4 <title>Theory and pragmatics of the tz code and data</title>
7 pre {margin-left: 2em; white-space: pre-wrap;}
12 <h1>Theory and pragmatics of the <code><abbr>tz</abbr></code> code and data</h1>
16 <li><a href="#scope">Scope of the <code><abbr>tz</abbr></code>
18 <li><a href="#naming">Timezone identifiers</a></li>
19 <li><a href="#abbreviations">Time zone abbreviations</a></li>
20 <li><a href="#accuracy">Accuracy of the <code><abbr>tz</abbr></code>
22 <li><a href="#functions">Time and date functions</a></li>
23 <li><a href="#stability">Interface stability</a></li>
24 <li><a href="#leapsec">Leap seconds</a></li>
25 <li><a href="#calendar">Calendrical issues</a></li>
26 <li><a href="#planets">Time and time zones on other planets</a></li>
31 <h2 id="scope">Scope of the <code><abbr>tz</abbr></code> database</h2>
34 href="https://www.iana.org/time-zones"><code><abbr>tz</abbr></code>
35 database</a> attempts to record the history and predicted future of
36 all computer-based clocks that track civil time.
37 It organizes <a href="tz-link.html">time zone and daylight saving time
38 data</a> by partitioning the world into <a
39 href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones"><dfn>timezones</dfn></a>
40 whose clocks all agree about timestamps that occur after the <a
41 href="https://en.wikipedia.org/wiki/Unix_time">POSIX Epoch</a>
42 (1970-01-01 00:00:00 <a
43 href="https://en.wikipedia.org/wiki/Coordinated_Universal_Time"><abbr
44 title="Coordinated Universal Time">UTC</abbr></a>).
45 The database labels each timezone with a notable location and
46 records all known clock transitions for that location.
47 Although 1970 is a somewhat-arbitrary cutoff, there are significant
48 challenges to moving the cutoff earlier even by a decade or two, due
49 to the wide variety of local practices before computer timekeeping
54 Each timezone typically corresponds to a geographical region that is
55 smaller than a traditional time zone, because clocks in a timezone
56 all agree after 1970 whereas a traditional time zone merely
57 specifies current standard time. For example, applications that deal
58 with current and future timestamps in the traditional North
59 American mountain time zone can choose from the timezones
60 <code>America/Denver</code> which observes US-style daylight saving
61 time, <code>America/Mazatlan</code> which observes Mexican-style DST,
62 and <code>America/Phoenix</code> which does not observe DST.
63 Applications that also deal with past timestamps in the mountain time
64 zone can choose from over a dozen timezones, such as
65 <code>America/Boise</code>, <code>America/Edmonton</code>, and
66 <code>America/Hermosillo</code>, each of which currently uses mountain
67 time but differs from other timezones for some timestamps after 1970.
71 Clock transitions before 1970 are recorded for each timezone,
72 because most systems support timestamps before 1970 and could
73 misbehave if data entries were omitted for pre-1970 transitions.
74 However, the database is not designed for and does not suffice for
75 applications requiring accurate handling of all past times everywhere,
76 as it would take far too much effort and guesswork to record all
77 details of pre-1970 civil timekeeping.
78 Although some information outside the scope of the database is
79 collected in a file <code>backzone</code> that is distributed along
80 with the database proper, this file is less reliable and does not
81 necessarily follow database guidelines.
85 As described below, reference source code for using the
86 <code><abbr>tz</abbr></code> database is also available.
87 The <code><abbr>tz</abbr></code> code is upwards compatible with <a
88 href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
90 href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
91 As of this writing, the current edition of POSIX is: <a
92 href="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
93 Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
95 Because the database's scope encompasses real-world changes to civil
96 timekeeping, its model for describing time is more complex than the
97 standard and daylight saving times supported by POSIX.
98 A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
99 have more than two changes per year, these changes need not merely
100 flip back and forth between two alternatives, and the rules themselves
102 Whether and when a timezone changes its clock,
103 and even the timezone's notional base offset from <abbr>UTC</abbr>,
105 It does not always make sense to talk about a timezone's
106 "base offset", which is not necessarily a single number.
112 <h2 id="naming">Timezone identifiers</h2>
114 Each timezone has a name that uniquely identifies the timezone.
115 Inexperienced users are not expected to select these names unaided.
116 Distributors should provide documentation and/or a simple selection
117 interface that explains each name via a map or via descriptive text like
118 "Ruthenia" instead of the timezone name "<code>Europe/Uzhgorod</code>".
119 If geolocation information is available, a selection interface can
120 locate the user on a timezone map or prioritize names that are
121 geographically close. For an example selection interface, see the
122 <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
123 The <a href="http://cldr.unicode.org/">Unicode Common Locale Data
124 Repository</a> contains data that may be useful for other selection
125 interfaces; it maps timezone names like <code>Europe/Uzhgorod</code>
126 to CLDR names like <code>uauzh</code> which are in turn mapped to
127 locale-dependent strings like "Uzhhorod", "Ungvár", "Ужгород", and
132 The naming conventions attempt to strike a balance
133 among the following goals:
138 Uniquely identify every timezone where clocks have agreed since 1970.
139 This is essential for the intended use: static clocks keeping local
143 Indicate to experts where the timezone's clocks typically are.
146 Be robust in the presence of political changes.
147 For example, names are typically not tied to countries, to avoid
148 incompatibilities when countries change their name (e.g.,
149 Swaziland→Eswatini) or when locations change countries (e.g., Hong
150 Kong from UK colony to China).
151 There is no requirement that every country or national
152 capital must have a timezone name.
155 Be portable to a wide variety of implementations.
158 Use a consistent naming conventions over the entire world.
163 Names normally have the form
164 <var>AREA</var><code>/</code><var>LOCATION</var>, where
165 <var>AREA</var> is a continent or ocean, and
166 <var>LOCATION</var> is a specific location within the area.
