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
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 "Czech Republic" instead of the timezone name "<code>Europe/Prague</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/Prague</code> to
126 locale-dependent strings like "Prague", "Praha", "Прага", and "布拉格".
130 The naming conventions attempt to strike a balance
131 among the following goals:
136 Uniquely identify every timezone where clocks have agreed since 1970.
137 This is essential for the intended use: static clocks keeping local
141 Indicate to experts where the timezone's clocks typically are.
144 Be robust in the presence of political changes.
145 For example, names are typically not tied to countries, to avoid
146 incompatibilities when countries change their name (e.g.,
147 Swaziland→Eswatini) or when locations change countries (e.g., Hong
148 Kong from UK colony to China).
149 There is no requirement that every country or national
150 capital must have a timezone name.
153 Be portable to a wide variety of implementations.
156 Use a consistent naming conventions over the entire world.
161 Names normally have the form
162 <var>AREA</var><code>/</code><var>LOCATION</var>, where
163 <var>AREA</var> is a continent or ocean, and
164 <var>LOCATION</var> is a specific location within the area.
165 North and South America share the same area, '<code>America</code>'.
166 Typical names are '<code>Africa/Cairo</code>',
167 '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
168 Some names are further qualified to help avoid confusion; for example,
169 '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
170 Indiana from other Petersburgs in America.
174 Here are the general guidelines used for
175 choosing timezone names,
176 in decreasing order of importance:
181 Use only valid POSIX file name components (i.e., the parts of
182 names other than '<code>/</code>').
183 Do not use the file name components '<code>.</code>' and
185 Within a file name component, use only <a
186 href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
187 '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
188 Do not use digits, as that might create an ambiguity with <a
189 href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX
190 <code>TZ</code> strings</a>.
191 A file name component must not exceed 14 characters or start with
193 E.g., prefer <code>Asia/Brunei</code> to
194 <code>Asia/Bandar_Seri_Begawan</code>.
195 Exceptions: see the discussion of legacy names below.
198 A name must not be empty, or contain '<code>//</code>', or
199 start or end with '<code>/</code>'.
202 Do not use names that differ only in case.
203 Although the reference implementation is case-sensitive, some
204 other implementations are not, and they would mishandle names
205 differing only in case.
208 If one name <var>A</var> is an initial prefix of another
209 name <var>AB</var> (ignoring case), then <var>B</var> must not
210 start with '<code>/</code>', as a regular file cannot have the
211 same name as a directory in POSIX.
212 For example, <code>America/New_York</code> precludes
213 <code>America/New_York/Bronx</code>.
216 Uninhabited regions like the North Pole and Bouvet Island
217 do not need locations, since local time is not defined there.
220 If all the clocks in a timezone have agreed since 1970,
221 do not bother to include more than one timezone
222 even if some of the clocks disagreed before 1970.
223 Otherwise these tables would become annoyingly large.
226 If boundaries between regions are fluid, such as during a war or
227 insurrection, do not bother to create a new timezone merely
228 because of yet another boundary change. This helps prevent table
229 bloat and simplifies maintenance.
232 If a name is ambiguous, use a less ambiguous alternative;
233 e.g., many cities are named San José and Georgetown, so
234 prefer <code>America/Costa_Rica</code> to
235 <code>America/San_Jose</code> and <code>America/Guyana</code>
236 to <code>America/Georgetown</code>.
239 Keep locations compact.
240 Use cities or small islands, not countries or regions, so that any
241 future changes do not split individual locations into different
243 E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
245 <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
246 has had multiple time zones</a>.
249 Use mainstream English spelling, e.g., prefer
250 <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
251 prefer <code>Europe/Athens</code> to the Greek
252 <code>Ευρώπη/Αθήνα</code> or the Romanized
253 <code>Evrópi/Athína</code>.
254 The POSIX file name restrictions encourage this guideline.
257 Use the most populous among locations in a region,
258 e.g., prefer <code>Asia/Shanghai</code> to
259 <code>Asia/Beijing</code>.
260 Among locations with similar populations, pick the best-known
261 location, e.g., prefer <code>Europe/Rome</code> to
262 <code>Europe/Milan</code>.
265 Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
266 <code>Atlantic/Canaries</code>.
269 Omit common suffixes like '<code>_Islands</code>' and
270 '<code>_City</code>', unless that would lead to ambiguity.
271 E.g., prefer <code>America/Cayman</code> to
272 <code>America/Cayman_Islands</code> and
273 <code>America/Guatemala</code> to
274 <code>America/Guatemala_City</code>, but prefer
275 <code>America/Mexico_City</code> to
276 <code>America/Mexico</code>
277 because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
278 country of Mexico has several time zones</a>.
281 Use '<code>_</code>' to represent a space.
284 Omit '<code>.</code>' from abbreviations in names.
285 E.g., prefer <code>Atlantic/St_Helena</code> to
286 <code>Atlantic/St._Helena</code>.
289 Do not change established names if they only marginally violate
290 the above guidelines.
291 For example, do not change the existing name <code>Europe/Rome</code> to
292 <code>Europe/Milan</code> merely because Milan's population has grown
293 to be somewhat greater than Rome's.
296 If a name is changed, put its old spelling in the
297 '<code>backward</code>' file.
298 This means old spellings will continue to work.
299 Ordinarily a name change should occur only in the rare case when
300 a location's consensus English-language spelling changes; for example,
301 in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
302 due to long-time widespread use of the new city name instead of the old.
