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 off earth</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 Although 1970 is a somewhat-arbitrary cutoff, there are significant
46 challenges to moving the cutoff earlier even by a decade or two, due
47 to the wide variety of local practices before computer timekeeping
49 Most timezones correspond to a notable location and the database
50 records all known clock transitions for that location;
51 some timezones correspond instead to a fixed <abbr>UTC</abbr> offset.
55 Each timezone typically corresponds to a geographical region that is
56 smaller than a traditional time zone, because clocks in a timezone
57 all agree after 1970 whereas a traditional time zone merely
58 specifies current standard time. For example, applications that deal
59 with current and future timestamps in the traditional North
60 American mountain time zone can choose from the timezones
61 <code>America/Denver</code> which observes US-style daylight saving
62 time (<abbr>DST</abbr>),
63 and <code>America/Phoenix</code> which does not observe <abbr>DST</abbr>.
64 Applications that also deal with past timestamps in the mountain time
65 zone can choose from over a dozen timezones, such as
66 <code>America/Boise</code>, <code>America/Edmonton</code>, and
67 <code>America/Hermosillo</code>, each of which currently uses mountain
68 time but differs from other timezones for some timestamps after 1970.
72 Clock transitions before 1970 are recorded for location-based timezones,
73 because most systems support timestamps before 1970 and could
74 misbehave if data entries were omitted for pre-1970 transitions.
75 However, the database is not designed for and does not suffice for
76 applications requiring accurate handling of all past times everywhere,
77 as it would take far too much effort and guesswork to record all
78 details of pre-1970 civil timekeeping.
79 Although some information outside the scope of the database is
80 collected in a file <code>backzone</code> that is distributed along
81 with the database proper, this file is less reliable and does not
82 necessarily follow database guidelines.
86 As described below, reference source code for using the
87 <code><abbr>tz</abbr></code> database is also available.
88 The <code><abbr>tz</abbr></code> code is upwards compatible with <a
89 href="https://en.wikipedia.org/wiki/POSIX">POSIX</a>, an international
91 href="https://en.wikipedia.org/wiki/Unix">UNIX</a>-like systems.
92 As of this writing, the current edition of POSIX is: <a
93 href="https://pubs.opengroup.org/onlinepubs/9699919799/"> The Open
94 Group Base Specifications Issue 7</a>, IEEE Std 1003.1-2017, 2018
96 Because the database's scope encompasses real-world changes to civil
97 timekeeping, its model for describing time is more complex than the
98 standard and daylight saving times supported by POSIX.1-2017.
99 A <code><abbr>tz</abbr></code> timezone corresponds to a ruleset that can
100 have more than two changes per year, these changes need not merely
101 flip back and forth between two alternatives, and the rules themselves
103 Whether and when a timezone changes its clock,
104 and even the timezone's notional base offset from <abbr>UTC</abbr>,
106 It does not always make sense to talk about a timezone's
107 "base offset", which is not necessarily a single number.
113 <h2 id="naming">Timezone identifiers</h2>
115 Each timezone has a name that uniquely identifies the timezone.
116 Inexperienced users are not expected to select these names unaided.
117 Distributors should provide documentation and/or a simple selection
118 interface that explains each name via a map or via descriptive text like
119 "Czech Republic" instead of the timezone name "<code>Europe/Prague</code>".
120 If geolocation information is available, a selection interface can
121 locate the user on a timezone map or prioritize names that are
122 geographically close. For an example selection interface, see the
123 <code>tzselect</code> program in the <code><abbr>tz</abbr></code> code.
124 The <a href="https://cldr.unicode.org">Unicode Common Locale Data
125 Repository</a> contains data that may be useful for other selection
126 interfaces; it maps timezone names like <code>Europe/Prague</code> to
127 locale-dependent strings like "Prague", "Praha", "Прага", and "布拉格".
131 The naming conventions attempt to strike a balance
132 among the following goals:
137 Uniquely identify every timezone where clocks have agreed since 1970.
138 This is essential for the intended use: static clocks keeping local
142 Indicate to experts where the timezone's clocks typically are.
145 Be robust in the presence of political changes.
146 For example, names are typically not tied to countries, to avoid
147 incompatibilities when countries change their name (e.g.,
148 Swaziland→Eswatini) or when locations change countries (e.g., Hong
149 Kong from UK colony to China).
150 There is no requirement that every country or national
151 capital must have a timezone name.
154 Be portable to a wide variety of implementations.
157 Use a consistent naming conventions over the entire world.
162 Names normally have the form
163 <var>AREA</var><code>/</code><var>LOCATION</var>, where
164 <var>AREA</var> is a continent or ocean, and
165 <var>LOCATION</var> is a specific location within the area.
166 North and South America share the same area, '<code>America</code>'.
167 Typical names are '<code>Africa/Cairo</code>',
168 '<code>America/New_York</code>', and '<code>Pacific/Honolulu</code>'.
169 Some names are further qualified to help avoid confusion; for example,
170 '<code>America/Indiana/Petersburg</code>' distinguishes Petersburg,
171 Indiana from other Petersburgs in America.
175 Here are the general guidelines used for
176 choosing timezone names,
177 in decreasing order of importance:
182 Use only valid POSIX file name components (i.e., the parts of
183 names other than '<code>/</code>').