167 North and South America share the same area, '<code>America</code>'.
168 Typical names are '<code>Africa/Cairo</code>',
169 '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
170 Some names are further qualified to help avoid confusion; for example,
171 '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
172 Indiana from other Petersburgs in America.
176 Here are the general guidelines used for
177 choosing timezone names,
178 in decreasing order of importance:
183 Use only valid POSIX file name components (i.e., the parts of
184 names other than '<code>/</code>').
185 Do not use the file name components '<code>.</code>' and
187 Within a file name component, use only <a
188 href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
189 '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
190 Do not use digits, as that might create an ambiguity with <a
191 href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
192 <code>TZ</code> strings</a>.
193 A file name component must not exceed 14 characters or start with
195 E.g., prefer <code>Asia/Brunei</code> to
196 <code>Asia/Bandar_Seri_Begawan</code>.
197 Exceptions: see the discussion of legacy names below.
200 A name must not be empty, or contain '<code>//</code>', or
201 start or end with '<code>/</code>'.
204 Do not use names that differ only in case.
205 Although the reference implementation is case-sensitive, some
206 other implementations are not, and they would mishandle names
207 differing only in case.
210 If one name <var>A</var> is an initial prefix of another
211 name <var>AB</var> (ignoring case), then <var>B</var> must not
212 start with '<code>/</code>', as a regular file cannot have the
213 same name as a directory in POSIX.
214 For example, <code>America/New_York</code> precludes
215 <code>America/New_York/Bronx</code>.
218 Uninhabited regions like the North Pole and Bouvet Island
219 do not need locations, since local time is not defined there.
222 If all the clocks in a timezone have agreed since 1970,
223 do not bother to include more than one timezone
224 even if some of the clocks disagreed before 1970.
225 Otherwise these tables would become annoyingly large.
228 If boundaries between regions are fluid, such as during a war or
229 insurrection, do not bother to create a new timezone merely
230 because of yet another boundary change. This helps prevent table
231 bloat and simplifies maintenance.
234 If a name is ambiguous, use a less ambiguous alternative;
235 e.g., many cities are named San José and Georgetown, so
236 prefer <code>America/Costa_Rica</code> to
237 <code>America/San_Jose</code> and <code>America/Guyana</code>
238 to <code>America/Georgetown</code>.
241 Keep locations compact.
242 Use cities or small islands, not countries or regions, so that any
243 future changes do not split individual locations into different
245 E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
247 <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
248 has had multiple time zones</a>.
251 Use mainstream English spelling, e.g., prefer
252 <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
253 prefer <code>Europe/Athens</code> to the Greek
254 <code>Ευρώπη/Αθήνα</code> or the Romanized
255 <code>Evrópi/Athína</code>.
256 The POSIX file name restrictions encourage this guideline.
259 Use the most populous among locations in a region,
260 e.g., prefer <code>Asia/Shanghai</code> to
261 <code>Asia/Beijing</code>.
262 Among locations with similar populations, pick the best-known
263 location, e.g., prefer <code>Europe/Rome</code> to
264 <code>Europe/Milan</code>.
267 Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
268 <code>Atlantic/Canaries</code>.
271 Omit common suffixes like '<code>_Islands</code>' and
272 '<code>_City</code>', unless that would lead to ambiguity.
273 E.g., prefer <code>America/Cayman</code> to
274 <code>America/Cayman_Islands</code> and
275 <code>America/Guatemala</code> to
276 <code>America/Guatemala_City</code>, but prefer
277 <code>America/Mexico_City</code> to
278 <code>America/Mexico</code>
279 because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
280 country of Mexico has several time zones</a>.
283 Use '<code>_</code>' to represent a space.
286 Omit '<code>.</code>' from abbreviations in names.
287 E.g., prefer <code>Atlantic/St_Helena</code> to
288 <code>Atlantic/St._Helena</code>.
291 Do not change established names if they only marginally violate
292 the above guidelines.
293 For example, do not change the existing name <code>Europe/Rome</code> to
294 <code>Europe/Milan</code> merely because Milan's population has grown
295 to be somewhat greater than Rome's.
298 If a name is changed, put its old spelling in the
299 '<code>backward</code>' file.
300 This means old spellings will continue to work.
301 Ordinarily a name change should occur only in the rare case when
302 a location's consensus English-language spelling changes; for example,
303 in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
304 due to long-time widespread use of the new city name instead of the old.
309 Guidelines have evolved with time, and names following old versions of
310 these guidelines might not follow the current version. When guidelines
311 have changed, old names continue to be supported. Guideline changes
312 have included the following:
317 Older versions of this package used a different naming scheme.
318 See the file '<code>backward</code>' for most of these older names
319 (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
320 The other old-fashioned names still supported are
321 '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
322 '<code>EET</code>' (see the file '<code>europe</code>').
326 Older versions of this package defined legacy names that are
327 incompatible with the first guideline of location names, but which are
329 These legacy names are mostly defined in the file
330 '<code>etcetera</code>'.
331 Also, the file '<code>backward</code>' defines the legacy names
332 '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
333 and the file '<code>northamerica</code>' defines the legacy names
334 '<code>EST5EDT</code>', '<code>CST6CDT</code>',
335 '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
339 Older versions of these guidelines said that
340 there should typically be at least one name for each <a
341 href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
342 title="International Organization for Standardization">ISO</abbr>
343 3166-1</a> officially assigned two-letter code for an inhabited
344 country or territory.
345 This old guideline has been dropped, as it was not needed to handle
346 timestamps correctly and it increased maintenance burden.
351 The file '<code>zone1970.tab</code>' lists geographical locations used
353 It is intended to be an exhaustive list of names for geographic
354 regions as described above; this is a subset of the timezones in the data.
