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9395fe2018-04-14Henrik Grubbström (Grubba) <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
af92cb2017-11-17Henrik Grubbström (Grubba)  <h3>Outline</h3> <nav> <ul>
9395fe2018-04-14Henrik Grubbström (Grubba)  <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code> database</a></li>
59b4e02019-05-06Henrik Grubbström (Grubba)  <li><a href="#naming">Timezone identifiers</a></li>
af92cb2017-11-17Henrik Grubbström (Grubba)  <li><a href="#abbreviations">Time zone abbreviations</a></li>
9395fe2018-04-14Henrik Grubbström (Grubba)  <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</a></li>
af92cb2017-11-17Henrik Grubbström (Grubba)  <li><a href="#functions">Time and date functions</a></li> <li><a href="#stability">Interface stability</a></li> <li><a href="#calendar">Calendrical issues</a></li> <li><a href="#planets">Time and time zones on other planets</a></li> </ul> </nav>
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The <a href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code> database</a> attempts to record the history and predicted future of all computer-based clocks that track civil time. It organizes <a href="tz-link.html">time zone and daylight saving time data</a> by partitioning the world into <a
aa9f722018-12-18Henrik Grubbström (Grubba) href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a>
01f1de2018-06-04Henrik Grubbström (Grubba) whose clocks all agree about timestamps that occur after the <a
9395fe2018-04-14Henrik Grubbström (Grubba) href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a> (1970-01-01 00:00:00 <a href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr title="Coordinated Universal Time">UTC</abbr></a>).
aa9f722018-12-18Henrik Grubbström (Grubba) The database labels each timezone with a notable location and
9395fe2018-04-14Henrik Grubbström (Grubba) records all known clock transitions for that location. Although 1970 is a somewhat-arbitrary cutoff, there are significant challenges to moving the cutoff earlier even by a decade or two, due to the wide variety of local practices before computer timekeeping became prevalent.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
aa9f722018-12-18Henrik Grubbström (Grubba) Each timezone typically corresponds to a geographical region that is smaller than a traditional time zone, because clocks in a timezone all agree after 1970 whereas a traditional time zone merely specifies current standard time. For example, applications that deal with current and future timestamps in the traditional North American mountain time zone can choose from the timezones <code>America/Denver</code> which observes US-style daylight saving time, <code>America/Mazatlan</code> which observes Mexican-style DST, and <code>America/Phoenix</code> which does not observe DST. Applications that also deal with past timestamps in the mountain time zone can choose from over a dozen timezones, such as <code>America/Boise</code>, <code>America/Edmonton</code>, and <code>America/Hermosillo</code>, each of which currently uses mountain time but differs from other timezones for some timestamps after 1970. </p> <p> Clock transitions before 1970 are recorded for each timezone,
af92cb2017-11-17Henrik Grubbström (Grubba) because most systems support timestamps before 1970 and could misbehave if data entries were omitted for pre-1970 transitions. However, the database is not designed for and does not suffice for applications requiring accurate handling of all past times everywhere, as it would take far too much effort and guesswork to record all details of pre-1970 civil timekeeping.
01f1de2018-06-04Henrik Grubbström (Grubba) Although some information outside the scope of the database is
80d0322018-01-29Henrik Grubbström (Grubba) collected in a file <code>backzone</code> that is distributed along with the database proper, this file is less reliable and does not necessarily follow database guidelines.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) As described below, reference source code for using the <code><abbr>tz</abbr></code> database is also available. The <code><abbr>tz</abbr></code> code is upwards compatible with <a href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international standard for <a href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems. As of this writing, the current edition of POSIX is: <a
29a9d82019-08-10Henrik Grubbström (Grubba) href="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
01f1de2018-06-04Henrik Grubbström (Grubba) Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
9395fe2018-04-14Henrik Grubbström (Grubba) Edition. Because the database's scope encompasses real-world changes to civil timekeeping, its model for describing time is more complex than the standard and daylight saving times supported by POSIX.
aa9f722018-12-18Henrik Grubbström (Grubba) A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
9395fe2018-04-14Henrik Grubbström (Grubba) have more than two changes per year, these changes need not merely flip back and forth between two alternatives, and the rules themselves can change at times.
aa9f722018-12-18Henrik Grubbström (Grubba) Whether and when a timezone changes its clock, and even the timezone's notional base offset from UTC, are variable. It does not always make sense to talk about a timezone's "base offset", which is not necessarily a single number.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section>
59b4e02019-05-06Henrik Grubbström (Grubba)  <h2 id="naming">Timezone identifiers</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
59b4e02019-05-06Henrik Grubbström (Grubba) Each timezone has a name that uniquely identifies the timezone.
af92cb2017-11-17Henrik Grubbström (Grubba) Inexperienced users are not expected to select these names unaided. Distributors should provide documentation and/or a simple selection
aa9f722018-12-18Henrik Grubbström (Grubba) interface that explains each name via a map or via descriptive text like "Ruthenia" instead of the timezone name "<code>Europe/Uzhgorod</code>". If geolocation information is available, a selection interface can locate the user on a timezone map or prioritize names that are geographically close. For an example selection interface, see the
01f1de2018-06-04Henrik Grubbström (Grubba) <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
9395fe2018-04-14Henrik Grubbström (Grubba) The <a href="http://cldr.unicode.org/">Unicode Common Locale Data Repository</a> contains data that may be useful for other selection
aa9f722018-12-18Henrik Grubbström (Grubba) interfaces; it maps timezone names like <code>Europe/Uzhgorod</code> to CLDR names like <code>uauzh</code> which are in turn mapped to locale-dependent strings like "Uzhhorod", "Ungvár", "Ужгород", and "乌日哥罗德".
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The naming conventions attempt to strike a balance
af92cb2017-11-17Henrik Grubbström (Grubba) among the following goals: </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  Uniquely identify every timezone where clocks have agreed since 1970.
9395fe2018-04-14Henrik Grubbström (Grubba)  This is essential for the intended use: static clocks keeping local civil time.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  Indicate to experts where the timezone's clocks typically are.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Be robust in the presence of political changes.
59b4e02019-05-06Henrik Grubbström (Grubba)  For example, names are typically not tied to countries, to avoid
9395fe2018-04-14Henrik Grubbström (Grubba)  incompatibilities when countries change their name (e.g.,
59b4e02019-05-06Henrik Grubbström (Grubba)  Swaziland&rarr;Eswatini) or when locations change countries (e.g., Hong
9395fe2018-04-14Henrik Grubbström (Grubba)  Kong from UK colony to China).
59b4e02019-05-06Henrik Grubbström (Grubba)  There is no requirement that every country or national capital must have a timezone name.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Be portable to a wide variety of implementations.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use a consistent naming conventions over the entire world.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Names normally have the form <var>AREA</var><code>/</code><var>LOCATION</var>, where
aa9f722018-12-18Henrik Grubbström (Grubba) <var>AREA</var> is a continent or ocean, and <var>LOCATION</var> is a specific location within the area.