307 Guidelines have evolved with time, and names following old versions of
308 these guidelines might not follow the current version. When guidelines
309 have changed, old names continue to be supported. Guideline changes
310 have included the following:
315 Older versions of this package used a different naming scheme.
316 See the file '<code>backward</code>' for most of these older names
317 (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
318 The other old-fashioned names still supported are
319 '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
320 '<code>EET</code>' (see the file '<code>europe</code>').
324 Older versions of this package defined legacy names that are
325 incompatible with the first guideline of location names, but which are
327 These legacy names are mostly defined in the file
328 '<code>etcetera</code>'.
329 Also, the file '<code>backward</code>' defines the legacy names
330 '<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>',
331 and the file '<code>northamerica</code>' defines the legacy names
332 '<code>EST5EDT</code>', '<code>CST6CDT</code>',
333 '<code>MST7MDT</code>', and '<code>PST8PDT</code>'.
337 Older versions of these guidelines said that
338 there should typically be at least one name for each <a
339 href="https://en.wikipedia.org/wiki/ISO_3166-1"><abbr
340 title="International Organization for Standardization">ISO</abbr>
341 3166-1</a> officially assigned two-letter code for an inhabited
342 country or territory.
343 This old guideline has been dropped, as it was not needed to handle
344 timestamps correctly and it increased maintenance burden.
349 The file '<code>zone1970.tab</code>' lists geographical locations used
351 It is intended to be an exhaustive list of names for geographic
352 regions as described above; this is a subset of the timezones in the data.
353 Although a '<code>zone1970.tab</code>' location's
354 <a href="https://en.wikipedia.org/wiki/Longitude">longitude</a>
356 its <a href="https://en.wikipedia.org/wiki/Local_mean_time">local mean
357 time (<abbr>LMT</abbr>)</a> offset with one hour for every 15°
358 east longitude, this relationship is not exact.
362 Excluding '<code>backward</code>' should not affect the other data.
363 If '<code>backward</code>' is excluded, excluding
364 '<code>etcetera</code>' should not affect the remaining data.
369 <h2 id="abbreviations">Time zone abbreviations</h2>
371 When this package is installed, it generates time zone abbreviations
372 like '<code>EST</code>' to be compatible with human tradition and POSIX.
373 Here are the general guidelines used for choosing time zone abbreviations,
374 in decreasing order of importance:
379 Use three to six characters that are ASCII alphanumerics or
380 '<code>+</code>' or '<code>-</code>'.
381 Previous editions of this database also used characters like
382 space and '<code>?</code>', but these characters have a
383 special meaning to the
384 <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
385 and cause commands like
386 '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
387 `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
388 to have unexpected effects.
389 Previous editions of this guideline required upper-case letters, but the
390 Congressman who introduced
391 <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
392 Standard Time</a> preferred "ChST", so lower-case letters are now
394 Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
395 '<code>+</code>', and alphanumeric characters from the portable
396 character set in the current locale.
397 In practice ASCII alphanumerics and '<code>+</code>' and
398 '<code>-</code>' are safe in all locales.
401 In other words, in the C locale the POSIX extended regular
402 expression <code>[-+[:alnum:]]{3,6}</code> should match the
404 This guarantees that all abbreviations could have been specified by a
405 POSIX <code>TZ</code> string.
409 Use abbreviations that are in common use among English-speakers,
410 e.g., 'EST' for Eastern Standard Time in North America.
411 We assume that applications translate them to other languages
412 as part of the normal localization process; for example,
413 a French application might translate 'EST' to 'HNE'.
416 <small>These abbreviations (for standard/daylight/etc. time) are:
417 ACST/ACDT Australian Central,
418 AST/ADT/APT/AWT/ADDT Atlantic,
419 AEST/AEDT Australian Eastern,
420 AHST/AHDT Alaska-Hawaii,
422 AWST/AWDT Australian Western,
424 CAT/CAST Central Africa,
425 CET/CEST/CEMT Central European,
427 CST/CDT/CWT/CPT/CDDT Central [North America],
429 GMT/BST/IST/BDST Greenwich,
431 EST/EDT/EWT/EPT/EDDT Eastern [North America],
432 EET/EEST Eastern European,
434 HST/HDT/HWT/HPT Hawaii,
435 HKT/HKST/HKWT Hong Kong,
441 MET/MEST Middle European (a backward-compatibility alias for
444 MST/MDT/MWT/MPT/MDDT Mountain,
445 NST/NDT/NWT/NPT/NDDT Newfoundland,
446 NST/NDT/NWT/NPT Nome,
447 NZMT/NZST New Zealand through 1945,
448 NZST/NZDT New Zealand 1946–present,
450 PST/PDT/PWT/PPT/PDDT Pacific,
454 WAT/WAST West Africa,
455 WET/WEST/WEMT Western European,
456 WIB Waktu Indonesia Barat,
457 WIT Waktu Indonesia Timur,
458 WITA Waktu Indonesia Tengah,
459 YST/YDT/YWT/YPT/YDDT Yukon</small>.
464 For times taken from a city's longitude, use the
465 traditional <var>x</var>MT notation.
466 The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
467 The others are for timestamps before 1960,
468 except that Monrovia Mean Time persisted until 1972.
469 Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
470 MMT) would cause trouble here, as the numeric strings would exceed
471 the POSIX length limit.