184 Do not use the file name components '<code>.</code>' and
186 Within a file name component, use only <a
187 href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> letters,
188 '<code>.</code>', '<code>-</code>' and '<code>_</code>'.
189 Do not use digits, as that might create an ambiguity with <a
190 href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03">POSIX.1-2017
191 <code>TZ</code> strings</a>.
192 A file name component must not exceed 14 characters or start with
194 E.g., prefer <code>America/Noronha</code> to
195 <code>America/Fernando_de_Noronha</code>.
196 Exceptions: see the discussion of legacy names below.
199 A name must not be empty, or contain '<code>//</code>', or
200 start or end with '<code>/</code>'.
203 Do not use names that differ only in case.
204 Although the reference implementation is case-sensitive, some
205 other implementations are not, and they would mishandle names
206 differing only in case.
209 If one name <var>A</var> is an initial prefix of another
210 name <var>AB</var> (ignoring case), then <var>B</var> must not
211 start with '<code>/</code>', as a regular file cannot have the
212 same name as a directory in POSIX.
213 For example, <code>America/New_York</code> precludes
214 <code>America/New_York/Bronx</code>.
217 Uninhabited regions like the North Pole and Bouvet Island
218 do not need locations, since local time is not defined there.
221 If all the clocks in a timezone have agreed since 1970,
222 do not bother to include more than one timezone
223 even if some of the clocks disagreed before 1970.
224 Otherwise these tables would become annoyingly large.
227 If boundaries between regions are fluid, such as during a war or
228 insurrection, do not bother to create a new timezone merely
229 because of yet another boundary change. This helps prevent table
230 bloat and simplifies maintenance.
233 If a name is ambiguous, use a less ambiguous alternative;
234 e.g., many cities are named San José and Georgetown, so
235 prefer <code>America/Costa_Rica</code> to
236 <code>America/San_Jose</code> and <code>America/Guyana</code>
237 to <code>America/Georgetown</code>.
240 Keep locations compact.
241 Use cities or small islands, not countries or regions, so that any
242 future changes do not split individual locations into different
244 E.g., prefer <code>Europe/Paris</code> to <code>Europe/France</code>,
246 <a href="https://en.wikipedia.org/wiki/Time_in_France#History">France
247 has had multiple time zones</a>.
250 Use mainstream English spelling, e.g., prefer
251 <code>Europe/Rome</code> to <code>Europa/Roma</code>, and
252 prefer <code>Europe/Athens</code> to the Greek
253 <code>Ευρώπη/Αθήνα</code> or the Romanized
254 <code>Evrópi/Athína</code>.
255 The POSIX file name restrictions encourage this guideline.
258 Use the most populous among locations in a region,
259 e.g., prefer <code>Asia/Shanghai</code> to
260 <code>Asia/Beijing</code>.
261 Among locations with similar populations, pick the best-known
262 location, e.g., prefer <code>Europe/Rome</code> to
263 <code>Europe/Milan</code>.
266 Use the singular form, e.g., prefer <code>Atlantic/Canary</code> to
267 <code>Atlantic/Canaries</code>.
270 Omit common suffixes like '<code>_Islands</code>' and
271 '<code>_City</code>', unless that would lead to ambiguity.
272 E.g., prefer <code>America/Cayman</code> to
273 <code>America/Cayman_Islands</code> and
274 <code>America/Guatemala</code> to
275 <code>America/Guatemala_City</code>, but prefer
276 <code>America/Mexico_City</code> to
277 <code>America/Mexico</code>
278 because <a href="https://en.wikipedia.org/wiki/Time_in_Mexico">the
279 country of Mexico has several time zones</a>.
282 Use '<code>_</code>' to represent a space.
285 Omit '<code>.</code>' from abbreviations in names.
286 E.g., prefer <code>Atlantic/St_Helena</code> to
287 <code>Atlantic/St._Helena</code>.
290 Do not change established names if they only marginally violate
291 the above guidelines.
292 For example, do not change the existing name <code>Europe/Rome</code> to
293 <code>Europe/Milan</code> merely because Milan's population has grown
294 to be somewhat greater than Rome's.
297 If a name is changed, put its old spelling in the
298 '<code>backward</code>' file as a link to the new spelling.
299 This means old spellings will continue to work.
300 Ordinarily a name change should occur only in the rare case when
301 a location's consensus English-language spelling changes; for example,
302 in 2008 <code>Asia/Calcutta</code> was renamed to <code>Asia/Kolkata</code>
303 due to long-time widespread use of the new city name instead of the old.
308 Guidelines have evolved with time, and names following old versions of
309 these guidelines might not follow the current version. When guidelines
310 have changed, old names continue to be supported. Guideline changes
311 have included the following:
316 Older versions of this package used a different naming scheme.
317 See the file '<code>backward</code>' for most of these older names
318 (e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>').
319 The other old-fashioned names still supported are
320 '<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and
321 '<code>EET</code>' (see the file '<code>europe</code>').
325 Older versions of this package defined legacy names that are
326 incompatible with the first guideline of location names, but which are
328 These legacy names are mostly defined in the file
329 '<code>etcetera</code>'.