355 Although a '<code>zone1970.tab</code>' location's
356 <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
358 its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
359 time (<abbr>LMT</abbr>)</a> offset with one hour for every 15°
360 east longitude, this relationship is not exact.
364 Excluding '<code>backward</code>' should not affect the other data.
365 If '<code>backward</code>' is excluded, excluding
366 '<code>etcetera</code>' should not affect the remaining data.
371 <h2 id="abbreviations">Time zone abbreviations</h2>
373 When this package is installed, it generates time zone abbreviations
374 like '<code>EST</code>' to be compatible with human tradition and POSIX.
375 Here are the general guidelines used for choosing time zone abbreviations,
376 in decreasing order of importance:
381 Use three to six characters that are ASCII alphanumerics or
382 '<code>+</code>' or '<code>-</code>'.
383 Previous editions of this database also used characters like
384 space and '<code>?</code>', but these characters have a
385 special meaning to the
386 <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
387 and cause commands like
388 '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
389 `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
390 to have unexpected effects.
391 Previous editions of this guideline required upper-case letters, but the
392 Congressman who introduced
393 <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
394 Standard Time</a> preferred "ChST", so lower-case letters are now
396 Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
397 '<code>+</code>', and alphanumeric characters from the portable
398 character set in the current locale.
399 In practice ASCII alphanumerics and '<code>+</code>' and
400 '<code>-</code>' are safe in all locales.
403 In other words, in the C locale the POSIX extended regular
404 expression <code>[-+[:alnum:]]{3,6}</code> should match the
406 This guarantees that all abbreviations could have been specified by a
407 POSIX <code>TZ</code> string.
411 Use abbreviations that are in common use among English-speakers,
412 e.g., 'EST' for Eastern Standard Time in North America.
413 We assume that applications translate them to other languages
414 as part of the normal localization process; for example,
415 a French application might translate 'EST' to 'HNE'.
418 <small>These abbreviations (for standard/daylight/etc. time) are:
419 ACST/ACDT Australian Central,
420 AST/ADT/APT/AWT/ADDT Atlantic,
421 AEST/AEDT Australian Eastern,
422 AHST/AHDT Alaska-Hawaii,
424 AWST/AWDT Australian Western,
426 CAT/CAST Central Africa,
427 CET/CEST/CEMT Central European,
429 CST/CDT/CWT/CPT/CDDT Central [North America],
431 GMT/BST/IST/BDST Greenwich,
433 EST/EDT/EWT/EPT/EDDT Eastern [North America],
434 EET/EEST Eastern European,
436 HST/HDT/HWT/HPT Hawaii,
437 HKT/HKST/HKWT Hong Kong,
443 MET/MEST Middle European (a backward-compatibility alias for
446 MST/MDT/MWT/MPT/MDDT Mountain,
447 NST/NDT/NWT/NPT/NDDT Newfoundland,
448 NST/NDT/NWT/NPT Nome,
449 NZMT/NZST New Zealand through 1945,
450 NZST/NZDT New Zealand 1946–present,
452 PST/PDT/PWT/PPT/PDDT Pacific,
456 WAT/WAST West Africa,
457 WET/WEST/WEMT Western European,
458 WIB Waktu Indonesia Barat,
459 WIT Waktu Indonesia Timur,
460 WITA Waktu Indonesia Tengah,
461 YST/YDT/YWT/YPT/YDDT Yukon</small>.
466 For times taken from a city's longitude, use the
467 traditional <var>x</var>MT notation.
468 The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
469 The others are for timestamps before 1960,
470 except that Monrovia Mean Time persisted until 1972.
471 Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
472 MMT) would cause trouble here, as the numeric strings would exceed
473 the POSIX length limit.
477 <small>These abbreviations are:
478 AMT Amsterdam, Asunción, Athens;
479 BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels,
481 CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba;
487 HMT Havana, Helsinki, Horta, Howrah;
488 IMT Irkutsk, Istanbul;
490 KMT Kaunas, Kiev, Kingston;
491 LMT Lima, Lisbon, local, Luanda;
492 MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo,
495 PMT Paramaribo, Paris, Perm, Pontianak, Prague;
498 RMT Rangoon, Riga, Rome;
501 SMT Santiago, Simferopol, Singapore, Stanley;
508 <small>A few abbreviations also follow the pattern that
509 <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
511 CMT/BST for Calamarca Mean Time and Bolivian Summer Time
513 DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
515 MMT/MST/MDST for Moscow 1880–1919, and
516 RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926.
517 An extra-special case is SET for Swedish Time (<em>svensk
518 normaltid</em>) 1879–1899, 3° west of the Stockholm
523 Use '<abbr>LMT</abbr>' for local mean time of locations before the
524 introduction of standard time; see "<a href="#scope">Scope of the
525 <code><abbr>tz</abbr></code> database</a>".
528 If there is no common English abbreviation, use numeric offsets like
529 <code>-</code>05 and <code>+</code>0530 that are generated
530 by <code>zic</code>'s <code>%z</code> notation.
533 Use current abbreviations for older timestamps to avoid confusion.
534 For example, in 1910 a common English abbreviation for time
535 in central Europe was 'MEZ' (short for both "Middle European
536 Zone" and for "Mitteleuropäische Zeit" in German).
537 Nowadays 'CET' ("Central European Time") is more common in
538 English, and the database uses 'CET' even for circa-1910
539 timestamps as this is less confusing for modern users and avoids
540 the need for determining when 'CET' supplanted 'MEZ' in common
544 Use a consistent style in a timezone's history.
545 For example, if a history tends to use numeric
546 abbreviations and a particular entry could go either way, use a
547 numeric abbreviation.
551 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
552 (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
553 locations while uninhabited.
554 The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
555 some sense undefined; this notation is derived
556 from <a href="https://tools.ietf.org/html/rfc3339">Internet
557 <abbr title="Request For Comments">RFC</abbr> 3339</a>.