475 <small>These abbreviations are:
476 AMT Amsterdam, Asunción, Athens;
477 BMT Baghdad, Bangkok, Batavia, Bern, Bogotá, Bridgetown, Brussels,
479 CMT Calamarca, Caracas, Chisinau, Colón, Copenhagen, Córdoba;
485 HMT Havana, Helsinki, Horta, Howrah;
486 IMT Irkutsk, Istanbul;
488 KMT Kaunas, Kiev, Kingston;
489 LMT Lima, Lisbon, local, Luanda;
490 MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo,
493 PMT Paramaribo, Paris, Perm, Pontianak, Prague;
496 RMT Rangoon, Riga, Rome;
499 SMT Santiago, Simferopol, Singapore, Stanley;
506 <small>A few abbreviations also follow the pattern that
507 <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
509 CMT/BST for Calamarca Mean Time and Bolivian Summer Time
511 DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
513 MMT/MST/MDST for Moscow 1880–1919, and
514 RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926.
515 An extra-special case is SET for Swedish Time (<em>svensk
516 normaltid</em>) 1879–1899, 3° west of the Stockholm
521 Use '<abbr>LMT</abbr>' for local mean time of locations before the
522 introduction of standard time; see "<a href="#scope">Scope of the
523 <code><abbr>tz</abbr></code> database</a>".
526 If there is no common English abbreviation, use numeric offsets like
527 <code>-</code>05 and <code>+</code>0530 that are generated
528 by <code>zic</code>'s <code>%z</code> notation.
531 Use current abbreviations for older timestamps to avoid confusion.
532 For example, in 1910 a common English abbreviation for time
533 in central Europe was 'MEZ' (short for both "Middle European
534 Zone" and for "Mitteleuropäische Zeit" in German).
535 Nowadays 'CET' ("Central European Time") is more common in
536 English, and the database uses 'CET' even for circa-1910
537 timestamps as this is less confusing for modern users and avoids
538 the need for determining when 'CET' supplanted 'MEZ' in common
542 Use a consistent style in a timezone's history.
543 For example, if a history tends to use numeric
544 abbreviations and a particular entry could go either way, use a
545 numeric abbreviation.
549 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
550 (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
551 locations while uninhabited.
552 The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
553 some sense undefined; this notation is derived
554 from <a href="https://tools.ietf.org/html/rfc3339">Internet
555 <abbr title="Request For Comments">RFC</abbr> 3339</a>.
560 Application writers should note that these abbreviations are ambiguous
561 in practice: e.g., 'CST' means one thing in China and something else
562 in North America, and 'IST' can refer to time in India, Ireland or
564 To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
565 '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
570 <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
572 The <code><abbr>tz</abbr></code> database is not authoritative, and it
574 Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
575 Users requiring authoritative data should consult national standards
576 bodies and the references cited in the database's comments.
580 Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
585 The <code><abbr>tz</abbr></code> database predicts future
586 timestamps, and current predictions
587 will be incorrect after future governments change the rules.
588 For example, if today someone schedules a meeting for 13:00 next
589 October 1, Casablanca time, and tomorrow Morocco changes its
590 daylight saving rules, software can mess up after the rule change
591 if it blithely relies on conversions made before the change.
594 The pre-1970 entries in this database cover only a tiny sliver of how
595 clocks actually behaved; the vast majority of the necessary
596 information was lost or never recorded.
597 Thousands more timezones would be needed if
598 the <code><abbr>tz</abbr></code> database's scope were extended to
599 cover even just the known or guessed history of standard time; for
600 example, the current single entry for France would need to split
601 into dozens of entries, perhaps hundreds.
602 And in most of the world even this approach would be misleading
603 due to widespread disagreement or indifference about what times
607 href="http://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
608 Global Transformation of Time, 1870–1950</a></cite>,
610 "Outside of Europe and North America there was no system of time
611 zones at all, often not even a stable landscape of mean times,
612 prior to the middle decades of the twentieth century".
613 See: Timothy Shenk, <a
614 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
615 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
618 Most of the pre-1970 data entries come from unreliable sources, often
619 astrology books that lack citations and whose compilers evidently
620 invented entries when the true facts were unknown, without
621 reporting which entries were known and which were invented.
622 These books often contradict each other or give implausible entries,
623 and on the rare occasions when they are checked they are
624 typically found to be incorrect.
627 For the UK the <code><abbr>tz</abbr></code> database relies on
628 years of first-class work done by
629 Joseph Myers and others; see
630 "<a href="https://www.polyomino.org.uk/british-time/">History of
631 legal time in Britain</a>".
632 Other countries are not done nearly as well.
635 Sometimes, different people in the same city maintain clocks
636 that differ significantly.
637 Historically, railway time was used by railroad companies (which
639 agree with each other), church-clock time was used for birth
641 More recently, competing political groups might disagree about
642 clock settings. Often this is merely common practice, but
643 sometimes it is set by law.
644 For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
645 was legally <abbr>UT</abbr> +00:09:21 outside train stations and
646 <abbr>UT</abbr> +00:04:21 inside. Other examples include
647 Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
651 Although a named location in the <code><abbr>tz</abbr></code>
652 database stands for the containing region, its pre-1970 data
653 entries are often accurate for only a small subset of that region.
654 For example, <code>Europe/London</code> stands for the United
655 Kingdom, but its pre-1847 times are valid only for locations that
656 have London's exact meridian, and its 1847 transition
657 to <abbr>GMT</abbr> is known to be valid only for the L&NW and
658 the Caledonian railways.