330 Also, the file '<code>backward</code>' defines the legacy names
331 '<code>Etc/GMT0</code>', '<code>Etc/GMT-0</code>', '<code>Etc/GMT+0</code>',
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.
361 The backward-compatibility file <code>zone.tab</code> is similar
362 but conforms to the older-version guidelines related to <abbr>ISO</abbr> 3166-1;
363 it lists only one country code per entry and unlike <code>zone1970.tab</code>
364 it can list names defined in <code>backward</code>.
365 Applications that process only timestamps from now on can instead use the file
366 <code>zonenow.tab</code>, which partitions the world more coarsely,
367 into regions where clocks agree now and in the predicted future;
368 this file is smaller and simpler than <code>zone1970.tab</code>
369 and <code>zone.tab</code>.
373 The database defines each timezone name to be a zone, or a link to a zone.
374 The source file <code>backward</code> defines links for backward
375 compatibility; it does not define zones.
376 Although <code>backward</code> was originally designed to be optional,
377 nowadays distributions typically use it
378 and no great weight should be attached to whether a link
379 is defined in <code>backward</code> or in some other file.
380 The source file <code>etcetera</code> defines names that may be useful
381 on platforms that do not support POSIX.1-2017-style <code>TZ</code> strings;
382 no other source file other than <code>backward</code>
383 contains links to its zones.
384 One of <code>etcetera</code>'s names is <code>Etc/UTC</code>,
385 used by functions like <code>gmtime</code> to obtain leap
386 second information on platforms that support leap seconds.
387 Another <code>etcetera</code> name, <code>GMT</code>,
388 is used by older code releases.
393 <h2 id="abbreviations">Time zone abbreviations</h2>
395 When this package is installed, it generates time zone abbreviations
396 like '<code>EST</code>' to be compatible with human tradition and POSIX.
397 Here are the general guidelines used for choosing time zone abbreviations,
398 in decreasing order of importance:
403 Use three to six characters that are ASCII alphanumerics or
404 '<code>+</code>' or '<code>-</code>'.
405 Previous editions of this database also used characters like
406 space and '<code>?</code>', but these characters have a
407 special meaning to the
408 <a href="https://en.wikipedia.org/wiki/Unix_shell">UNIX shell</a>
409 and cause commands like
410 '<code><a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#set">set</a>
411 `<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html">date</a>`</code>'
412 to have unexpected effects.
413 Previous editions of this guideline required upper-case letters, but the
414 Congressman who introduced
415 <a href="https://en.wikipedia.org/wiki/Chamorro_Time_Zone">Chamorro
416 Standard Time</a> preferred "ChST", so lower-case letters are now
418 Also, POSIX from 2001 on relaxed the rule to allow '<code>-</code>',
419 '<code>+</code>', and alphanumeric characters from the portable
420 character set in the current locale.
421 In practice ASCII alphanumerics and '<code>+</code>' and
422 '<code>-</code>' are safe in all locales.
425 In other words, in the C locale the POSIX extended regular
426 expression <code>[-+[:alnum:]]{3,6}</code> should match the
428 This guarantees that all abbreviations could have been specified by a
429 POSIX.1-2017 <code>TZ</code> string.
433 Use abbreviations that are in common use among English-speakers,
434 e.g., 'EST' for Eastern Standard Time in North America.
435 We assume that applications translate them to other languages
436 as part of the normal localization process; for example,
437 a French application might translate 'EST' to 'HNE'.
440 <small>These abbreviations (for standard/daylight/etc. time) are:
441 ACST/ACDT Australian Central,
442 AST/ADT/APT/AWT/ADDT Atlantic,
443 AEST/AEDT Australian Eastern,
444 AHST/AHDT Alaska-Hawaii,
446 AWST/AWDT Australian Western,
448 CAT/CAST Central Africa,
449 CET/CEST/CEMT Central European,
451 CST/CDT/CWT/CPT Central [North America],
453 GMT/BST/IST/BDST Greenwich,
455 EST/EDT/EWT/EPT Eastern [North America],
456 EET/EEST Eastern European,
458 HST/HDT/HWT/HPT Hawaii,
459 HKT/HKST/HKWT Hong Kong,
465 MET/MEST Middle European (a backward-compatibility alias for
468 MST/MDT/MWT/MPT Mountain,
469 NST/NDT/NWT/NPT/NDDT Newfoundland,
470 NST/NDT/NWT/NPT Nome,
471 NZMT/NZST New Zealand through 1945,
472 NZST/NZDT New Zealand 1946–present,
474 PST/PDT/PWT/PPT Pacific,
479 WAT/WAST West Africa,
480 WET/WEST/WEMT Western European,
481 WIB Waktu Indonesia Barat,
482 WIT Waktu Indonesia Timur,
483 WITA Waktu Indonesia Tengah,
484 YST/YDT/YWT/YPT/YDDT Yukon</small>.
489 For times taken from a city's longitude, use the
490 traditional <var>x</var>MT notation.
491 The only abbreviation like this in current use is '<abbr>GMT</abbr>'.
492 The others are for timestamps before 1960,
493 except that Monrovia Mean Time persisted until 1972.