9395fe2018-04-14Henrik Grubbström (Grubba) North and South America share the same area, '<code>America</code>'. Typical names are '<code>Africa/Cairo</code>', '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
01f1de2018-06-04Henrik Grubbström (Grubba) Some names are further qualified to help avoid confusion; for example, '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg, Indiana from other Petersburgs in America.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Here are the general guidelines used for
aa9f722018-12-18Henrik Grubbström (Grubba) choosing timezone names,
af92cb2017-11-17Henrik Grubbström (Grubba) in decreasing order of importance: </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use only valid POSIX file name components (i.e., the parts of names other than '<code>/</code>'). Do not use the file name components '<code>.</code>' and '<code>..</code>'. Within a file name component, use only <a href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters, '<code>.</code>', '<code>-</code>' and '<code>_</code>'. Do not use digits, as that might create an ambiguity with <a
29a9d82019-08-10Henrik Grubbström (Grubba)  href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
9395fe2018-04-14Henrik Grubbström (Grubba)  <code>TZ</code> strings</a>. A file name component must not exceed 14 characters or start with '<code>-</code>'.
01f1de2018-06-04Henrik Grubbström (Grubba)  E.g., prefer <code>Asia/Brunei</code> to <code>Asia/Bandar_Seri_Begawan</code>.
9395fe2018-04-14Henrik Grubbström (Grubba)  Exceptions: see the discussion of legacy names below.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  A name must not be empty, or contain '<code>//</code>', or start or end with '<code>/</code>'.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Do not use names that differ only in case. Although the reference implementation is case-sensitive, some other implementations are not, and they would mishandle names differing only in case.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  If one name <var>A</var> is an initial prefix of another name <var>AB</var> (ignoring case), then <var>B</var> must not start with '<code>/</code>', as a regular file cannot have the same name as a directory in POSIX.
01f1de2018-06-04Henrik Grubbström (Grubba)  For example, <code>America/New_York</code> precludes <code>America/New_York/Bronx</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Uninhabited regions like the North Pole and Bouvet Island do not need locations, since local time is not defined there.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  If all the clocks in a timezone have agreed since 1970, do not bother to include more than one timezone even if some of the clocks disagreed before 1970.
9395fe2018-04-14Henrik Grubbström (Grubba)  Otherwise these tables would become annoyingly large.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
59b4e02019-05-06Henrik Grubbström (Grubba)  If boundaries between regions are fluid, such as during a war or insurrection, do not bother to create a new timezone merely because of yet another boundary change. This helps prevent table bloat and simplifies maintenance. </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  If a name is ambiguous, use a less ambiguous alternative; e.g., many cities are named San José and Georgetown, so
01f1de2018-06-04Henrik Grubbström (Grubba)  prefer <code>America/Costa_Rica</code> to <code>America/San_Jose</code> and <code>America/Guyana</code> to <code>America/Georgetown</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Keep locations compact. Use cities or small islands, not countries or regions, so that any future changes do not split individual locations into different
aa9f722018-12-18Henrik Grubbström (Grubba)  timezones.
01f1de2018-06-04Henrik Grubbström (Grubba)  E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>, since
9395fe2018-04-14Henrik Grubbström (Grubba)  <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France has had multiple time zones</a>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
01f1de2018-06-04Henrik Grubbström (Grubba)  Use mainstream English spelling, e.g., prefer
aa9f722018-12-18Henrik Grubbström (Grubba)  <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
01f1de2018-06-04Henrik Grubbström (Grubba)  prefer <code>Europe/Athens</code> to the Greek
aa9f722018-12-18Henrik Grubbström (Grubba)  <code>Ευρώπη/Αθήνα</code> or the Romanized <code>Evrópi/Athína</code>.
9395fe2018-04-14Henrik Grubbström (Grubba)  The POSIX file name restrictions encourage this guideline.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use the most populous among locations in a region,
01f1de2018-06-04Henrik Grubbström (Grubba)  e.g., prefer <code>Asia/Shanghai</code> to <code>Asia/Beijing</code>.
9395fe2018-04-14Henrik Grubbström (Grubba)  Among locations with similar populations, pick the best-known
01f1de2018-06-04Henrik Grubbström (Grubba)  location, e.g., prefer <code>Europe/Rome</code> to <code>Europe/Milan</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
01f1de2018-06-04Henrik Grubbström (Grubba)  Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to <code>Atlantic/Canaries</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Omit common suffixes like '<code>_Islands</code>' and '<code>_City</code>', unless that would lead to ambiguity.
01f1de2018-06-04Henrik Grubbström (Grubba)  E.g., prefer <code>America/Cayman</code> to <code>America/Cayman_Islands</code> and <code>America/Guatemala</code> to <code>America/Guatemala_City</code>, but prefer <code>America/Mexico_City</code> to <code>America/Mexico</code>
9395fe2018-04-14Henrik Grubbström (Grubba)  because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the country of Mexico has several time zones</a>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use '<code>_</code>' to represent a space.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Omit '<code>.</code>' from abbreviations in names.
01f1de2018-06-04Henrik Grubbström (Grubba)  E.g., prefer <code>Atlantic/St_Helena</code> to <code>Atlantic/St._Helena</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Do not change established names if they only marginally violate the above guidelines.
01f1de2018-06-04Henrik Grubbström (Grubba)  For example, do not change the existing name <code>Europe/Rome</code> to <code>Europe/Milan</code> merely because Milan's population has grown
9395fe2018-04-14Henrik Grubbström (Grubba)  to be somewhat greater than Rome's.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  If a name is changed, put its old spelling in the '<code>backward</code>' file. This means old spellings will continue to work.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul> <p>
59b4e02019-05-06Henrik Grubbström (Grubba) Guidelines have evolved with time, and names following old versions of
29a9d82019-08-10Henrik Grubbström (Grubba) these guidelines might not follow the current version. When guidelines
59b4e02019-05-06Henrik Grubbström (Grubba) have changed, old names continue to be supported. Guideline changes have included the following:
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
59b4e02019-05-06Henrik Grubbström (Grubba) <ul> <li> Older versions of this package used a different naming scheme.
af92cb2017-11-17Henrik Grubbström (Grubba) See the file '<code>backward</code>' for most of these older names (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). The other old-fashioned names still supported are
9395fe2018-04-14Henrik Grubbström (Grubba) '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>').