661 The <code><abbr>tz</abbr></code> database does not record the
662 earliest time for which a timezone's
663 data entries are thereafter valid for every location in the region.
664 For example, <code>Europe/London</code> is valid for all locations
665 in its region after <abbr>GMT</abbr> was made the standard time,
666 but the date of standardization (1880-08-02) is not in the
667 <code><abbr>tz</abbr></code> database, other than in commentary.
668 For many timezones the earliest time of
672 The <code><abbr>tz</abbr></code> database does not record a
673 region's boundaries, and in many cases the boundaries are not known.
674 For example, the timezone
675 <code>America/Kentucky/Louisville</code> represents a region
676 around the city of Louisville, the boundaries of which are
680 Changes that are modeled as instantaneous transitions in the
681 <code><abbr>tz</abbr></code>
682 database were often spread out over hours, days, or even decades.
685 Even if the time is specified by law, locations sometimes
686 deliberately flout the law.
689 Early timekeeping practices, even assuming perfect clocks, were
690 often not specified to the accuracy that the
691 <code><abbr>tz</abbr></code> database requires.
694 The <code><abbr>tz</abbr></code> database cannot represent stopped clocks.
695 However, on 1911-03-11 at 00:00, some public-facing French clocks
696 were changed by stopping them for a few minutes to effect a transition.
697 The <code><abbr>tz</abbr></code> database models this via a
698 backward transition; the relevant French legislation does not
699 specify exactly how the transition was to occur.
702 Sometimes historical timekeeping was specified more precisely
703 than what the <code><abbr>tz</abbr></code> code can handle.
704 For example, from 1909 to 1937 <a
705 href="https://www.staff.science.uu.nl/~gent0113/wettijd/wettijd.htm"
706 hreflang="nl">Netherlands clocks</a> were legally Amsterdam Mean
707 Time (estimated to be <abbr>UT</abbr>
708 +00:19:32.13), but the <code><abbr>tz</abbr></code>
709 code cannot represent the fractional second.
710 In practice these old specifications were rarely if ever
711 implemented to subsecond precision.
714 Even when all the timestamp transitions recorded by the
715 <code><abbr>tz</abbr></code> database are correct, the
716 <code><abbr>tz</abbr></code> rules that generate them may not
717 faithfully reflect the historical rules.
718 For example, from 1922 until World War II the UK moved clocks
719 forward the day following the third Saturday in April unless that
720 was Easter, in which case it moved clocks forward the previous
722 Because the <code><abbr>tz</abbr></code> database has no
723 way to specify Easter, these exceptional years are entered as
724 separate <code><abbr>tz</abbr> Rule</code> lines, even though the
725 legal rules did not change.
726 When transitions are known but the historical rules behind them are not,
727 the database contains <code>Zone</code> and <code>Rule</code>
728 entries that are intended to represent only the generated
729 transitions, not any underlying historical rules; however, this
730 intent is recorded at best only in commentary.
733 The <code><abbr>tz</abbr></code> database models time
735 href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
736 Gregorian calendar</a> with days containing 24 equal-length hours
737 numbered 00 through 23, except when clock transitions occur.
738 Pre-standard time is modeled as local mean time.
739 However, historically many people used other calendars and other timescales.
740 For example, the Roman Empire used
741 the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
743 and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
744 timekeeping</a> had twelve varying-length daytime hours with a
745 non-hour-based system at night.
746 And even today, some local practices diverge from the Gregorian
747 calendar with 24-hour days. These divergences range from
748 relatively minor, such as Japanese bars giving times like "24:30" for the
749 wee hours of the morning, to more-significant differences such as <a
750 href="https://www.pri.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the
751 east African practice of starting the day at dawn</a>, renumbering
752 the Western 06:00 to be 12:00. These practices are largely outside
753 the scope of the <code><abbr>tz</abbr></code> code and data, which
754 provide only limited support for date and time localization
755 such as that required by POSIX. If DST is not used a different time zone
756 can often do the trick; for example, in Kenya a <code>TZ</code> setting
757 like <code><-03>3</code> or <code>America/Cayenne</code> starts
758 the day six hours later than <code>Africa/Nairobi</code> does.
761 Early clocks were less reliable, and data entries do not represent
765 The <code><abbr>tz</abbr></code> database assumes Universal Time
766 (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
767 standardized for older timestamps.
768 In the <code><abbr>tz</abbr></code> database commentary,
769 <abbr>UT</abbr> denotes a family of time standards that includes
770 Coordinated Universal Time (<abbr>UTC</abbr>) along with other
771 variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
772 with days starting at midnight.
773 Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
774 timestamps, <abbr>UTC</abbr> was not defined until 1960, so
775 commentary uses the more-general abbreviation <abbr>UT</abbr> for
776 timestamps that might predate 1960.
777 Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
778 and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
779 interpretation of older timestamps can be problematic when
780 subsecond accuracy is needed.
783 Civil time was not based on atomic time before 1972, and we do not
785 <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
786 rotation</a> accurately enough to map <a
787 href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
788 title="International System of Units">SI</abbr></a> seconds to
789 historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
790 to more than about one-hour accuracy.
791 See: Stephenson FR, Morrison LV, Hohenkerk CY.
792 <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
793 the Earth's rotation: 720 BC to AD 2015</a>.
794 <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404.
795 Also see: Espenak F. <a
796 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
800 The relationship between POSIX time (that is, <abbr>UTC</abbr> but
801 ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
802 seconds</a>) and <abbr>UTC</abbr> is not agreed upon after 1972.
