother than the mission itself. 12301.1Schristos</p> 12311.1Schristos 12321.1Schristos<p> 12331.1SchristosMany calendars have been proposed for Mars, but none have achieved 12341.3Schristoswide acceptance. 12351.3SchristosAstronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a 12361.1Schristossequential count of Mars solar days elapsed since about 1873-12-29 12371.3Schristos12:00 <abbr>GMT</abbr>. 12381.1Schristos</p> 12391.1Schristos 12401.1Schristos<p> 12411.1SchristosIn our solar system, Mars is the planet with time and calendar most 12421.3Schristoslike Earth's. 12431.3SchristosOn other planets, Sun-based time and calendars would work quite 12441.3Schristosdifferently. 12451.3SchristosFor example, although Mercury's 12461.3Schristos<a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal 12471.3Schristosrotation period</a> is 58.646 Earth days, Mercury revolves around the 12481.3SchristosSun so rapidly that an observer on Mercury's equator would see a 12491.3Schristossunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a 12501.3SchristosMercury day. 12511.3SchristosVenus is more complicated, partly because its rotation is slightly 12521.3Schristos<a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>: 12531.3Schristosits year is 1.92 of its days. 12541.3SchristosGas giants like Jupiter are trickier still, as their polar and 12551.3Schristosequatorial regions rotate at different rates, so that the length of a 12561.3Schristosday depends on latitude. 12571.3SchristosThis effect is most pronounced on Neptune, where the day is about 12 12581.3Schristoshours at the poles and 18 hours at the equator. 12591.1Schristos</p> 12601.1Schristos 12611.1Schristos<p> 12621.3SchristosAlthough the <code><abbr>tz</abbr></code> database does not support 12631.3Schristostime on other planets, it is documented here in the hopes that support 12641.3Schristoswill be added eventually. 12651.1Schristos</p> 12661.1Schristos 12671.1Schristos<p> 12681.3SchristosSources for time on other planets: 12691.1Schristos</p> 12701.3Schristos 12711.1Schristos<ul> 12721.1Schristos <li> 12731.3Schristos Michael Allison and Robert Schmunk, 12741.3Schristos "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical 12751.3Schristos Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" 12761.3Schristos (2015-06-30). 12771.1Schristos </li> 12781.1Schristos <li> 12791.3Schristos Jia-Rui Chong, 12801.3Schristos "<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays 12811.3Schristos Fit for a Martian</a>", <cite>Los Angeles Times</cite> 12821.3Schristos (2004-01-14), pp A1, A20–A21. 12831.1Schristos </li> 12841.1Schristos <li> 12851.3Schristos Tom Chmielewski, 12861.3Schristos "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet 12871.3Schristos Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26) 12881.1Schristos </li> 12891.1Schristos <li> 12901.3Schristos Matt Williams, 12911.3Schristos "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How 12921.3Schristos long is a day on the other planets of the solar system?</a>" 12931.3Schristos (2017-04-27). 12941.1Schristos </li> 12951.1Schristos</ul> 12961.3Schristos</section> 12971.1Schristos 12981.3Schristos<footer> 12991.3Schristos <hr> 13001.3Schristos This file is in the public domain, so clarified as of 2009-05-17 by 13011.3Schristos Arthur David Olson. 13021.3Schristos</footer> 13031.1Schristos</body> 13041.1Schristos</html> 1305