59b4e02019-05-06Henrik Grubbström (Grubba) </li>
af92cb2017-11-17Henrik Grubbström (Grubba) 
59b4e02019-05-06Henrik Grubbström (Grubba) <li>
af92cb2017-11-17Henrik Grubbström (Grubba) Older versions of this package defined legacy names that are
9395fe2018-04-14Henrik Grubbström (Grubba) incompatible with the first guideline of location names, but which are still supported. These legacy names are mostly defined in the file '<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
59b4e02019-05-06Henrik Grubbström (Grubba) </li> <li>
29a9d82019-08-10Henrik Grubbström (Grubba) Older versions of these guidelines said that
59b4e02019-05-06Henrik Grubbström (Grubba) there should typically be at least one name for each <a href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr title="International Organization for Standardization">ISO</abbr> 3166-1</a> officially assigned two-letter code for an inhabited country or territory. This old guideline has been dropped, as it was not needed to handle timestamps correctly and it increased maintenance burden. </li> </ul> <p> The file '<code>zone1970.tab</code>' lists geographical locations used to name timezones. It is intended to be an exhaustive list of names for geographic regions as described above; this is a subset of the timezones in the data. Although a '<code>zone1970.tab</code>' location's <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a> corresponds to its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean time (<abbr>LMT</abbr>)</a> offset with one hour for every 15&deg; east longitude, this relationship is not exact.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Excluding '<code>backward</code>' should not affect the other data. If '<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the remaining data.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="abbreviations">Time zone abbreviations</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p> When this package is installed, it generates time zone abbreviations like '<code>EST</code>' to be compatible with human tradition and POSIX.
9395fe2018-04-14Henrik Grubbström (Grubba) Here are the general guidelines used for choosing time zone abbreviations,
af92cb2017-11-17Henrik Grubbström (Grubba) in decreasing order of importance:
9395fe2018-04-14Henrik Grubbström (Grubba) </p>
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use three to six characters that are ASCII alphanumerics or '<code>+</code>' or '<code>-</code>'. Previous editions of this database also used characters like
01f1de2018-06-04Henrik Grubbström (Grubba)  space and '<code>?</code>', but these characters have a special meaning to the <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a> and cause commands like
29a9d82019-08-10Henrik Grubbström (Grubba)  '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a> `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
9395fe2018-04-14Henrik Grubbström (Grubba)  to have unexpected effects. Previous editions of this guideline required upper-case letters, but the Congressman who introduced <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro Standard Time</a> preferred "ChST", so lower-case letters are now allowed. Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>', '<code>+</code>', and alphanumeric characters from the portable character set in the current locale. In practice ASCII alphanumerics and '<code>+</code>' and '<code>-</code>' are safe in all locales.
80d0322018-01-29Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <p> In other words, in the C locale the POSIX extended regular expression <code>[-+[:alnum:]]{3,6}</code> should match the abbreviation. This guarantees that all abbreviations could have been specified by a POSIX <code>TZ</code> string. </p> </li> <li> Use abbreviations that are in common use among English-speakers, e.g., 'EST' for Eastern Standard Time in North America. We assume that applications translate them to other languages as part of the normal localization process; for example, a French application might translate 'EST' to 'HNE'.
80d0322018-01-29Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <p> <small>These abbreviations (for standard/daylight/etc. time) are: ACST/ACDT Australian Central, AST/ADT/APT/AWT/ADDT Atlantic, AEST/AEDT Australian Eastern, AHST/AHDT Alaska-Hawaii, AKST/AKDT Alaska, AWST/AWDT Australian Western, BST/BDT Bering, CAT/CAST Central Africa, CET/CEST/CEMT Central European, ChST Chamorro, CST/CDT/CWT/CPT/CDDT Central [North America], CST/CDT China, GMT/BST/IST/BDST Greenwich, EAT East Africa, EST/EDT/EWT/EPT/EDDT Eastern [North America], EET/EEST Eastern European,
36e21c2019-01-08Henrik Grubbström (Grubba)  GST/GDT Guam,
aa9f722018-12-18Henrik Grubbström (Grubba)  HST/HDT/HWT/HPT Hawaii,
9395fe2018-04-14Henrik Grubbström (Grubba)  HKT/HKST Hong Kong, IST India, IST/GMT Irish, IST/IDT/IDDT Israel, JST/JDT Japan, KST/KDT Korea, MET/MEST Middle European (a backward-compatibility alias for Central European), MSK/MSD Moscow, MST/MDT/MWT/MPT/MDDT Mountain, NST/NDT/NWT/NPT/NDDT Newfoundland, NST/NDT/NWT/NPT Nome, NZMT/NZST New Zealand through 1945, NZST/NZDT New Zealand 1946&ndash;present, PKT/PKST Pakistan, PST/PDT/PWT/PPT/PDDT Pacific,
aa9f722018-12-18Henrik Grubbström (Grubba)  PST/PDT Philippine,
9395fe2018-04-14Henrik Grubbström (Grubba)  SAST South Africa, SST Samoa, WAT/WAST West Africa, WET/WEST/WEMT Western European, WIB Waktu Indonesia Barat, WIT Waktu Indonesia Timur, WITA Waktu Indonesia Tengah, YST/YDT/YWT/YPT/YDDT Yukon</small>. </p> </li> <li> <p> For times taken from a city's longitude, use the traditional <var>x</var>MT notation. The only abbreviation like this in current use is '<abbr>GMT</abbr>'. The others are for timestamps before 1960, except that Monrovia Mean Time persisted until 1972. Typically, numeric abbreviations (e.g., '<code>-</code>004430' for MMT) would cause trouble here, as the numeric strings would exceed the POSIX length limit. </p>
80d0322018-01-29Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <p> <small>These abbreviations are: AMT Amsterdam, Asunción, Athens; BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels, Bucharest; CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba; DMT Dublin/Dunsink; EMT Easter; FFMT Fort-de-France; FMT Funchal; GMT Greenwich; HMT Havana, Helsinki, Horta, Howrah; IMT Irkutsk, Istanbul; JMT Jerusalem; KMT Kaunas, Kiev, Kingston; LMT Lima, Lisbon, local, Luanda; MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo, Moratuwa, Moscow; PLMT Phù Liễn; PMT Paramaribo, Paris, Perm, Pontianak, Prague; PMMT Port Moresby; QMT Quito; RMT Rangoon, Riga, Rome; SDMT Santo Domingo; SJMT San José; SMT Santiago, Simferopol, Singapore, Stanley; TBMT Tbilisi; TMT Tallinn, Tehran; WMT Warsaw</small>. </p>
80d0322018-01-29Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <p> <small>A few abbreviations also follow the pattern that
aa9f722018-12-18Henrik Grubbström (Grubba)  <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
9395fe2018-04-14Henrik Grubbström (Grubba)  They are: CMT/BST for Calamarca Mean Time and Bolivian Summer Time 1890&ndash;1932, DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time 1880&ndash;1916, MMT/MST/MDST for Moscow 1880&ndash;1919, and RMT/LST for Riga Mean Time and Latvian Summer time 1880&ndash;1926. An extra-special case is SET for Swedish Time (<em>svensk normaltid</em>) 1879&ndash;1899, 3&deg; west of the Stockholm Observatory.</small> </p>
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use '<abbr>LMT</abbr>' for local mean time of locations before the introduction of standard time; see "<a href="#scope">Scope of the <code><abbr>tz</abbr></code> database</a>".