804 clock officially stops during an inserted leap second, at least one
805 proposed standard has it jumping back a second instead; and in
806 practice POSIX clocks more typically either progress glacially during
807 a leap second, or are slightly slowed while near a leap second.
810 The <code><abbr>tz</abbr></code> database does not represent how
811 uncertain its information is.
812 Ideally it would contain information about when data entries are
814 Partial temporal knowledge is a field of active research, though,
815 and it is not clear how to apply it here.
820 In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
821 database's pre-1970 and future timestamps are either wrong or
823 Any attempt to pass the
824 <code><abbr>tz</abbr></code> database off as the definition of time
825 should be unacceptable to anybody who cares about the facts.
826 In particular, the <code><abbr>tz</abbr></code> database's
827 <abbr>LMT</abbr> offsets should not be considered meaningful, and
828 should not prompt creation of timezones
829 merely because two locations
830 differ in <abbr>LMT</abbr> or transitioned to standard time at
836 <h2 id="functions">Time and date functions</h2>
838 The <code><abbr>tz</abbr></code> code contains time and date functions
839 that are upwards compatible with those of POSIX.
840 Code compatible with this package is already
841 <a href="tz-link.html#tzdb">part of many platforms</a>, where the
842 primary use of this package is to update obsolete time-related files.
843 To do this, you may need to compile the time zone compiler
844 '<code>zic</code>' supplied with this package instead of using the
845 system '<code>zic</code>', since the format of <code>zic</code>'s
846 input is occasionally extended, and a platform may still be shipping
847 an older <code>zic</code>.
850 <h3 id="POSIX">POSIX properties and limitations</h3>
854 In POSIX, time display in a process is controlled by the
855 environment variable <code>TZ</code>.
856 Unfortunately, the POSIX
857 <code>TZ</code> string takes a form that is hard to describe and
858 is error-prone in practice.
859 Also, POSIX <code>TZ</code> strings cannot deal with daylight
860 saving time rules not based on the Gregorian calendar (as in
861 Iran), or with situations where more than two time zone
862 abbreviations or <abbr>UT</abbr> offsets are used in an area.
866 The POSIX <code>TZ</code> string takes the following form:
870 <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>]]]
878 <dt><var>std</var> and <var>dst</var></dt><dd>
879 are 3 or more characters specifying the standard
880 and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
881 Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
882 may also be in a quoted form like '<code><+09></code>';
883 this allows "<code>+</code>" and "<code>-</code>" in the names.
885 <dt><var>offset</var></dt><dd>
887 '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
888 and specifies the offset west of <abbr>UT</abbr>.
889 '<var>hh</var>' may be a single digit;
890 0≤<var>hh</var>≤24.
891 The default <abbr>DST</abbr> offset is one hour ahead of
894 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
895 specifies the beginning and end of <abbr>DST</abbr>.
896 If this is absent, the system supplies its own ruleset
897 for <abbr>DST</abbr>, and its rules can differ from year to year;
898 typically <abbr>US</abbr> <abbr>DST</abbr> rules are used.
900 <dt><var>time</var></dt><dd>
902 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
903 and defaults to 02:00.
904 This is the same format as the offset, except that a
905 leading '<code>+</code>' or '<code>-</code>' is not allowed.
907 <dt><var>date</var></dt><dd>
908 takes one of the following forms:
910 <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd>
911 origin-1 day number not counting February 29
913 <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd>
914 origin-0 day number counting February 29 if present
916 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
917 (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5,
918 1≤<var>m</var>≤12)</dt><dd>
919 for the <var>d</var>th day of week <var>n</var> of
920 month <var>m</var> of the year, where week 1 is the first
921 week in which day <var>d</var> appears, and
922 '<code>5</code>' stands for the last week in which
923 day <var>d</var> appears (which may be either the 4th or
925 Typically, this is the only useful form; the <var>n</var>
926 and <code>J</code><var>n</var> forms are rarely used.
933 Here is an example POSIX <code>TZ</code> string for New
935 It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
936 of <abbr>UT</abbr>, and that daylight saving time
937 (<abbr>NZDT</abbr>) is observed from September's last Sunday at
938 02:00 until April's first Sunday at 03:00:
941 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
944 This POSIX <code>TZ</code> string is hard to remember, and
945 mishandles some timestamps before 2008.
946 With this package you can use this instead:
949 <pre><code>TZ='Pacific/Auckland'</code></pre>
952 POSIX does not define the <abbr>DST</abbr> transitions
953 for <code>TZ</code> values like
954 "<code>EST5EDT</code>".
955 Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
956 were used to interpret such values, but this meant that the
957 <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
958 program that did time conversion. This meant that when
959 <abbr>US</abbr> time conversion rules changed (as in the United
960 States in 1987), all programs that did time conversion had to be
961 recompiled to ensure proper results.
964 The <code>TZ</code> environment variable is process-global, which
965 makes it hard to write efficient, thread-safe applications that
966 need access to multiple timezones.
969 In POSIX, there is no tamper-proof way for a process to learn the
970 system's best idea of local (wall clock) time.
971 This is important for applications that an administrator wants
972 used only at certain times – without regard to whether the
974 <code>TZ</code> environment variable.
975 While an administrator can "do everything in <abbr>UT</abbr>" to
976 get around the problem, doing so is inconvenient and precludes
977 handling daylight saving time shifts – as might be required to
978 limit phone calls to off-peak hours.