494 Typically, numeric abbreviations (e.g., '<code>-</code>004430' for
495 MMT) would cause trouble here, as the numeric strings would exceed
496 the POSIX length limit.
500 <small>These abbreviations are:
501 AMT Asunción, Athens;
502 BMT Baghdad, Bangkok, Batavia, Bermuda, Bern, Bogotá,
504 CMT Calamarca, Caracas, Chisinau, Colón, Córdoba;
510 HMT Havana, Helsinki, Horta, Howrah;
511 IMT Irkutsk, Istanbul;
513 KMT Kaunas, Kyiv, Kingston;
514 LMT Lima, Lisbon, local;
515 MMT Macassar, Madras, Malé, Managua, Minsk, Monrovia, Montevideo,
518 PMT Paramaribo, Paris, Perm, Pontianak, Prague;
522 RMT Rangoon, Riga, Rome;
525 SMT Santiago, Simferopol, Singapore, Stanley;
532 <small>A few abbreviations also follow the pattern that
533 <abbr>GMT</abbr>/<abbr>BST</abbr> established for time in the UK.
535 BMT/BST for Bermuda 1890–1930,
536 CMT/BST for Calamarca Mean Time and Bolivian Summer Time
538 DMT/IST for Dublin/Dunsink Mean Time and Irish Summer Time
540 MMT/MST/MDST for Moscow 1880–1919, and
541 RMT/LST for Riga Mean Time and Latvian Summer time 1880–1926.
546 Use '<abbr>LMT</abbr>' for local mean time of locations before the
547 introduction of standard time; see "<a href="#scope">Scope of the
548 <code><abbr>tz</abbr></code> database</a>".
551 If there is no common English abbreviation, use numeric offsets like
552 <code>-</code>05 and <code>+</code>0530 that are generated
553 by <code>zic</code>'s <code>%z</code> notation.
556 Use current abbreviations for older timestamps to avoid confusion.
557 For example, in 1910 a common English abbreviation for time
558 in central Europe was 'MEZ' (short for both "Middle European
559 Zone" and for "Mitteleuropäische Zeit" in German).
560 Nowadays 'CET' ("Central European Time") is more common in
561 English, and the database uses 'CET' even for circa-1910
562 timestamps as this is less confusing for modern users and avoids
563 the need for determining when 'CET' supplanted 'MEZ' in common
567 Use a consistent style in a timezone's history.
568 For example, if a history tends to use numeric
569 abbreviations and a particular entry could go either way, use a
570 numeric abbreviation.
574 <a href="https://en.wikipedia.org/wiki/Universal_Time">Universal Time</a>
575 (<abbr>UT</abbr>) (with time zone abbreviation '<code>-</code>00') for
576 locations while uninhabited.
577 The leading '<code>-</code>' is a flag that the <abbr>UT</abbr> offset is in
578 some sense undefined; this notation is derived
579 from <a href="https://datatracker.ietf.org/doc/html/rfc3339">Internet
580 <abbr title="Request For Comments">RFC</abbr> 3339</a>.
585 Application writers should note that these abbreviations are ambiguous
586 in practice: e.g., 'CST' means one thing in China and something else
587 in North America, and 'IST' can refer to time in India, Ireland or
589 To avoid ambiguity, use numeric <abbr>UT</abbr> offsets like
590 '<code>-</code>0600' instead of time zone abbreviations like 'CST'.
595 <h2 id="accuracy">Accuracy of the <code><abbr>tz</abbr></code> database</h2>
597 The <code><abbr>tz</abbr></code> database is not authoritative, and it
599 Corrections are welcome and encouraged; see the file <code>CONTRIBUTING</code>.
600 Users requiring authoritative data should consult national standards
601 bodies and the references cited in the database's comments.
605 Errors in the <code><abbr>tz</abbr></code> database arise from many sources:
610 The <code><abbr>tz</abbr></code> database predicts future
611 timestamps, and current predictions
612 will be incorrect after future governments change the rules.
613 For example, if today someone schedules a meeting for 13:00 next
614 October 1, Casablanca time, and tomorrow Morocco changes its
615 daylight saving rules, software can mess up after the rule change
616 if it blithely relies on conversions made before the change.
619 The pre-1970 entries in this database cover only a tiny sliver of how
620 clocks actually behaved; the vast majority of the necessary
621 information was lost or never recorded.
622 Thousands more timezones would be needed if
623 the <code><abbr>tz</abbr></code> database's scope were extended to
624 cover even just the known or guessed history of standard time; for
625 example, the current single entry for France would need to split
626 into dozens of entries, perhaps hundreds.
627 And in most of the world even this approach would be misleading
628 due to widespread disagreement or indifference about what times
632 href="https://www.hup.harvard.edu/catalog.php?isbn=9780674286146">The
633 Global Transformation of Time, 1870–1950</a></cite>,
635 "Outside of Europe and North America there was no system of time
636 zones at all, often not even a stable landscape of mean times,
637 prior to the middle decades of the twentieth century".
638 See: Timothy Shenk, <a
639 href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked:
640 A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17.
643 Most of the pre-1970 data entries come from unreliable sources, often
644 astrology books that lack citations and whose compilers evidently
645 invented entries when the true facts were unknown, without
646 reporting which entries were known and which were invented.