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  If there is no common English abbreviation, use numeric offsets like
aa9f722018-12-18Henrik Grubbström (Grubba)  <code>-</code>05 and <code>+</code>0530 that are generated
9395fe2018-04-14Henrik Grubbström (Grubba)  by <code>zic</code>'s <code>%z</code> notation.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use current abbreviations for older timestamps to avoid confusion. For example, in 1910 a common English abbreviation for time in central Europe was 'MEZ' (short for both "Middle European Zone" and for "Mitteleuropäische Zeit" in German). Nowadays 'CET' ("Central European Time") is more common in English, and the database uses 'CET' even for circa-1910 timestamps as this is less confusing for modern users and avoids the need for determining when 'CET' supplanted 'MEZ' in common usage.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  Use a consistent style in a timezone's history. For example, if a history tends to use numeric
9395fe2018-04-14Henrik Grubbström (Grubba)  abbreviations and a particular entry could go either way, use a numeric abbreviation.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Use <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a> (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for locations while uninhabited. The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in some sense undefined; this notation is derived from <a href="https://tools.ietf.org/html/rfc3339">Internet
aa9f722018-12-18Henrik Grubbström (Grubba)  <abbr title="Request For Comments">RFC</abbr> 3339</a>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <p> Application writers should note that these abbreviations are ambiguous
80d0322018-01-29Henrik Grubbström (Grubba) in practice: e.g., 'CST' means one thing in China and something else in North America, and 'IST' can refer to time in India, Ireland or
9395fe2018-04-14Henrik Grubbström (Grubba) Israel. To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
80d0322018-01-29Henrik Grubbström (Grubba) '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The <code><abbr>tz</abbr></code> database is not authoritative, and it surely has errors.
80d0322018-01-29Henrik Grubbström (Grubba) Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
af92cb2017-11-17Henrik Grubbström (Grubba) Users requiring authoritative data should consult national standards bodies and the references cited in the database's comments. </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The <code><abbr>tz</abbr></code> database predicts future timestamps, and current predictions will be incorrect after future governments change the rules. For example, if today someone schedules a meeting for 13:00 next October 1, Casablanca time, and tomorrow Morocco changes its daylight saving rules, software can mess up after the rule change if it blithely relies on conversions made before the change. </li> <li> The pre-1970 entries in this database cover only a tiny sliver of how clocks actually behaved; the vast majority of the necessary information was lost or never recorded.
aa9f722018-12-18Henrik Grubbström (Grubba)  Thousands more timezones would be needed if
9395fe2018-04-14Henrik Grubbström (Grubba)  the <code><abbr>tz</abbr></code> database's scope were extended to cover even just the known or guessed history of standard time; for example, the current single entry for France would need to split into dozens of entries, perhaps hundreds. And in most of the world even this approach would be misleading due to widespread disagreement or indifference about what times should be observed. In her 2015 book <cite><a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The Global Transformation of Time, 1870&ndash;1950</a></cite>, Vanessa Ogle writes "Outside of Europe and North America there was no system of time zones at all, often not even a stable landscape of mean times, prior to the middle decades of the twentieth century". See: Timothy Shenk, <a href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. </li> <li> Most of the pre-1970 data entries come from unreliable sources, often astrology books that lack citations and whose compilers evidently invented entries when the true facts were unknown, without reporting which entries were known and which were invented. These books often contradict each other or give implausible entries, and on the rare occasions when they are checked they are typically found to be incorrect. </li> <li> For the UK the <code><abbr>tz</abbr></code> database relies on years of first-class work done by Joseph Myers and others; see "<a href="https://www.polyomino.org.uk/british-time/">History of legal time in Britain</a>". Other countries are not done nearly as well. </li> <li> Sometimes, different people in the same city maintain clocks that differ significantly. Historically, railway time was used by railroad companies (which did not always agree with each other), church-clock time was used for birth certificates, etc. More recently, competing political groups might disagree about clock settings. Often this is merely common practice, but sometimes it is set by law. For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France was legally <abbr>UT</abbr> +00:09:21 outside train stations and <abbr>UT</abbr> +00:04:21 inside. Other examples include Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and Ürümqi to this day. </li> <li> Although a named location in the <code><abbr>tz</abbr></code> database stands for the containing region, its pre-1970 data entries are often accurate for only a small subset of that region. For example, <code>Europe/London</code> stands for the United Kingdom, but its pre-1847 times are valid only for locations that have London's exact meridian, and its 1847 transition to <abbr>GMT</abbr> is known to be valid only for the L&amp;NW and the Caledonian railways. </li> <li> The <code><abbr>tz</abbr></code> database does not record the
aa9f722018-12-18Henrik Grubbström (Grubba)  earliest time for which a timezone's
9395fe2018-04-14Henrik Grubbström (Grubba)  data entries are thereafter valid for every location in the region. For example, <code>Europe/London</code> is valid for all locations in its region after <abbr>GMT</abbr> was made the standard time, but the date of standardization (1880-08-02) is not in the <code><abbr>tz</abbr></code> database, other than in commentary.
aa9f722018-12-18Henrik Grubbström (Grubba)  For many timezones the earliest time of
9395fe2018-04-14Henrik Grubbström (Grubba)  validity is unknown. </li> <li> The <code><abbr>tz</abbr></code> database does not record a region's boundaries, and in many cases the boundaries are not known.
aa9f722018-12-18Henrik Grubbström (Grubba)  For example, the timezone
9395fe2018-04-14Henrik Grubbström (Grubba)  <code>America/Kentucky/Louisville</code> represents a region around the city of Louisville, the boundaries of which are unclear. </li> <li> Changes that are modeled as instantaneous transitions in the <code><abbr>tz</abbr></code> database were often spread out over hours, days, or even decades. </li> <li> Even if the time is specified by law, locations sometimes deliberately flout the law. </li> <li> Early timekeeping practices, even assuming perfect clocks, were often not specified to the accuracy that the <code><abbr>tz</abbr></code> database requires. </li> <li> Sometimes historical timekeeping was specified more precisely than what the <code><abbr>tz</abbr></code> code can handle. For example, from 1909 to 1937 <a href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm" hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean Time (estimated to be <abbr>UT</abbr> +00:19:32.13), but the <code><abbr>tz</abbr></code> code cannot represent the fractional second. In practice these old specifications were rarely if ever implemented to subsecond precision. </li> <li> Even when all the timestamp transitions recorded by the <code><abbr>tz</abbr></code> database are correct, the <code><abbr>tz</abbr></code> rules that generate them may not faithfully reflect the historical rules. For example, from 1922 until World War II the UK moved clocks forward the day following the third Saturday in April unless that was Easter, in which case it moved clocks forward the previous Sunday. Because the <code><abbr>tz</abbr></code> database has no way to specify Easter, these exceptional years are entered as separate <code><abbr>tz</abbr> Rule</code> lines, even though the legal rules did not change.
aa9f722018-12-18Henrik Grubbström (Grubba)  When transitions are known but the historical rules behind them are not, the database contains <code>Zone</code> and <code>Rule</code> entries that are intended to represent only the generated transitions, not any underlying historical rules; however, this intent is recorded at best only in commentary.