981 POSIX provides no convenient and efficient way to determine
982 the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
983 timestamps, particularly for timezones
984 that do not fit into the POSIX model.
987 POSIX requires that <code>time_t</code> clock counts exclude leap
991 The <code><abbr>tz</abbr></code> code attempts to support all the
992 <code>time_t</code> implementations allowed by POSIX.
993 The <code>time_t</code> type represents a nonnegative count of seconds
994 since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
995 In practice, <code>time_t</code> is usually a signed 64- or 32-bit
996 integer; 32-bit signed <code>time_t</code> values stop working after
997 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
998 days typically use a signed 64-bit integer.
999 Unsigned 32-bit integers are used on one or two platforms, and 36-bit
1000 and 40-bit integers are also used occasionally.
1001 Although earlier POSIX versions allowed <code>time_t</code> to be a
1002 floating-point type, this was not supported by any practical system,
1003 and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
1004 require <code>time_t</code> to be an integer type.
1008 <h3 id="POSIX-extensions">Extensions to POSIX in the
1009 <code><abbr>tz</abbr></code> code</h3>
1013 The <code>TZ</code> environment variable is used in generating
1014 the name of a file from which time-related information is read
1015 (or is interpreted à la POSIX); <code>TZ</code> is no longer
1016 constrained to be a string containing abbreviations
1017 and numeric data as described <a href="#POSIX">above</a>.
1018 The file's format is <dfn><abbr>TZif</abbr></dfn>,
1019 a timezone information format that contains binary data; see
1020 <a href="https://tools.ietf.org/html/8536">Internet
1021 <abbr>RFC</abbr> 8536</a>.
1022 The daylight saving time rules to be used for a
1023 particular timezone are encoded in the
1024 <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>,
1025 Australian, and other rules to be encoded, and
1026 allows for situations where more than two time zone
1027 abbreviations are used.
1030 It was recognized that allowing the <code>TZ</code> environment
1031 variable to take on values such as '<code>America/New_York</code>'
1032 might cause "old" programs (that expect <code>TZ</code> to have a
1033 certain form) to operate incorrectly; consideration was given to using
1034 some other environment variable (for example, <code>TIMEZONE</code>)
1035 to hold the string used to generate the <abbr>TZif</abbr> file's name.
1036 In the end, however, it was decided to continue using
1037 <code>TZ</code>: it is widely used for time zone purposes;
1038 separately maintaining both <code>TZ</code>
1039 and <code>TIMEZONE</code> seemed a nuisance; and systems where
1040 "new" forms of <code>TZ</code> might cause problems can simply
1041 use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which
1042 can be used by "new" programs as well as by "old" programs that
1043 assume pre-POSIX <code>TZ</code> values.
1047 The code supports platforms with a <abbr>UT</abbr> offset member
1048 in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>.
1051 The code supports platforms with a time zone abbreviation member in
1052 <code>struct tm</code>, e.g., <code>tm_zone</code>.
1055 Functions <code>tzalloc</code>, <code>tzfree</code>,
1056 <code>localtime_rz</code>, and <code>mktime_z</code> for
1057 more-efficient thread-safe applications that need to use multiple
1059 The <code>tzalloc</code> and <code>tzfree</code> functions
1060 allocate and free objects of type <code>timezone_t</code>,
1061 and <code>localtime_rz</code> and <code>mktime_z</code> are
1062 like <code>localtime_r</code> and <code>mktime</code> with an
1063 extra <code>timezone_t</code> argument.
1064 The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
1067 Negative <code>time_t</code> values are supported, on systems
1068 where <code>time_t</code> is signed.
1071 These functions can account for leap seconds;
1072 see <a href="#leapsec">Leap seconds</a> below.
1076 <h3 id="vestigial">POSIX features no longer needed</h3>
1078 POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
1079 define some <a href="https://en.wikipedia.org/wiki/API"><abbr
1080 title="application programming interface">API</abbr>s</a> that are vestigial:
1081 they are not needed, and are relics of a too-simple model that does
1082 not suffice to handle many real-world timestamps.
1083 Although the <code><abbr>tz</abbr></code> code supports these
1084 vestigial <abbr>API</abbr>s for backwards compatibility, they should
1085 be avoided in portable applications.
1086 The vestigial <abbr>API</abbr>s are:
1090 The POSIX <code>tzname</code> variable does not suffice and is no
1092 To get a timestamp's time zone abbreviation, consult
1093 the <code>tm_zone</code> member if available; otherwise,
1094 use <code>strftime</code>'s <code>"%Z"</code> conversion
1098 The POSIX <code>daylight</code> and <code>timezone</code>
1099 variables do not suffice and are no longer needed.
1100 To get a timestamp's <abbr>UT</abbr> offset, consult
1101 the <code>tm_gmtoff</code> member if available; otherwise,
1102 subtract values returned by <code>localtime</code>
1103 and <code>gmtime</code> using the rules of the Gregorian calendar,
1104 or use <code>strftime</code>'s <code>"%z"</code> conversion
1105 specification if a string like <code>"+0900"</code> suffices.
1108 The <code>tm_isdst</code> member is almost never needed and most of
1109 its uses should be discouraged in favor of the abovementioned
1111 Although it can still be used in arguments to
1112 <code>mktime</code> to disambiguate timestamps near
1113 a <abbr>DST</abbr> transition when the clock jumps back, this
1114 disambiguation does not work when standard time itself jumps back,
1115 which can occur when a location changes to a time zone with a
1116 lesser <abbr>UT</abbr> offset.