647 These books often contradict each other or give implausible entries,
648 and on the rare occasions when they are checked they are
649 typically found to be incorrect.
652 For the UK the <code><abbr>tz</abbr></code> database relies on
653 years of first-class work done by
654 Joseph Myers and others; see
655 "<a href="https://www.polyomino.org.uk/british-time/">History of
656 legal time in Britain</a>".
657 Other countries are not done nearly as well.
660 Sometimes, different people in the same city maintain clocks
661 that differ significantly.
662 Historically, railway time was used by railroad companies (which
664 agree with each other), church-clock time was used for birth
666 More recently, competing political groups might disagree about
667 clock settings. Often this is merely common practice, but
668 sometimes it is set by law.
669 For example, from 1891 to 1911 the <abbr>UT</abbr> offset in France
670 was legally <abbr>UT</abbr> +00:09:21 outside train stations and
671 <abbr>UT</abbr> +00:04:21 inside. Other examples include
672 Chillicothe in 1920, Palm Springs in 1946/7, and Jerusalem and
676 Although a named location in the <code><abbr>tz</abbr></code>
677 database stands for the containing region, its pre-1970 data
678 entries are often accurate for only a small subset of that region.
679 For example, <code>Europe/London</code> stands for the United
680 Kingdom, but its pre-1847 times are valid only for locations that
681 have London's exact meridian, and its 1847 transition
682 to <abbr>GMT</abbr> is known to be valid only for the L&NW and
683 the Caledonian railways.
686 The <code><abbr>tz</abbr></code> database does not record the
687 earliest time for which a timezone's
688 data entries are thereafter valid for every location in the region.
689 For example, <code>Europe/London</code> is valid for all locations
690 in its region after <abbr>GMT</abbr> was made the standard time,
691 but the date of standardization (1880-08-02) is not in the
692 <code><abbr>tz</abbr></code> database, other than in commentary.
693 For many timezones the earliest time of
697 The <code><abbr>tz</abbr></code> database does not record a
698 region's boundaries, and in many cases the boundaries are not known.
699 For example, the timezone
700 <code>America/Kentucky/Louisville</code> represents a region
701 around the city of Louisville, the boundaries of which are
705 Changes that are modeled as instantaneous transitions in the
706 <code><abbr>tz</abbr></code>
707 database were often spread out over hours, days, or even decades.
710 Even if the time is specified by law, locations sometimes
711 deliberately flout the law.
714 Early timekeeping practices, even assuming perfect clocks, were
715 often not specified to the accuracy that the
716 <code><abbr>tz</abbr></code> database requires.
719 The <code><abbr>tz</abbr></code> database cannot represent stopped clocks.
720 However, on 1911-03-11 at 00:00, some public-facing French clocks
721 were changed by stopping them for a few minutes to effect a transition.
722 The <code><abbr>tz</abbr></code> database models this via a
723 backward transition; the relevant French legislation does not
724 specify exactly how the transition was to occur.
727 Sometimes historical timekeeping was specified more precisely
728 than what the <code><abbr>tz</abbr></code> code can handle.
729 For example, from 1880 to 1916 clocks in Ireland observed Dublin Mean
730 Time (estimated to be <abbr>UT</abbr>
731 −00:25:21.1); although the <code><abbr>tz</abbr></code>
732 source data can represent the .1 second, TZif files and the code cannot.
733 In practice these old specifications were rarely if ever
734 implemented to subsecond precision.
737 Even when all the timestamp transitions recorded by the
738 <code><abbr>tz</abbr></code> database are correct, the
739 <code><abbr>tz</abbr></code> rules that generate them may not
740 faithfully reflect the historical rules.
741 For example, from 1922 until World War II the UK moved clocks
742 forward the day following the third Saturday in April unless that
743 was Easter, in which case it moved clocks forward the previous
745 Because the <code><abbr>tz</abbr></code> database has no
746 way to specify Easter, these exceptional years are entered as
747 separate <code><abbr>tz</abbr> Rule</code> lines, even though the
748 legal rules did not change.
749 When transitions are known but the historical rules behind them are not,
750 the database contains <code>Zone</code> and <code>Rule</code>
751 entries that are intended to represent only the generated
752 transitions, not any underlying historical rules; however, this
753 intent is recorded at best only in commentary.
756 The <code><abbr>tz</abbr></code> database models time
758 href="https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">proleptic
759 Gregorian calendar</a> with days containing 24 equal-length hours
760 numbered 00 through 23, except when clock transitions occur.
761 Pre-standard time is modeled as local mean time.
762 However, historically many people used other calendars and other timescales.
763 For example, the Roman Empire used
764 the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
766 and <a href="https://en.wikipedia.org/wiki/Roman_timekeeping">Roman
767 timekeeping</a> had twelve varying-length daytime hours with a
768 non-hour-based system at night.
769 And even today, some local practices diverge from the Gregorian
770 calendar with 24-hour days. These divergences range from
771 relatively minor, such as Japanese bars giving times like "24:30" for the
772 wee hours of the morning, to more-significant differences such as <a
773 href="https://theworld.org/stories/2015-01-30/if-you-have-meeting-ethiopia-you-better-double-check-time">the
774 east African practice of starting the day at dawn</a>, renumbering
775 the Western 06:00 to be 12:00. These practices are largely outside
776 the scope of the <code><abbr>tz</abbr></code> code and data, which
777 provide only limited support for date and time localization
778 such as that required by POSIX.1-2017.