9395fe2018-04-14Henrik Grubbström (Grubba)  </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  The <code><abbr>tz</abbr></code> database models time
9395fe2018-04-14Henrik Grubbström (Grubba)  using the <a href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
aa9f722018-12-18Henrik Grubbström (Grubba)  Gregorian calendar</a> with days containing 24 equal-length hours numbered 00 through 23, except when clock transitions occur. Pre-standard time is modeled as local mean time. However, historically many people used other calendars and other timescales.
9395fe2018-04-14Henrik Grubbström (Grubba)  For example, the Roman Empire used the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian calendar</a>, and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman timekeeping</a> had twelve varying-length daytime hours with a non-hour-based system at night.
aa9f722018-12-18Henrik Grubbström (Grubba)  And even today, some local practices diverge from the Gregorian calendar with 24-hour days. These divergences range from relatively minor, such as Japanese bars giving times like "24:30" for the wee hours of the morning, to more-significant differences such as <a href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the east African practice of starting the day at dawn</a>, renumbering the Western 06:00 to be 12:00. These practices are largely outside the scope of the <code><abbr>tz</abbr></code> code and data, which provide only limited support for date and time localization such as that required by POSIX. If DST is not used a different time zone can often do the trick; for example, in Kenya a <code>TZ</code> setting like <code>&lt;-03&gt;3</code> or <code>America/Cayenne</code> starts the day six hours later than <code>Africa/Nairobi</code> does.
9395fe2018-04-14Henrik Grubbström (Grubba)  </li> <li> Early clocks were less reliable, and data entries do not represent clock error. </li> <li> The <code><abbr>tz</abbr></code> database assumes Universal Time (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not standardized for older timestamps. In the <code><abbr>tz</abbr></code> database commentary, <abbr>UT</abbr> denotes a family of time standards that includes Coordinated Universal Time (<abbr>UTC</abbr>) along with other variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>, with days starting at midnight. Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern timestamps, <abbr>UTC</abbr> was not defined until 1960, so commentary uses the more-general abbreviation <abbr>UT</abbr> for timestamps that might predate 1960. Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly, and since pre-1972 <abbr>UTC</abbr> seconds varied in length, interpretation of older timestamps can be problematic when subsecond accuracy is needed. </li> <li>
01f1de2018-06-04Henrik Grubbström (Grubba)  Civil time was not based on atomic time before 1972, and we do not
9395fe2018-04-14Henrik Grubbström (Grubba)  know the history of <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's rotation</a> accurately enough to map <a href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr title="International System of Units">SI</abbr></a> seconds to historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a> to more than about one-hour accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY.
aa9f722018-12-18Henrik Grubbström (Grubba)  <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
9395fe2018-04-14Henrik Grubbström (Grubba)  the Earth's rotation: 720 BC to AD 2015</a>. <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. Also see: Espenak F. <a href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty in Delta T (ΔT)</a>. </li> <li> The relationship between POSIX time (that is, <abbr>UTC</abbr> but ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972. Although the POSIX clock officially stops during an inserted leap second, at least one proposed standard has it jumping back a second instead; and in practice POSIX clocks more typically either progress glacially during a leap second, or are slightly slowed while near a leap second. </li> <li> The <code><abbr>tz</abbr></code> database does not represent how uncertain its information is. Ideally it would contain information about when data entries are incomplete or dicey. Partial temporal knowledge is a field of active research, though,
01f1de2018-06-04Henrik Grubbström (Grubba)  and it is not clear how to apply it here.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) In short, many, perhaps most, of the <code><abbr>tz</abbr></code> database's pre-1970 and future timestamps are either wrong or misleading. Any attempt to pass the <code><abbr>tz</abbr></code> database off as the definition of time should be unacceptable to anybody who cares about the facts. In particular, the <code><abbr>tz</abbr></code> database's <abbr>LMT</abbr> offsets should not be considered meaningful, and
aa9f722018-12-18Henrik Grubbström (Grubba) should not prompt creation of timezones
9395fe2018-04-14Henrik Grubbström (Grubba) merely because two locations differ in <abbr>LMT</abbr> or transitioned to standard time at different dates.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="functions">Time and date functions</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The <code><abbr>tz</abbr></code> code contains time and date functions that are upwards compatible with those of POSIX. Code compatible with this package is already <a href="tz-link.html#tzdb">part of many platforms</a>, where the primary use of this package is to update obsolete time-related files. To do this, you may need to compile the time zone compiler '<code>zic</code>' supplied with this package instead of using the system '<code>zic</code>', since the format of <code>zic</code>'s input is occasionally extended, and a platform may still be shipping an older <code>zic</code>.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba)  <h3 id="POSIX">POSIX properties and limitations</h3>
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li> <p>
9395fe2018-04-14Henrik Grubbström (Grubba)  In POSIX, time display in a process is controlled by the environment variable <code>TZ</code>. Unfortunately, the POSIX <code>TZ</code> string takes a form that is hard to describe and is error-prone in practice.
01f1de2018-06-04Henrik Grubbström (Grubba)  Also, POSIX <code>TZ</code> strings cannot deal with daylight
9395fe2018-04-14Henrik Grubbström (Grubba)  saving time rules not based on the Gregorian calendar (as in Iran), or with situations where more than two time zone abbreviations or <abbr>UT</abbr> offsets are used in an area.
af92cb2017-11-17Henrik Grubbström (Grubba)  </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba)  <p>
9395fe2018-04-14Henrik Grubbström (Grubba)  The POSIX <code>TZ</code> string takes the following form:
af92cb2017-11-17Henrik Grubbström (Grubba)  </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba)  <p>
9395fe2018-04-14Henrik Grubbström (Grubba)  <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>]]]
af92cb2017-11-17Henrik Grubbström (Grubba)  </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba)  <p>
9395fe2018-04-14Henrik Grubbström (Grubba)  where: </p>
af92cb2017-11-17Henrik Grubbström (Grubba)  <dl> <dt><var>std</var> and <var>dst</var></dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  are 3 or more characters specifying the standard
aa9f722018-12-18Henrik Grubbström (Grubba)  and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
9395fe2018-04-14Henrik Grubbström (Grubba)  Starting with POSIX.1-2001, <var>std</var> and <var>dst</var> may also be in a quoted form like '<code>&lt;+09&gt;</code>'; this allows "<code>+</code>" and "<code>-</code>" in the names.
af92cb2017-11-17Henrik Grubbström (Grubba)  </dd> <dt><var>offset</var></dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  is of the form '<code>[&plusmn;]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' and specifies the offset west of <abbr>UT</abbr>. '<var>hh</var>' may be a single digit; 0&le;<var>hh</var>&le;24. The default <abbr>DST</abbr> offset is one hour ahead of standard time.