562 Application writers should note that these abbreviations are ambiguous
563 in practice: e.g., 'CST' means one thing in China and something else
564 in North America, and 'IST' can refer to time in India, Ireland or
566 To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
567 '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
572 <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
574 The <code><abbr>tz</abbr></code> database is not authoritative, and it
576 Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
577 Users requiring authoritative data should consult national standards
578 bodies and the references cited in the database's comments.
582 Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
587 The <code><abbr>tz</abbr></code> database predicts future
588 timestamps, and current predictions
589 will be incorrect after future governments change the rules.
590 For example, if today someone schedules a meeting for 13:00 next
591 October 1, Casablanca time, and tomorrow Morocco changes its
592 daylight saving rules, software can mess up after the rule change
593 if it blithely relies on conversions made before the change.
596 The pre-1970 entries in this database cover only a tiny sliver of how
597 clocks actually behaved; the vast majority of the necessary
598 information was lost or never recorded.
599 Thousands more timezones would be needed if
600 the <code><abbr>tz</abbr></code> database's scope were extended to
601 cover even just the known or guessed history of standard time; for
602 example, the current single entry for France would need to split
603 into dozens of entries, perhaps hundreds.
604 And in most of the world even this approach would be misleading
605 due to widespread disagreement or indifference about what times
609 href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
610 Global Transformation of Time, 1870–1950</a></cite>,
612 "Outside of Europe and North America there was no system of time
613 zones at all, often not even a stable landscape of mean times,
614 prior to the middle decades of the twentieth century".
615 See: Timothy Shenk, <a
616 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
617 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
620 Most of the pre-1970 data entries come from unreliable sources, often
621 astrology books that lack citations and whose compilers evidently
622 invented entries when the true facts were unknown, without
623 reporting which entries were known and which were invented.
624 These books often contradict each other or give implausible entries,
625 and on the rare occasions when they are checked they are
626 typically found to be incorrect.
629 For the UK the <code><abbr>tz</abbr></code> database relies on
630 years of first-class work done by
631 Joseph Myers and others; see
632 "<a href="https://www.polyomino.org.uk/british-time/">History of
633 legal time in Britain</a>".
634 Other countries are not done nearly as well.
637 Sometimes, different people in the same city maintain clocks
638 that differ significantly.
639 Historically, railway time was used by railroad companies (which
641 agree with each other), church-clock time was used for birth
643 More recently, competing political groups might disagree about
644 clock settings. Often this is merely common practice, but
645 sometimes it is set by law.
646 For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
647 was legally <abbr>UT</abbr> +00:09:21 outside train stations and
648 <abbr>UT</abbr> +00:04:21 inside. Other examples include
649 Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
653 Although a named location in the <code><abbr>tz</abbr></code>
654 database stands for the containing region, its pre-1970 data
655 entries are often accurate for only a small subset of that region.
656 For example, <code>Europe/London</code> stands for the United
657 Kingdom, but its pre-1847 times are valid only for locations that
658 have London's exact meridian, and its 1847 transition
659 to <abbr>GMT</abbr> is known to be valid only for the L&NW and
660 the Caledonian railways.
663 The <code><abbr>tz</abbr></code> database does not record the
664 earliest time for which a timezone's
665 data entries are thereafter valid for every location in the region.
666 For example, <code>Europe/London</code> is valid for all locations
667 in its region after <abbr>GMT</abbr> was made the standard time,
668 but the date of standardization (1880-08-02) is not in the
669 <code><abbr>tz</abbr></code> database, other than in commentary.
670 For many timezones the earliest time of
674 The <code><abbr>tz</abbr></code> database does not record a
675 region's boundaries, and in many cases the boundaries are not known.
676 For example, the timezone
677 <code>America/Kentucky/Louisville</code> represents a region
678 around the city of Louisville, the boundaries of which are
682 Changes that are modeled as instantaneous transitions in the
683 <code><abbr>tz</abbr></code>
684 database were often spread out over hours, days, or even decades.
687 Even if the time is specified by law, locations sometimes
688 deliberately flout the law.
691 Early timekeeping practices, even assuming perfect clocks, were
692 often not specified to the accuracy that the
693 <code><abbr>tz</abbr></code> database requires.
696 Sometimes historical timekeeping was specified more precisely
697 than what the <code><abbr>tz</abbr></code> code can handle.
698 For example, from 1909 to 1937 <a
699 href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
700 hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean
701 Time (estimated to be <abbr>UT</abbr>
702 +00:19:32.13), but the <code><abbr>tz</abbr></code>
703 code cannot represent the fractional second.
704 In practice these old specifications were rarely if ever
705 implemented to subsecond precision.
708 Even when all the timestamp transitions recorded by the
709 <code><abbr>tz</abbr></code> database are correct, the
710 <code><abbr>tz</abbr></code> rules that generate them may not
711 faithfully reflect the historical rules.
712 For example, from 1922 until World War II the UK moved clocks
713 forward the day following the third Saturday in April unless that
714 was Easter, in which case it moved clocks forward the previous
716 Because the <code><abbr>tz</abbr></code> database has no
717 way to specify Easter, these exceptional years are entered as
718 separate <code><abbr>tz</abbr> Rule</code> lines, even though the
719 legal rules did not change.
720 When transitions are known but the historical rules behind them are not,
721 the database contains <code>Zone</code> and <code>Rule</code>
722 entries that are intended to represent only the generated
723 transitions, not any underlying historical rules; however, this
724 intent is recorded at best only in commentary.
727 The <code><abbr>tz</abbr></code> database models time
729 href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
730 Gregorian calendar</a> with days containing 24 equal-length hours
731 numbered 00 through 23, except when clock transitions occur.
732 Pre-standard time is modeled as local mean time.
733 However, historically many people used other calendars and other timescales.
734 For example, the Roman Empire used
735 the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
737 and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
738 timekeeping</a> had twelve varying-length daytime hours with a
739 non-hour-based system at night.