1120 <h3 id="other-portability">Other portability notes</h3>
1123 The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
1124 UNIX</a> <code>timezone</code> function is not present in this
1125 package; it is impossible to reliably map <code>timezone</code>'s
1126 arguments (a "minutes west of <abbr>GMT</abbr>" value and a
1127 "daylight saving time in effect" flag) to a time zone
1128 abbreviation, and we refuse to guess.
1129 Programs that in the past used the <code>timezone</code> function
1130 may now examine <code>localtime(&clock)->tm_zone</code>
1131 (if <code>TM_ZONE</code> is defined) or
1132 <code>tzname[localtime(&clock)->tm_isdst]</code>
1133 (if <code>HAVE_TZNAME</code> is nonzero) to learn the correct time
1134 zone abbreviation to use.
1138 href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
1139 <code>gettimeofday</code> function is not
1140 used in this package.
1141 This formerly let users obtain the current <abbr>UTC</abbr> offset
1142 and <abbr>DST</abbr> flag, but this functionality was removed in
1143 later versions of <abbr>BSD</abbr>.
1146 In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
1147 near-maximum <code>time_t</code> values when doing conversions
1148 for places that do not use <abbr>UT</abbr>.
1149 This package takes care to do these conversions correctly.
1150 A comment in the source code tells how to get compatibly wrong
1154 The functions that are conditionally compiled
1155 if <code>STD_INSPIRED</code> is defined should, at this point, be
1156 looked on primarily as food for thought.
1157 They are not in any sense "standard compatible" – some are
1158 not, in fact, specified in <em>any</em> standard.
1159 They do, however, represent responses of various authors to
1160 standardization proposals.
1163 Other time conversion proposals, in particular those supported by the
1164 <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
1165 Database Parser</a>, offer a wider selection of functions
1166 that provide capabilities beyond those provided here.
1167 The absence of such functions from this package is not meant to
1168 discourage the development, standardization, or use of such
1170 Rather, their absence reflects the decision to make this package
1171 contain valid extensions to POSIX, to ensure its broad
1173 If more powerful time conversion functions can be standardized, so
1180 <h2 id="stability">Interface stability</h2>
1182 The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
1187 A set of timezone names as per
1188 "<a href="#naming">Timezone identifiers</a>" above.
1191 Library functions described in "<a href="#functions">Time and date
1192 functions</a>" above.
1195 The programs <code>tzselect</code>, <code>zdump</code>,
1196 and <code>zic</code>, documented in their man pages.
1199 The format of <code>zic</code> input files, documented in
1200 the <code>zic</code> man page.
1203 The format of <code>zic</code> output files, documented in
1204 the <code>tzfile</code> man page.
1207 The format of zone table files, documented in <code>zone1970.tab</code>.
1210 The format of the country code file, documented in <code>iso3166.tab</code>.
1213 The version number of the code and data, as the first line of
1214 the text file '<code>version</code>' in each release.
1219 Interface changes in a release attempt to preserve compatibility with
1221 For example, <code><abbr>tz</abbr></code> data files typically do not
1222 rely on recently-added <code>zic</code> features, so that users can
1223 run older <code>zic</code> versions to process newer data files.
1224 <a href="tz-link.html#download">Downloading
1225 the <code><abbr>tz</abbr></code> database</a> describes how releases
1226 are tagged and distributed.
1230 Interfaces not listed above are less stable.
1231 For example, users should not rely on particular <abbr>UT</abbr>
1232 offsets or abbreviations for timestamps, as data entries are often
1233 based on guesswork and these guesses may be corrected or improved.
1237 Timezone boundaries are not part of the stable interface.
1238 For example, even though the <samp>Asia/Bangkok</samp> timezone
1239 currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part
1240 of the stable interface and the timezone can split at any time.
1241 If a calendar application records a future event in some location other
1242 than Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record,
1243 the application should be robust in the presence of timezone splits
1244 between now and the future time.
1249 <h2 id="leapsec">Leap seconds</h2>
1251 The <code><abbr>tz</abbr></code> code and data can account for leap seconds,
1252 thanks to code contributed by Bradley White.
1253 However, the leap second support of this package is rarely used directly
1254 because POSIX requires leap seconds to be excluded and many
1255 software packages would mishandle leap seconds if they were present.
1256 Instead, leap seconds are more commonly handled by occasionally adjusting
1257 the operating system kernel clock as described in
1258 <a href="tz-link.html#precision">Precision timekeeping</a>,
1259 and this package by default installs a <samp>leapseconds</samp> file
1261 <a href="http://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a>
1262 software that adjusts the kernel clock.
1263 However, kernel-clock twiddling approximates UTC only roughly,
1264 and systems needing more-precise UTC can use this package's leap
1265 second support directly.
1269 The directly-supported mechanism assumes that <code>time_t</code>
1270 counts of seconds since the POSIX epoch normally include leap seconds,
1271 as opposed to POSIX <code>time_t</code> counts which exclude leap seconds.
1272 This modified timescale is converted to <abbr>UTC</abbr>
1273 at the same point that time zone and DST adjustments are applied –
1274 namely, at calls to <code>localtime</code> and analogous functions –
1275 and the process is driven by leap second information
1276 stored in alternate versions of the <abbr>TZif</abbr> files.