af92cb2017-11-17Henrik Grubbström (Grubba)  </dd> <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  specifies the beginning and end of <abbr>DST</abbr>. If this is absent, the system supplies its own ruleset for <abbr>DST</abbr>, and its rules can differ from year to year; typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
af92cb2017-11-17Henrik Grubbström (Grubba)  </dd> <dt><var>time</var></dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  takes the form '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' and defaults to 02:00. This is the same format as the offset, except that a leading '<code>+</code>' or '<code>-</code>' is not allowed.
af92cb2017-11-17Henrik Grubbström (Grubba)  </dd> <dt><var>date</var></dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  takes one of the following forms:
af92cb2017-11-17Henrik Grubbström (Grubba)  <dl> <dt>J<var>n</var> (1&le;<var>n</var>&le;365)</dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  origin-1 day number not counting February 29 </dd>
af92cb2017-11-17Henrik Grubbström (Grubba)  <dt><var>n</var> (0&le;<var>n</var>&le;365)</dt><dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  origin-0 day number counting February 29 if present </dd> <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> (0[Sunday]&le;<var>d</var>&le;6[Saturday], 1&le;<var>n</var>&le;5, 1&le;<var>m</var>&le;12)</dt><dd> for the <var>d</var>th day of week <var>n</var> of month <var>m</var> of the year, where week 1 is the first week in which day <var>d</var> appears, and '<code>5</code>' stands for the last week in which day <var>d</var> appears (which may be either the 4th or 5th week). Typically, this is the only useful form; the <var>n</var> and <code>J</code><var>n</var> forms are rarely used.
af92cb2017-11-17Henrik Grubbström (Grubba)  </dd>
9395fe2018-04-14Henrik Grubbström (Grubba)  </dl> </dd> </dl>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <p> Here is an example POSIX <code>TZ</code> string for New Zealand after 2007. It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead of <abbr>UT</abbr>, and that daylight saving time (<abbr>NZDT</abbr>) is observed from September's last Sunday at 02:00 until April's first Sunday at 03:00: </p> <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> <p> This POSIX <code>TZ</code> string is hard to remember, and mishandles some timestamps before 2008. With this package you can use this instead: </p>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba)  <pre><code>TZ='Pacific/Auckland'</code></pre> </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  POSIX does not define the <abbr>DST</abbr> transitions for <code>TZ</code> values like
9395fe2018-04-14Henrik Grubbström (Grubba)  "<code>EST5EDT</code>".
aa9f722018-12-18Henrik Grubbström (Grubba)  Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules were used to interpret such values, but this meant that the <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each program that did time conversion. This meant that when <abbr>US</abbr> time conversion rules changed (as in the United States in 1987), all programs that did time conversion had to be
9395fe2018-04-14Henrik Grubbström (Grubba)  recompiled to ensure proper results. </li> <li> The <code>TZ</code> environment variable is process-global, which makes it hard to write efficient, thread-safe applications that
aa9f722018-12-18Henrik Grubbström (Grubba)  need access to multiple timezones.
9395fe2018-04-14Henrik Grubbström (Grubba)  </li> <li>
01f1de2018-06-04Henrik Grubbström (Grubba)  In POSIX, there is no tamper-proof way for a process to learn the
29a9d82019-08-10Henrik Grubbström (Grubba)  system's best idea of local (wall clock) time.
aa9f722018-12-18Henrik Grubbström (Grubba)  This is important for applications that an administrator wants
9395fe2018-04-14Henrik Grubbström (Grubba)  used only at certain times &ndash; without regard to whether the user has fiddled the <code>TZ</code> environment variable. While an administrator can "do everything in <abbr>UT</abbr>" to get around the problem, doing so is inconvenient and precludes
aa9f722018-12-18Henrik Grubbström (Grubba)  handling daylight saving time shifts &ndash; as might be required to limit phone calls to off-peak hours.
9395fe2018-04-14Henrik Grubbström (Grubba)  </li> <li> POSIX provides no convenient and efficient way to determine the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
aa9f722018-12-18Henrik Grubbström (Grubba)  timestamps, particularly for timezones
9395fe2018-04-14Henrik Grubbström (Grubba)  that do not fit into the POSIX model. </li> <li> POSIX requires that systems ignore leap seconds. </li> <li> The <code><abbr>tz</abbr></code> code attempts to support all the <code>time_t</code> implementations allowed by POSIX. The <code>time_t</code> type represents a nonnegative count of seconds since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds. In practice, <code>time_t</code> is usually a signed 64- or 32-bit integer; 32-bit signed <code>time_t</code> values stop working after 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these days typically use a signed 64-bit integer. Unsigned 32-bit integers are used on one or two platforms, and 36-bit and 40-bit integers are also used occasionally. Although earlier POSIX versions allowed <code>time_t</code> to be a
aa9f722018-12-18Henrik Grubbström (Grubba)  floating-point type, this was not supported by any practical system,
9395fe2018-04-14Henrik Grubbström (Grubba)  and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both require <code>time_t</code> to be an integer type. </li> </ul>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <h3 id="POSIX-extensions">Extensions to POSIX in the <code><abbr>tz</abbr></code> code</h3> <ul> <li> <p> The <code>TZ</code> environment variable is used in generating
aa9f722018-12-18Henrik Grubbström (Grubba)  the name of a file from which time-related information is read
9395fe2018-04-14Henrik Grubbström (Grubba)  (or is interpreted à la POSIX); <code>TZ</code> is no longer
aa9f722018-12-18Henrik Grubbström (Grubba)  constrained to be a string containing abbreviations and numeric data as described <a href="#POSIX">above</a>. The file's format is <dfn><abbr>TZif</abbr></dfn>,
59b4e02019-05-06Henrik Grubbström (Grubba)  a timezone information format that contains binary data; see <a href="https://tools.ietf.org/html/8536">Internet <abbr>RFC</abbr> 8536</a>.
9395fe2018-04-14Henrik Grubbström (Grubba)  The daylight saving time rules to be used for a
aa9f722018-12-18Henrik Grubbström (Grubba)  particular timezone are encoded in the <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>, Australian, and other rules to be encoded, and
9395fe2018-04-14Henrik Grubbström (Grubba)  allows for situations where more than two time zone abbreviations are used. </p> <p> It was recognized that allowing the <code>TZ</code> environment variable to take on values such as '<code>America/New_York</code>' might cause "old" programs (that expect <code>TZ</code> to have a certain form) to operate incorrectly; consideration was given to using some other environment variable (for example, <code>TIMEZONE</code>)
aa9f722018-12-18Henrik Grubbström (Grubba)  to hold the string used to generate the <abbr>TZif</abbr> file's name.
9395fe2018-04-14Henrik Grubbström (Grubba)  In the end, however, it was decided to continue using <code>TZ</code>: it is widely used for time zone purposes; separately maintaining both <code>TZ</code> and <code>TIMEZONE</code> seemed a nuisance; and systems where "new" forms of <code>TZ</code> might cause problems can simply
aa9f722018-12-18Henrik Grubbström (Grubba)  use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which can be used by "new" programs as well as by "old" programs that assume pre-POSIX <code>TZ</code> values.