740 And even today, some local practices diverge from the Gregorian
741 calendar with 24-hour days. These divergences range from
742 relatively minor, such as Japanese bars giving times like "24:30" for the
743 wee hours of the morning, to more-significant differences such as <a
744 href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the
745 east African practice of starting the day at dawn</a>, renumbering
746 the Western 06:00 to be 12:00. These practices are largely outside
747 the scope of the <code><abbr>tz</abbr></code> code and data, which
748 provide only limited support for date and time localization
749 such as that required by POSIX. If DST is not used a different time zone
750 can often do the trick; for example, in Kenya a <code>TZ</code> setting
751 like <code><-03>3</code> or <code>America/Cayenne</code> starts
752 the day six hours later than <code>Africa/Nairobi</code> does.
755 Early clocks were less reliable, and data entries do not represent
759 The <code><abbr>tz</abbr></code> database assumes Universal Time
760 (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
761 standardized for older timestamps.
762 In the <code><abbr>tz</abbr></code> database commentary,
763 <abbr>UT</abbr> denotes a family of time standards that includes
764 Coordinated Universal Time (<abbr>UTC</abbr>) along with other
765 variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
766 with days starting at midnight.
767 Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
768 timestamps, <abbr>UTC</abbr> was not defined until 1960, so
769 commentary uses the more-general abbreviation <abbr>UT</abbr> for
770 timestamps that might predate 1960.
771 Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
772 and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
773 interpretation of older timestamps can be problematic when
774 subsecond accuracy is needed.
777 Civil time was not based on atomic time before 1972, and we do not
779 <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
780 rotation</a> accurately enough to map <a
781 href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
782 title="International System of Units">SI</abbr></a> seconds to
783 historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
784 to more than about one-hour accuracy.
785 See: Stephenson FR, Morrison LV, Hohenkerk CY.
786 <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
787 the Earth's rotation: 720 BC to AD 2015</a>.
788 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
789 Also see: Espenak F. <a
790 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
794 The relationship between POSIX time (that is, <abbr>UTC</abbr> but
795 ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
796 seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
798 clock officially stops during an inserted leap second, at least one
799 proposed standard has it jumping back a second instead; and in
800 practice POSIX clocks more typically either progress glacially during
801 a leap second, or are slightly slowed while near a leap second.
804 The <code><abbr>tz</abbr></code> database does not represent how
805 uncertain its information is.
806 Ideally it would contain information about when data entries are
808 Partial temporal knowledge is a field of active research, though,
809 and it is not clear how to apply it here.
814 In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
815 database's pre-1970 and future timestamps are either wrong or
817 Any attempt to pass the
818 <code><abbr>tz</abbr></code> database off as the definition of time
819 should be unacceptable to anybody who cares about the facts.
820 In particular, the <code><abbr>tz</abbr></code> database's
821 <abbr>LMT</abbr> offsets should not be considered meaningful, and
822 should not prompt creation of timezones
823 merely because two locations
824 differ in <abbr>LMT</abbr> or transitioned to standard time at
830 <h2 id="functions">Time and date functions</h2>
832 The <code><abbr>tz</abbr></code> code contains time and date functions
833 that are upwards compatible with those of POSIX.
834 Code compatible with this package is already
835 <a href="tz-link.html#tzdb">part of many platforms</a>, where the
836 primary use of this package is to update obsolete time-related files.
837 To do this, you may need to compile the time zone compiler
838 '<code>zic</code>' supplied with this package instead of using the
839 system '<code>zic</code>', since the format of <code>zic</code>'s
840 input is occasionally extended, and a platform may still be shipping
841 an older <code>zic</code>.
844 <h3 id="POSIX">POSIX properties and limitations</h3>
848 In POSIX, time display in a process is controlled by the
849 environment variable <code>TZ</code>.
850 Unfortunately, the POSIX
851 <code>TZ</code> string takes a form that is hard to describe and
852 is error-prone in practice.
853 Also, POSIX <code>TZ</code> strings cannot deal with daylight
854 saving time rules not based on the Gregorian calendar (as in
855 Iran), or with situations where more than two time zone
856 abbreviations or <abbr>UT</abbr> offsets are used in an area.
860 The POSIX <code>TZ</code> string takes the following form:
864 <var>stdoffset</var>[<var>dst</var>[<var>offset</var>][<code>,</code><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]]]
872 <dt><var>std</var> and <var>dst</var></dt><dd>
873 are 3 or more characters specifying the standard
874 and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
875 Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
876 may also be in a quoted form like '<code><+09></code>';
877 this allows "<code>+</code>" and "<code>-</code>" in the names.
879 <dt><var>offset</var></dt><dd>
881 '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
882 and specifies the offset west of <abbr>UT</abbr>.
883 '<var>hh</var>' may be a single digit;
884 0≤<var>hh</var>≤24.
885 The default <abbr>DST</abbr> offset is one hour ahead of
888 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
889 specifies the beginning and end of <abbr>DST</abbr>.
890 If this is absent, the system supplies its own ruleset
891 for <abbr>DST</abbr>, and its rules can differ from year to year;
892 typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
894 <dt><var>time</var></dt><dd>
896 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
897 and defaults to 02:00.
898 This is the same format as the offset, except that a
899 leading '<code>+</code>' or '<code>-</code>' is not allowed.
901 <dt><var>date</var></dt><dd>
902 takes one of the following forms:
904 <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd>
905 origin-1 day number not counting February 29
907 <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd>
908 origin-0 day number counting February 29 if present
910 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
911 (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5,
912 1≤<var>m</var>≤12)</dt><dd>
913 for the <var>d</var>th day of week <var>n</var> of
914 month <var>m</var> of the year, where week 1 is the first
915 week in which day <var>d</var> appears, and
916 '<code>5</code>' stands for the last week in which
917 day <var>d</var> appears (which may be either the 4th or
919 Typically, this is the only useful form; the <var>n</var>
920 and <code>J</code><var>n</var> forms are rarely used.