779 If <abbr>DST</abbr> is not used a different time zone
780 can often do the trick; for example, in Kenya a <code>TZ</code> setting
781 like <code><-03>3</code> or <code>America/Cayenne</code> starts
782 the day six hours later than <code>Africa/Nairobi</code> does.
785 Early clocks were less reliable, and data entries do not represent
789 The <code><abbr>tz</abbr></code> database assumes Universal Time
790 (<abbr>UT</abbr>) as an origin, even though <abbr>UT</abbr> is not
791 standardized for older timestamps.
792 In the <code><abbr>tz</abbr></code> database commentary,
793 <abbr>UT</abbr> denotes a family of time standards that includes
794 Coordinated Universal Time (<abbr>UTC</abbr>) along with other
795 variants such as <abbr>UT1</abbr> and <abbr>GMT</abbr>,
796 with days starting at midnight.
797 Although <abbr>UT</abbr> equals <abbr>UTC</abbr> for modern
798 timestamps, <abbr>UTC</abbr> was not defined until 1960, so
799 commentary uses the more general abbreviation <abbr>UT</abbr> for
800 timestamps that might predate 1960.
801 Since <abbr>UT</abbr>, <abbr>UT1</abbr>, etc. disagree slightly,
802 and since pre-1972 <abbr>UTC</abbr> seconds varied in length,
803 interpretation of older timestamps can be problematic when
804 subsecond accuracy is needed.
807 Civil time was not based on atomic time before 1972, and we do not
809 <a href="https://en.wikipedia.org/wiki/Earth's_rotation">earth's
810 rotation</a> accurately enough to map <a
811 href="https://en.wikipedia.org/wiki/International_System_of_Units"><abbr
812 title="International System of Units">SI</abbr></a> seconds to
813 historical <a href="https://en.wikipedia.org/wiki/Solar_time">solar time</a>
814 to more than about one-hour accuracy.
815 See: Stephenson FR, Morrison LV, Hohenkerk CY.
816 <a href="https://dx.doi.org/10.1098/rspa.2016.0404">Measurement of
817 the Earth's rotation: 720 BC to AD 2015</a>.
818 <cite>Proc Royal Soc A</cite>. 2016;472:20160404.
819 Also see: Espenak F. <a
820 href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty
824 The relationship between POSIX time (that is, <abbr>UTC</abbr> but
825 ignoring <a href="https://en.wikipedia.org/wiki/Leap_second">leap
826 seconds</a>) and <abbr>UTC</abbr> is not agreed upon.
827 This affects time stamps during the leap second era (1972–2035).
829 clock officially stops during an inserted leap second, at least one
830 proposed standard has it jumping back a second instead; and in
831 practice POSIX clocks more typically either progress glacially during
832 a leap second, or are slightly slowed while near a leap second.
835 The <code><abbr>tz</abbr></code> database does not represent how
836 uncertain its information is.
837 Ideally it would contain information about when data entries are
839 Partial temporal knowledge is a field of active research, though,
840 and it is not clear how to apply it here.
845 In short, many, perhaps most, of the <code><abbr>tz</abbr></code>
846 database's pre-1970 and future timestamps are either wrong or
848 Any attempt to pass the
849 <code><abbr>tz</abbr></code> database off as the definition of time
850 should be unacceptable to anybody who cares about the facts.
851 In particular, the <code><abbr>tz</abbr></code> database's
852 <abbr>LMT</abbr> offsets should not be considered meaningful, and
853 should not prompt creation of timezones
854 merely because two locations
855 differ in <abbr>LMT</abbr> or transitioned to standard time at
861 <h2 id="functions">Time and date functions</h2>
863 The <code><abbr>tz</abbr></code> code contains time and date functions
864 that are upwards compatible with those of POSIX.
865 Code compatible with this package is already
866 <a href="tz-link.html#tzdb">part of many platforms</a>, where the
867 primary use of this package is to update obsolete time-related files.
868 To do this, you may need to compile the time zone compiler
869 '<code>zic</code>' supplied with this package instead of using the
870 system '<code>zic</code>', since the format of <code>zic</code>'s
871 input is occasionally extended, and a platform may still be shipping
872 an older <code>zic</code>.
875 <h3 id="POSIX">POSIX.1-2017 properties and limitations</h3>
879 In POSIX.1-2017, time display in a process is controlled by the
880 environment variable <code>TZ</code>.
881 Unfortunately, the POSIX.1-2017
882 <code>TZ</code> string takes a form that is hard to describe and
883 is error-prone in practice.
884 Also, POSIX.1-2017 <code>TZ</code> strings cannot deal with daylight
885 saving time rules not based on the Gregorian calendar (as in
886 Morocco), or with situations where more than two time zone
887 abbreviations or <abbr>UT</abbr> offsets are used in an area.