1277 Because a leap second adjustment may be needed even
1278 if no time zone correction is desired,
1279 calls to <code>gmtime</code>-like functions
1280 also need to consult a <abbr>TZif</abbr> file,
1281 conventionally named <samp><abbr>GMT</abbr></samp>,
1282 to see whether leap second corrections are needed.
1283 To convert an application's <code>time_t</code> timestamps to or from
1284 POSIX <code>time_t</code> timestamps (for use when, say,
1285 embedding or interpreting timestamps in portable
1286 <a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a>
1288 the application can call the utility functions
1289 <code>time2posix</code> and <code>posix2time</code>
1290 included with this package.
1294 If the POSIX-compatible <abbr>TZif</abbr> file set is installed
1295 in a directory whose basename is <samp>zoneinfo</samp>, the
1296 leap-second-aware file set is by default installed in a separate
1297 directory <samp>zoneinfo-leaps</samp>.
1298 Although each process can have its own time zone by setting
1299 its <code>TZ</code> environment variable, there is no support for some
1300 processes being leap-second aware while other processes are
1301 POSIX-compatible; the leap-second choice is system-wide.
1302 So if you configure your kernel to count leap seconds, you should also
1303 discard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp>
1304 to <samp>zoneinfo</samp>.
1305 Alternatively, you can install just one set of <abbr>TZif</abbr> files
1306 in the first place; see the <code>REDO</code> variable in this package's
1307 <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>.
1312 <h2 id="calendar">Calendrical issues</h2>
1314 Calendrical issues are a bit out of scope for a time zone database,
1315 but they indicate the sort of problems that we would run into if we
1316 extended the time zone database further into the past.
1317 An excellent resource in this area is Edward M. Reingold
1318 and Nachum Dershowitz, <cite><a
1319 href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
1320 Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
1321 Other information and sources are given in the file '<code>calendars</code>'
1322 in the <code><abbr>tz</abbr></code> distribution.
1323 They sometimes disagree.
1328 <h2 id="planets">Time and time zones on other planets</h2>
1330 Some people's work schedules have used
1331 <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
1332 Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
1334 <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
1335 Pathfinder</a> mission (1997).
1336 Some of their family members also adapted to Mars time.
1337 Dozens of special Mars watches were built for JPL workers who kept
1338 Mars time during the
1339 <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
1340 Exploration Rovers (MER)</a> mission (2004–2018).
1341 These timepieces looked like normal Seikos and Citizens but were adjusted
1342 to use Mars seconds rather than terrestrial seconds, although
1343 unfortunately the adjusted watches were unreliable and appear to have
1344 had only limited use.
1348 A Mars solar day is called a "sol" and has a mean period equal to
1349 about 24 hours 39 minutes 35.244 seconds in terrestrial time.
1350 It is divided into a conventional 24-hour clock, so each Mars second
1351 equals about 1.02749125 terrestrial seconds.
1352 (One MER worker noted, "If I am working Mars hours, and Mars hours are
1353 2.5% more than Earth hours, shouldn't I get an extra 2.5% pay raise?")
1357 The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
1358 meridian</a> of Mars goes through the center of the crater
1359 <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
1360 honor of the British astronomer who built the Greenwich telescope that
1361 defines Earth's prime meridian.
1362 Mean solar time on the Mars prime meridian is
1363 called Mars Coordinated Time (<abbr>MTC</abbr>).
1367 Each landed mission on Mars has adopted a different reference for
1368 solar timekeeping, so there is no real standard for Mars time zones.
1369 For example, the MER mission defined two time zones "Local
1370 Solar Time A" and "Local Solar Time B" for its two missions, each zone
1371 designed so that its time equals local true solar time at
1372 approximately the middle of the nominal mission.
1373 The A and B zones differ enough so that an MER worker assigned to
1374 the A zone might suffer "Mars lag" when switching to work in the B zone.
1375 Such a "time zone" is not particularly suited for any application
1376 other than the mission itself.
1380 Many calendars have been proposed for Mars, but none have achieved
1382 Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
1383 sequential count of Mars solar days elapsed since about 1873-12-29
1384 12:00 <abbr>GMT</abbr>.
1388 In our solar system, Mars is the planet with time and calendar most
1390 On other planets, Sun-based time and calendars would work quite
1392 For example, although Mercury's
1393 <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
1394 rotation period</a> is 58.646 Earth days, Mercury revolves around the
1395 Sun so rapidly that an observer on Mercury's equator would see a
1396 sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
1398 Venus is more complicated, partly because its rotation is slightly
1399 <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
1400 its year is 1.92 of its days.
1401 Gas giants like Jupiter are trickier still, as their polar and
1402 equatorial regions rotate at different rates, so that the length of a
1403 day depends on latitude.
1404 This effect is most pronounced on Neptune, where the day is about 12
1405 hours at the poles and 18 hours at the equator.
1409 Although the <code><abbr>tz</abbr></code> database does not support
1410 time on other planets, it is documented here in the hopes that support
1411 will be added eventually.
1415 Sources for time on other planets:
1420 Michael Allison and Robert Schmunk,
1421 "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
1422 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
1427 <em><a href="https://mitpress.mit.edu/books/making-time-mars">Making
1428 Time on Mars</a></em>, MIT Press (March 2020), ISBN 978-0262043854.
1432 "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
1433 Fit for a Martian</a>", <cite>Los Angeles Times</cite>
1434 (2004-01-14), pp A1, A20–A21.
1438 "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
1439 Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
1443 "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
1444 long is a day on the other planets of the solar system?</a>"
1452 This file is in the public domain, so clarified as of 2009-05-17 by