927 Here is an example POSIX <code>TZ</code> string for New
929 It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
930 of <abbr>UT</abbr>, and that daylight saving time
931 (<abbr>NZDT</abbr>) is observed from September's last Sunday at
932 02:00 until April's first Sunday at 03:00:
935 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
938 This POSIX <code>TZ</code> string is hard to remember, and
939 mishandles some timestamps before 2008.
940 With this package you can use this instead:
943 <pre><code>TZ='Pacific/Auckland'</code></pre>
946 POSIX does not define the <abbr>DST</abbr> transitions
947 for <code>TZ</code> values like
948 "<code>EST5EDT</code>".
949 Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
950 were used to interpret such values, but this meant that the
951 <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
952 program that did time conversion. This meant that when
953 <abbr>US</abbr> time conversion rules changed (as in the United
954 States in 1987), all programs that did time conversion had to be
955 recompiled to ensure proper results.
958 The <code>TZ</code> environment variable is process-global, which
959 makes it hard to write efficient, thread-safe applications that
960 need access to multiple timezones.
963 In POSIX, there is no tamper-proof way for a process to learn the
964 system's best idea of local (wall clock) time.
965 This is important for applications that an administrator wants
966 used only at certain times – without regard to whether the
968 <code>TZ</code> environment variable.
969 While an administrator can "do everything in <abbr>UT</abbr>" to
970 get around the problem, doing so is inconvenient and precludes
971 handling daylight saving time shifts – as might be required to
972 limit phone calls to off-peak hours.
975 POSIX provides no convenient and efficient way to determine
976 the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
977 timestamps, particularly for timezones
978 that do not fit into the POSIX model.
981 POSIX requires that <code>time_t</code> clock counts exclude leap
985 The <code><abbr>tz</abbr></code> code attempts to support all the
986 <code>time_t</code> implementations allowed by POSIX.
987 The <code>time_t</code> type represents a nonnegative count of seconds
988 since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
989 In practice, <code>time_t</code> is usually a signed 64- or 32-bit
990 integer; 32-bit signed <code>time_t</code> values stop working after
991 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
992 days typically use a signed 64-bit integer.
993 Unsigned 32-bit integers are used on one or two platforms, and 36-bit
994 and 40-bit integers are also used occasionally.
995 Although earlier POSIX versions allowed <code>time_t</code> to be a
996 floating-point type, this was not supported by any practical system,
997 and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
998 require <code>time_t</code> to be an integer type.
1002 <h3 id="POSIX-extensions">Extensions to POSIX in the
1003 <code><abbr>tz</abbr></code> code</h3>
1007 The <code>TZ</code> environment variable is used in generating
1008 the name of a file from which time-related information is read
1009 (or is interpreted à la POSIX); <code>TZ</code> is no longer
1010 constrained to be a string containing abbreviations
1011 and numeric data as described <a href="#POSIX">above</a>.
1012 The file's format is <dfn><abbr>TZif</abbr></dfn>,
1013 a timezone information format that contains binary data; see
1014 <a href="https://tools.ietf.org/html/8536">Internet
1015 <abbr>RFC</abbr> 8536</a>.
1016 The daylight saving time rules to be used for a
1017 particular timezone are encoded in the
1018 <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>,
1019 Australian, and other rules to be encoded, and
1020 allows for situations where more than two time zone
1021 abbreviations are used.
1024 It was recognized that allowing the <code>TZ</code> environment
1025 variable to take on values such as '<code>America/New_York</code>'
1026 might cause "old" programs (that expect <code>TZ</code> to have a
1027 certain form) to operate incorrectly; consideration was given to using
1028 some other environment variable (for example, <code>TIMEZONE</code>)
1029 to hold the string used to generate the <abbr>TZif</abbr> file's name.
1030 In the end, however, it was decided to continue using
1031 <code>TZ</code>: it is widely used for time zone purposes;
1032 separately maintaining both <code>TZ</code>
1033 and <code>TIMEZONE</code> seemed a nuisance; and systems where
1034 "new" forms of <code>TZ</code> might cause problems can simply
1035 use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which
1036 can be used by "new" programs as well as by "old" programs that
1037 assume pre-POSIX <code>TZ</code> values.
1041 The code supports platforms with a <abbr>UT</abbr> offset member
1042 in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
1045 The code supports platforms with a time zone abbreviation member in
1046 <code>struct tm</code>, e.g., <code>tm_zone</code>.
1049 Functions <code>tzalloc</code>, <code>tzfree</code>,
1050 <code>localtime_rz</code>, and <code>mktime_z</code> for
1051 more-efficient thread-safe applications that need to use multiple
1053 The <code>tzalloc</code> and <code>tzfree</code> functions
1054 allocate and free objects of type <code>timezone_t</code>,
1055 and <code>localtime_rz</code> and <code>mktime_z</code> are
1056 like <code>localtime_r</code> and <code>mktime</code> with an
1057 extra <code>timezone_t</code> argument.
1058 The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
1061 Negative <code>time_t</code> values are supported, on systems
1062 where <code>time_t</code> is signed.
1065 These functions can account for leap seconds;
1066 see <a href="#leapsec">Leap seconds</a> below.
1070 <h3 id="vestigial">POSIX features no longer needed</h3>
1072 POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
1073 define some <a href="https://en.wikipedia.org/wiki/API"><abbr
1074 title="application programming interface">API</abbr>s</a> that are vestigial:
1075 they are not needed, and are relics of a too-simple model that does
1076 not suffice to handle many real-world timestamps.