891 The POSIX.1-2017 <code>TZ</code> string takes the following form:
895 <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>]]]
903 <dt><var>std</var> and <var>dst</var></dt><dd>
904 are 3 or more characters specifying the standard
905 and daylight saving time (<abbr>DST</abbr>) zone abbreviations.
906 Starting with POSIX.1-2001, <var>std</var> and <var>dst</var>
907 may also be in a quoted form like '<code><+09></code>';
908 this allows "<code>+</code>" and "<code>-</code>" in the names.
910 <dt><var>offset</var></dt><dd>
912 '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>'
913 and specifies the offset west of <abbr>UT</abbr>.
914 '<var>hh</var>' may be a single digit;
915 0≤<var>hh</var>≤24.
916 The default <abbr>DST</abbr> offset is one hour ahead of
919 <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd>
920 specifies the beginning and end of <abbr>DST</abbr>.
921 If this is absent, the system supplies its own ruleset
922 for <abbr>DST</abbr>, typically current <abbr>US</abbr>
923 <abbr>DST</abbr> rules.
925 <dt><var>time</var></dt><dd>
927 '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]'
928 and defaults to 02:00.
929 This is the same format as the offset, except that a
930 leading '<code>+</code>' or '<code>-</code>' is not allowed.
932 <dt><var>date</var></dt><dd>
933 takes one of the following forms:
935 <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd>
936 origin-1 day number not counting February 29
938 <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd>
939 origin-0 day number counting February 29 if present
941 <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var>
942 (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5,
943 1≤<var>m</var>≤12)</dt><dd>
944 for the <var>d</var>th day of week <var>n</var> of
945 month <var>m</var> of the year, where week 1 is the first
946 week in which day <var>d</var> appears, and
947 '<code>5</code>' stands for the last week in which
948 day <var>d</var> appears (which may be either the 4th or
950 Typically, this is the only useful form; the <var>n</var>
951 and <code>J</code><var>n</var> forms are rarely used.
958 Here is an example POSIX.1-2017 <code>TZ</code> string for New
960 It says that standard time (<abbr>NZST</abbr>) is 12 hours ahead
961 of <abbr>UT</abbr>, and that daylight saving time
962 (<abbr>NZDT</abbr>) is observed from September's last Sunday at
963 02:00 until April's first Sunday at 03:00:
966 <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre>
969 This POSIX.1-2017 <code>TZ</code> string is hard to remember, and
970 mishandles some timestamps before 2008.
971 With this package you can use this instead:
974 <pre><code>TZ='Pacific/Auckland'</code></pre>
977 POSIX does not define the <abbr>DST</abbr> transitions
978 for <code>TZ</code> values like
979 "<code>EST5EDT</code>".
980 Traditionally the current <abbr>US</abbr> <abbr>DST</abbr> rules
981 were used to interpret such values, but this meant that the
982 <abbr>US</abbr> <abbr>DST</abbr> rules were compiled into each
983 time conversion package, and when
984 <abbr>US</abbr> time conversion rules changed (as in the United
985 States in 1987 and again in 2007), all packages that
986 interpreted <code>TZ</code> values had to be updated
987 to ensure proper results.
990 The <code>TZ</code> environment variable is process-global, which
991 makes it hard to write efficient, thread-safe applications that
992 need access to multiple timezones.
995 In POSIX, there is no tamper-proof way for a process to learn the
996 system's best idea of local (wall clock) time.
997 This is important for applications that an administrator wants
998 used only at certain times – without regard to whether the
1000 <code>TZ</code> environment variable.
1001 While an administrator can "do everything in <abbr>UT</abbr>" to
1002 get around the problem, doing so is inconvenient and precludes
1003 handling daylight saving time shifts – as might be required to
1004 limit phone calls to off-peak hours.
1007 POSIX.1-2017 provides no convenient and efficient way to determine
1008 the <abbr>UT</abbr> offset and time zone abbreviation of arbitrary
1009 timestamps, particularly for timezones
1010 that do not fit into the POSIX model.
1013 POSIX requires that <code>time_t</code> clock counts exclude leap
1017 The <code><abbr>tz</abbr></code> code attempts to support all the
1018 <code>time_t</code> implementations allowed by POSIX.
1019 The <code>time_t</code> type represents a nonnegative count of seconds
1020 since 1970-01-01 00:00:00 <abbr>UTC</abbr>, ignoring leap seconds.
1021 In practice, <code>time_t</code> is usually a signed 64- or 32-bit
1022 integer; 32-bit signed <code>time_t</code> values stop working after
1023 2038-01-19 03:14:07 <abbr>UTC</abbr>, so new implementations these
1024 days typically use a signed 64-bit integer.
1025 Unsigned 32-bit integers are used on one or two platforms, and 36-bit
1026 and 40-bit integers are also used occasionally.
1027 Although earlier POSIX versions allowed <code>time_t</code> to be a
1028 floating-point type, this was not supported by any practical system,
1029 and POSIX.1-2013 and the <code><abbr>tz</abbr></code> code both
1030 require <code>time_t</code> to be an integer type.
1034 <h3 id="POSIX-extensions">Extensions to POSIX.1-2017 in the
1035 <code><abbr>tz</abbr></code> code</h3>
1039 The <code>TZ</code> environment variable is used in generating
1040 the name of a file from which time-related information is read
1041 (or is interpreted à la POSIX.1-2017); <code>TZ</code> is no longer
1042 constrained to be a string containing abbreviations
1043 and numeric data as described <a href="#POSIX">above</a>.