9395fe2018-04-14Henrik Grubbström (Grubba)  </p>
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The code supports platforms with a <abbr>UT</abbr> offset member in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The code supports platforms with a time zone abbreviation member in <code>struct tm</code>, e.g., <code>tm_zone</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Functions <code>tzalloc</code>, <code>tzfree</code>, <code>localtime_rz</code>, and <code>mktime_z</code> for more-efficient thread-safe applications that need to use multiple
aa9f722018-12-18Henrik Grubbström (Grubba)  timezones.
9395fe2018-04-14Henrik Grubbström (Grubba)  The <code>tzalloc</code> and <code>tzfree</code> functions allocate and free objects of type <code>timezone_t</code>, and <code>localtime_rz</code> and <code>mktime_z</code> are like <code>localtime_r</code> and <code>mktime</code> with an extra <code>timezone_t</code> argument. The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  A function <code>tzsetwall</code> has been added to arrange for the
29a9d82019-08-10Henrik Grubbström (Grubba)  system's best approximation to local (wall clock) time to be delivered
9395fe2018-04-14Henrik Grubbström (Grubba)  by subsequent calls to <code>localtime</code>.
29a9d82019-08-10Henrik Grubbström (Grubba)  Source code for portable applications that "must" run on local time should call <code>tzsetwall</code>;
01f1de2018-06-04Henrik Grubbström (Grubba)  if such code is moved to "old" systems that do not provide <code>tzsetwall</code>, you will not be able to generate an
9395fe2018-04-14Henrik Grubbström (Grubba)  executable program.
29a9d82019-08-10Henrik Grubbström (Grubba)  (These functions also arrange for local time to
9395fe2018-04-14Henrik Grubbström (Grubba)  be used if <code>tzset</code> is called &ndash; directly or
01f1de2018-06-04Henrik Grubbström (Grubba)  indirectly &ndash; and there is no <code>TZ</code> environment
9395fe2018-04-14Henrik Grubbström (Grubba)  variable; portable applications should not, however, rely on this
01f1de2018-06-04Henrik Grubbström (Grubba)  behavior since it is not the way <a href="https://en.wikipedia.org/wiki/UNIX_System_V#SVR2"><abbr>SVR2</abbr></a> systems behave.)
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Negative <code>time_t</code> values are supported, on systems where <code>time_t</code> is signed.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  These functions can account for leap seconds, thanks to Bradley White.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba)  <h3 id="vestigial">POSIX features no longer needed</h3>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a> define some <a href="https://en.wikipedia.org/wiki/API"><abbr title="application programming interface">API</abbr>s</a> that are vestigial: they are not needed, and are relics of a too-simple model that does not suffice to handle many real-world timestamps. Although the <code><abbr>tz</abbr></code> code supports these vestigial <abbr>API</abbr>s for backwards compatibility, they should be avoided in portable applications. The vestigial <abbr>API</abbr>s are:
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The POSIX <code>tzname</code> variable does not suffice and is no longer needed. To get a timestamp's time zone abbreviation, consult the <code>tm_zone</code> member if available; otherwise, use <code>strftime</code>'s <code>"%Z"</code> conversion specification. </li> <li> The POSIX <code>daylight</code> and <code>timezone</code> variables do not suffice and are no longer needed. To get a timestamp's <abbr>UT</abbr> offset, consult the <code>tm_gmtoff</code> member if available; otherwise, subtract values returned by <code>localtime</code> and <code>gmtime</code> using the rules of the Gregorian calendar, or use <code>strftime</code>'s <code>"%z"</code> conversion specification if a string like <code>"+0900"</code> suffices. </li> <li> The <code>tm_isdst</code> member is almost never needed and most of its uses should be discouraged in favor of the abovementioned <abbr>API</abbr>s. Although it can still be used in arguments to <code>mktime</code> to disambiguate timestamps near a <abbr>DST</abbr> transition when the clock jumps back, this disambiguation does not work when standard time itself jumps back, which can occur when a location changes to a time zone with a lesser <abbr>UT</abbr> offset. </li>
af92cb2017-11-17Henrik Grubbström (Grubba) </ul>
9395fe2018-04-14Henrik Grubbström (Grubba)  <h3 id="other-portability">Other portability notes</h3>
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition UNIX</a> <code>timezone</code> function is not present in this
01f1de2018-06-04Henrik Grubbström (Grubba)  package; it is impossible to reliably map <code>timezone</code>'s
9395fe2018-04-14Henrik Grubbström (Grubba)  arguments (a "minutes west of <abbr>GMT</abbr>" value and a "daylight saving time in effect" flag) to a time zone abbreviation, and we refuse to guess. Programs that in the past used the <code>timezone</code> function may now examine <code>localtime(&amp;clock)-&gt;tm_zone</code> (if <code>TM_ZONE</code> is defined) or <code>tzname[localtime(&amp;clock)-&gt;tm_isdst]</code> (if <code>HAVE_TZNAME</code> is defined) to learn the correct time zone abbreviation to use. </li> <li>
01f1de2018-06-04Henrik Grubbström (Grubba)  The <a href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a> <code>gettimeofday</code> function is not
9395fe2018-04-14Henrik Grubbström (Grubba)  used in this package. This formerly let users obtain the current <abbr>UTC</abbr> offset and <abbr>DST</abbr> flag, but this functionality was removed in later versions of <abbr>BSD</abbr>. </li> <li> In <abbr>SVR2</abbr>, time conversion fails for near-minimum or near-maximum <code>time_t</code> values when doing conversions
01f1de2018-06-04Henrik Grubbström (Grubba)  for places that do not use <abbr>UT</abbr>.