1077 Although the <code><abbr>tz</abbr></code> code supports these
1078 vestigial <abbr>API</abbr>s for backwards compatibility, they should
1079 be avoided in portable applications.
1080 The vestigial <abbr>API</abbr>s are:
1084 The POSIX <code>tzname</code> variable does not suffice and is no
1086 To get a timestamp's time zone abbreviation, consult
1087 the <code>tm_zone</code> member if available; otherwise,
1088 use <code>strftime</code>'s <code>"%Z"</code> conversion
1092 The POSIX <code>daylight</code> and <code>timezone</code>
1093 variables do not suffice and are no longer needed.
1094 To get a timestamp's <abbr>UT</abbr> offset, consult
1095 the <code>tm_gmtoff</code> member if available; otherwise,
1096 subtract values returned by <code>localtime</code>
1097 and <code>gmtime</code> using the rules of the Gregorian calendar,
1098 or use <code>strftime</code>'s <code>"%z"</code> conversion
1099 specification if a string like <code>"+0900"</code> suffices.
1102 The <code>tm_isdst</code> member is almost never needed and most of
1103 its uses should be discouraged in favor of the abovementioned
1105 Although it can still be used in arguments to
1106 <code>mktime</code> to disambiguate timestamps near
1107 a <abbr>DST</abbr> transition when the clock jumps back, this
1108 disambiguation does not work when standard time itself jumps back,
1109 which can occur when a location changes to a time zone with a
1110 lesser <abbr>UT</abbr> offset.
1114 <h3 id="other-portability">Other portability notes</h3>
1117 The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
1118 UNIX</a> <code>timezone</code> function is not present in this
1119 package; it is impossible to reliably map <code>timezone</code>'s
1120 arguments (a "minutes west of <abbr>GMT</abbr>" value and a
1121 "daylight saving time in effect" flag) to a time zone
1122 abbreviation, and we refuse to guess.
1123 Programs that in the past used the <code>timezone</code> function
1124 may now examine <code>localtime(&clock)->tm_zone</code>
1125 (if <code>TM_ZONE</code> is defined) or
1126 <code>tzname[localtime(&clock)->tm_isdst]</code>
1127 (if <code>HAVE_TZNAME</code> is nonzero) to learn the correct time
1128 zone abbreviation to use.
1132 href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
1133 <code>gettimeofday</code> function is not
1134 used in this package.
1135 This formerly let users obtain the current <abbr>UTC</abbr> offset
1136 and <abbr>DST</abbr> flag, but this functionality was removed in
1137 later versions of <abbr>BSD</abbr>.
1140 In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
1141 near-maximum <code>time_t</code> values when doing conversions
1142 for places that do not use <abbr>UT</abbr>.
1143 This package takes care to do these conversions correctly.
1144 A comment in the source code tells how to get compatibly wrong
1148 The functions that are conditionally compiled
1149 if <code>STD_INSPIRED</code> is defined should, at this point, be
1150 looked on primarily as food for thought.
1151 They are not in any sense "standard compatible" – some are
1152 not, in fact, specified in <em>any</em> standard.
1153 They do, however, represent responses of various authors to
1154 standardization proposals.
1157 Other time conversion proposals, in particular those supported by the
1158 <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
1159 Database Parser</a>, offer a wider selection of functions
1160 that provide capabilities beyond those provided here.
1161 The absence of such functions from this package is not meant to
1162 discourage the development, standardization, or use of such
1164 Rather, their absence reflects the decision to make this package
1165 contain valid extensions to POSIX, to ensure its broad
1167 If more powerful time conversion functions can be standardized, so
1174 <h2 id="stability">Interface stability</h2>
1176 The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
1181 A set of timezone names as per
1182 "<a href="#naming">Timezone identifiers</a>" above.
1185 Library functions described in "<a href="#functions">Time and date
1186 functions</a>" above.
1189 The programs <code>tzselect</code>, <code>zdump</code>,
1190 and <code>zic</code>, documented in their man pages.
1193 The format of <code>zic</code> input files, documented in
1194 the <code>zic</code> man page.
1197 The format of <code>zic</code> output files, documented in
1198 the <code>tzfile</code> man page.
1201 The format of zone table files, documented in <code>zone1970.tab</code>.
1204 The format of the country code file, documented in <code>iso3166.tab</code>.
1207 The version number of the code and data, as the first line of
1208 the text file '<code>version</code>' in each release.
1213 Interface changes in a release attempt to preserve compatibility with
1215 For example, <code><abbr>tz</abbr></code> data files typically do not
1216 rely on recently-added <code>zic</code> features, so that users can
1217 run older <code>zic</code> versions to process newer data files.
1218 <a href="tz-link.html#download">Downloading
1219 the <code><abbr>tz</abbr></code> database</a> describes how releases
1220 are tagged and distributed.
1224 Interfaces not listed above are less stable.
1225 For example, users should not rely on particular <abbr>UT</abbr>
1226 offsets or abbreviations for timestamps, as data entries are often
1227 based on guesswork and these guesses may be corrected or improved.
1231 Timezone boundaries are not part of the stable interface.
1232 For example, even though the <samp>Asia/Bangkok</samp> timezone
1233 currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part
1234 of the stable interface and the timezone can split at any time.
1235 If a calendar application records a future event in some location other
1236 than Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record,
1237 the application should be robust in the presence of timezone splits
1238 between now and the future time.
1243 <h2 id="leapsec">Leap seconds</h2>
1245 The <code><abbr>tz</abbr></code> code and data can account for leap seconds,
1246 thanks to code contributed by Bradley White.
1247 However, the leap second support of this package is rarely used directly
1248 because POSIX requires leap seconds to be excluded and many
1249 software packages would mishandle leap seconds if they were present.
1250 Instead, leap seconds are more commonly handled by occasionally adjusting
1251 the operating system kernel clock as described in
1252 <a href="tz-link.html#precision">Precision timekeeping</a>,
1253 and this package by default installs a <samp>leapseconds</samp> file
1255 <a href="http://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a>
1256 software that adjusts the kernel clock.
1257 However, kernel-clock twiddling approximates UTC only roughly,
1258 and systems needing more-precise UTC can use this package's leap
1259 second support directly.
1263 The directly-supported mechanism assumes that <code>time_t</code>
1264 counts of seconds since the POSIX epoch normally include leap seconds,
1265 as opposed to POSIX <code>time_t</code> counts which exclude leap seconds.
1266 This modified timescale is converted to <abbr>UTC</abbr>
1267 at the same point that time zone and DST adjustments are applied –
1268 namely, at calls to <code>localtime</code> and analogous functions –
1269 and the process is driven by leap second information
1270 stored in alternate versions of the <abbr>TZif</abbr> files.
1271 Because a leap second adjustment may be needed even
1272 if no time zone correction is desired,
1273 calls to <code>gmtime</code>-like functions
1274 also need to consult a <abbr>TZif</abbr> file,
1275 conventionally named <samp><abbr>GMT</abbr></samp>,
1276 to see whether leap second corrections are needed.
1277 To convert an application's <code>time_t</code> timestamps to or from
1278 POSIX <code>time_t</code> timestamps (for use when, say,
1279 embedding or interpreting timestamps in portable
1280 <a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a>
1282 the application can call the utility functions
1283 <code>time2posix</code> and <code>posix2time</code>
1284 included with this package.
1288 If the POSIX-compatible <abbr>TZif</abbr> file set is installed
1289 in a directory whose basename is <samp>zoneinfo</samp>, the
1290 leap-second-aware file set is by default installed in a separate
1291 directory <samp>zoneinfo-leaps</samp>.
1292 Although each process can have its own time zone by setting
1293 its <code>TZ</code> environment variable, there is no support for some
1294 processes being leap-second aware while other processes are
1295 POSIX-compatible; the leap-second choice is system-wide.
1296 So if you configure your kernel to count leap seconds, you should also
1297 discard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp>
1298 to <samp>zoneinfo</samp>.
1299 Alternatively, you can install just one set of <abbr>TZif</abbr> files
1300 in the first place; see the <code>REDO</code> variable in this package's
1301 <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>.
1306 <h2 id="calendar">Calendrical issues</h2>
1308 Calendrical issues are a bit out of scope for a time zone database,
1309 but they indicate the sort of problems that we would run into if we
1310 extended the time zone database further into the past.
1311 An excellent resource in this area is Edward M. Reingold
1312 and Nachum Dershowitz, <cite><a
1313 href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
1314 Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
1315 Other information and sources are given in the file '<code>calendars</code>'
1316 in the <code><abbr>tz</abbr></code> distribution.
1317 They sometimes disagree.
1322 <h2 id="planets">Time and time zones on other planets</h2>
1324 Some people's work schedules
1325 use <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
1326 Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
1328 <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
1329 Pathfinder</a> mission.
1330 Some of their family members also adapted to Mars time.
1331 Dozens of special Mars watches were built for JPL workers who kept
1332 Mars time during the Mars Exploration Rovers mission (2004).
1333 These timepieces look like normal Seikos and Citizens but use Mars
1334 seconds rather than terrestrial seconds.
1338 A Mars solar day is called a "sol" and has a mean period equal to
1339 about 24 hours 39 minutes 35.244 seconds in terrestrial time.
1340 It is divided into a conventional 24-hour clock, so each Mars second
1341 equals about 1.02749125 terrestrial seconds.
1345 The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
1346 meridian</a> of Mars goes through the center of the crater
1347 <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
1348 honor of the British astronomer who built the Greenwich telescope that
1349 defines Earth's prime meridian.
1350 Mean solar time on the Mars prime meridian is
1351 called Mars Coordinated Time (<abbr>MTC</abbr>).
1355 Each landed mission on Mars has adopted a different reference for
1356 solar timekeeping, so there is no real standard for Mars time zones.
1358 <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
1359 Exploration Rover</a> project (2004) defined two time zones "Local
1360 Solar Time A" and "Local Solar Time B" for its two missions, each zone
1361 designed so that its time equals local true solar time at
1362 approximately the middle of the nominal mission.
1363 Such a "time zone" is not particularly suited for any application
1364 other than the mission itself.
1368 Many calendars have been proposed for Mars, but none have achieved
1370 Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
1371 sequential count of Mars solar days elapsed since about 1873-12-29
1372 12:00 <abbr>GMT</abbr>.
1376 In our solar system, Mars is the planet with time and calendar most
1378 On other planets, Sun-based time and calendars would work quite
1380 For example, although Mercury's
1381 <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
1382 rotation period</a> is 58.646 Earth days, Mercury revolves around the
1383 Sun so rapidly that an observer on Mercury's equator would see a
1384 sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
1386 Venus is more complicated, partly because its rotation is slightly
1387 <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
1388 its year is 1.92 of its days.
1389 Gas giants like Jupiter are trickier still, as their polar and
1390 equatorial regions rotate at different rates, so that the length of a
1391 day depends on latitude.
1392 This effect is most pronounced on Neptune, where the day is about 12
1393 hours at the poles and 18 hours at the equator.
1397 Although the <code><abbr>tz</abbr></code> database does not support
1398 time on other planets, it is documented here in the hopes that support
1399 will be added eventually.
1403 Sources for time on other planets:
1408 Michael Allison and Robert Schmunk,
1409 "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
1410 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
1415 "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
1416 Fit for a Martian</a>", <cite>Los Angeles Times</cite>
1417 (2004-01-14), pp A1, A20–A21.
1421 "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
1422 Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
1426 "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
1427 long is a day on the other planets of the solar system?</a>"
1435 This file is in the public domain, so clarified as of 2009-05-17 by