9395fe2018-04-14Henrik Grubbström (Grubba)  This package takes care to do these conversions correctly. A comment in the source code tells how to get compatibly wrong results. </li> <li> The functions that are conditionally compiled if <code>STD_INSPIRED</code> is defined should, at this point, be looked on primarily as food for thought. They are not in any sense "standard compatible" &ndash; some are not, in fact, specified in <em>any</em> standard. They do, however, represent responses of various authors to standardization proposals. </li> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  Other time conversion proposals, in particular those supported by the <a href="https://howardhinnant.github.io/date/tz.html">Time Zone Database Parser</a>, offer a wider selection of functions
9395fe2018-04-14Henrik Grubbström (Grubba)  that provide capabilities beyond those provided here. The absence of such functions from this package is not meant to discourage the development, standardization, or use of such functions. Rather, their absence reflects the decision to make this package contain valid extensions to POSIX, to ensure its broad acceptability. If more powerful time conversion functions can be standardized, so much the better.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="stability">Interface stability</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <ul> <li>
aa9f722018-12-18Henrik Grubbström (Grubba)  A set of timezone names as per
59b4e02019-05-06Henrik Grubbström (Grubba)  "<a href="#naming">Timezone identifiers</a>" above.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Library functions described in "<a href="#functions">Time and date functions</a>" above.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The programs <code>tzselect</code>, <code>zdump</code>, and <code>zic</code>, documented in their man pages.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The format of <code>zic</code> input files, documented in the <code>zic</code> man page.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The format of <code>zic</code> output files, documented in the <code>tzfile</code> man page.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The format of zone table files, documented in <code>zone1970.tab</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The format of the country code file, documented in <code>iso3166.tab</code>.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  The version number of the code and data, as the first line of the text file '<code>version</code>' in each release.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <p> Interface changes in a release attempt to preserve compatibility with
9395fe2018-04-14Henrik Grubbström (Grubba) recent releases. For example, <code><abbr>tz</abbr></code> data files typically do not rely on recently-added <code>zic</code> features, so that users can run older <code>zic</code> versions to process newer data files. <a href="tz-link.html#download">Downloading the <code><abbr>tz</abbr></code> database</a> describes how releases are tagged and distributed.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Interfaces not listed above are less stable. For example, users should not rely on particular <abbr>UT</abbr> offsets or abbreviations for timestamps, as data entries are often based on guesswork and these guesses may be corrected or improved.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
59b4e02019-05-06Henrik Grubbström (Grubba)  <p> Timezone boundaries are not part of the stable interface. For example, even though the <samp>Asia/Bangkok</samp> timezone currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part of the stable interface and the timezone can split at any time. If a calendar application records a future event in some location other than Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record, the application should be robust in the presence of timezone splits between now and the future time. </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="calendar">Calendrical issues</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p> Calendrical issues are a bit out of scope for a time zone database, but they indicate the sort of problems that we would run into if we
9395fe2018-04-14Henrik Grubbström (Grubba) extended the time zone database further into the past.
01f1de2018-06-04Henrik Grubbström (Grubba) An excellent resource in this area is Edward M. Reingold and Nachum Dershowitz, <cite><a href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
9395fe2018-04-14Henrik Grubbström (Grubba) Other information and sources are given in the file '<code>calendars</code>' in the <code><abbr>tz</abbr></code> distribution. They sometimes disagree.
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <section> <h2 id="planets">Time and time zones on other planets</h2>
af92cb2017-11-17Henrik Grubbström (Grubba) <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Some people's work schedules
aa9f722018-12-18Henrik Grubbström (Grubba) use <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
01f1de2018-06-04Henrik Grubbström (Grubba) Jet Propulsion Laboratory (JPL) coordinators kept Mars time on and off during the
36e21c2019-01-08Henrik Grubbström (Grubba) <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
9395fe2018-04-14Henrik Grubbström (Grubba) Pathfinder</a> mission.
01f1de2018-06-04Henrik Grubbström (Grubba) Some of their family members also adapted to Mars time.
9395fe2018-04-14Henrik Grubbström (Grubba) Dozens of special Mars watches were built for JPL workers who kept Mars time during the Mars Exploration Rovers mission (2004). These timepieces look like normal Seikos and Citizens but use Mars seconds rather than terrestrial seconds.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p> A Mars solar day is called a "sol" and has a mean period equal to
9395fe2018-04-14Henrik Grubbström (Grubba) about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is divided into a conventional 24-hour clock, so each Mars second equals about 1.02749125 terrestrial seconds.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime meridian</a> of Mars goes through the center of the crater <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in honor of the British astronomer who built the Greenwich telescope that defines Earth's prime meridian. Mean solar time on the Mars prime meridian is
36e21c2019-01-08Henrik Grubbström (Grubba) called Mars Coordinated Time (<abbr>MTC</abbr>).
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p> Each landed mission on Mars has adopted a different reference for
aa9f722018-12-18Henrik Grubbström (Grubba) solar timekeeping, so there is no real standard for Mars time zones.
9395fe2018-04-14Henrik Grubbström (Grubba) For example, the <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars Exploration Rover</a> project (2004) defined two time zones "Local Solar Time A" and "Local Solar Time B" for its two missions, each zone designed so that its time equals local true solar time at approximately the middle of the nominal mission. Such a "time zone" is not particularly suited for any application other than the mission itself.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p> Many calendars have been proposed for Mars, but none have achieved
9395fe2018-04-14Henrik Grubbström (Grubba) wide acceptance. Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
af92cb2017-11-17Henrik Grubbström (Grubba) sequential count of Mars solar days elapsed since about 1873-12-29
9395fe2018-04-14Henrik Grubbström (Grubba) 12:00 <abbr>GMT</abbr>.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p> In our solar system, Mars is the planet with time and calendar most
9395fe2018-04-14Henrik Grubbström (Grubba) like Earth's. On other planets, Sun-based time and calendars would work quite differently. For example, although Mercury's <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal rotation period</a> is 58.646 Earth days, Mercury revolves around the Sun so rapidly that an observer on Mercury's equator would see a sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury day. Venus is more complicated, partly because its rotation is slightly <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>: its year is 1.92 of its days. Gas giants like Jupiter are trickier still, as their polar and equatorial regions rotate at different rates, so that the length of a day depends on latitude. This effect is most pronounced on Neptune, where the day is about 12 hours at the poles and 18 hours at the equator.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Although the <code><abbr>tz</abbr></code> database does not support time on other planets, it is documented here in the hopes that support will be added eventually.
af92cb2017-11-17Henrik Grubbström (Grubba) </p> <p>
9395fe2018-04-14Henrik Grubbström (Grubba) Sources for time on other planets:
af92cb2017-11-17Henrik Grubbström (Grubba) </p>
9395fe2018-04-14Henrik Grubbström (Grubba) 
af92cb2017-11-17Henrik Grubbström (Grubba) <ul> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Michael Allison and Robert Schmunk, "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
29a9d82019-08-10Henrik Grubbström (Grubba)  (2018-12-13).
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Jia-Rui Chong,
29a9d82019-08-10Henrik Grubbström (Grubba)  "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
9395fe2018-04-14Henrik Grubbström (Grubba)  Fit for a Martian</a>", <cite>Los Angeles Times</cite>
01f1de2018-06-04Henrik Grubbström (Grubba)  (2004-01-14), pp A1, A20&ndash;A21.
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Tom Chmielewski, "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> <li>
9395fe2018-04-14Henrik Grubbström (Grubba)  Matt Williams, "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How long is a day on the other planets of the solar system?</a>"
aa9f722018-12-18Henrik Grubbström (Grubba)  (2016-01-20).
af92cb2017-11-17Henrik Grubbström (Grubba)  </li> </ul>
9395fe2018-04-14Henrik Grubbström (Grubba) </section>
af92cb2017-11-17Henrik Grubbström (Grubba) 
9395fe2018-04-14Henrik Grubbström (Grubba) <footer> <hr> This file is in the public domain, so clarified as of 2009-05-17 by Arthur David Olson. </footer>
af92cb2017-11-17Henrik Grubbström (Grubba) </body> </html>