1044 The file's format is <dfn><abbr>TZif</abbr></dfn>,
1045 a timezone information format that contains binary data; see
1046 <a href="https://datatracker.ietf.org/doc/html/8536">Internet
1047 <abbr>RFC</abbr> 8536</a>.
1048 The daylight saving time rules to be used for a
1049 particular timezone are encoded in the
1050 <abbr>TZif</abbr> file; the format of the file allows <abbr>US</abbr>,
1051 Australian, and other rules to be encoded, and
1052 allows for situations where more than two time zone
1053 abbreviations are used.
1056 It was recognized that allowing the <code>TZ</code> environment
1057 variable to take on values such as '<code>America/New_York</code>'
1058 might cause "old" programs (that expect <code>TZ</code> to have a
1059 certain form) to operate incorrectly; consideration was given to using
1060 some other environment variable (for example, <code>TIMEZONE</code>)
1061 to hold the string used to generate the <abbr>TZif</abbr> file's name.
1062 In the end, however, it was decided to continue using
1063 <code>TZ</code>: it is widely used for time zone purposes;
1064 separately maintaining both <code>TZ</code>
1065 and <code>TIMEZONE</code> seemed a nuisance; and systems where
1066 "new" forms of <code>TZ</code> might cause problems can simply
1067 use legacy <code>TZ</code> values such as "<code>EST5EDT</code>" which
1068 can be used by "new" programs as well as by "old" programs that
1069 assume pre-POSIX <code>TZ</code> values.
1073 The code supports platforms with a <abbr>UT</abbr> offset member
1074 in <code>struct tm</code>, e.g., <code>tm_gmtoff</code>,
1075 or with a time zone abbreviation member in
1076 <code>struct tm</code>, e.g., <code>tm_zone</code>. As noted
1077 in <a href="https://austingroupbugs.net/view.php?id=1533">Austin
1078 Group defect 1533</a>, a future version of POSIX is planned to
1079 require <code>tm_gmtoff</code> and <code>tm_zone</code>.
1082 Functions <code>tzalloc</code>, <code>tzfree</code>,
1083 <code>localtime_rz</code>, and <code>mktime_z</code> for
1084 more-efficient thread-safe applications that need to use multiple
1086 The <code>tzalloc</code> and <code>tzfree</code> functions
1087 allocate and free objects of type <code>timezone_t</code>,
1088 and <code>localtime_rz</code> and <code>mktime_z</code> are
1089 like <code>localtime_r</code> and <code>mktime</code> with an
1090 extra <code>timezone_t</code> argument.
1091 The functions were inspired by <a href="https://netbsd.org/">NetBSD</a>.
1094 Negative <code>time_t</code> values are supported, on systems
1095 where <code>time_t</code> is signed.
1098 These functions can account for leap seconds;
1099 see <a href="#leapsec">Leap seconds</a> below.
1103 <h3 id="vestigial">POSIX features no longer needed</h3>
1105 POSIX and <a href="https://en.wikipedia.org/wiki/ISO_C"><abbr>ISO</abbr> C</a>
1106 define some <a href="https://en.wikipedia.org/wiki/API"><abbr
1107 title="application programming interface">API</abbr>s</a> that are vestigial:
1108 they are not needed, and are relics of a too-simple model that does
1109 not suffice to handle many real-world timestamps.
1110 Although the <code><abbr>tz</abbr></code> code supports these
1111 vestigial <abbr>API</abbr>s for backwards compatibility, they should
1112 be avoided in portable applications.
1113 The vestigial <abbr>API</abbr>s are:
1117 The POSIX <code>tzname</code> variable does not suffice and is no
1119 To get a timestamp's time zone abbreviation, consult
1120 the <code>tm_zone</code> member if available; otherwise,
1121 use <code>strftime</code>'s <code>"%Z"</code> conversion
1125 The POSIX <code>daylight</code> and <code>timezone</code>
1126 variables do not suffice and are no longer needed.
1127 To get a timestamp's <abbr>UT</abbr> offset, consult
1128 the <code>tm_gmtoff</code> member if available; otherwise,
1129 subtract values returned by <code>localtime</code>
1130 and <code>gmtime</code> using the rules of the Gregorian calendar,
1131 or use <code>strftime</code>'s <code>"%z"</code> conversion
1132 specification if a string like <code>"+0900"</code> suffices.
1135 The <code>tm_isdst</code> member is almost never needed and most of
1136 its uses should be discouraged in favor of the abovementioned
1138 Although it can still be used in arguments to
1139 <code>mktime</code> to disambiguate timestamps near
1140 a <abbr>DST</abbr> transition when the clock jumps back on
1141 platforms lacking <code>tm_gmtoff</code>, this
1142 disambiguation does not work when standard time itself jumps back,
1143 which can occur when a location changes to a time zone with a
1144 lesser <abbr>UT</abbr> offset.
1148 <h3 id="other-portability">Other portability notes</h3>
1151 The <a href="https://en.wikipedia.org/wiki/Version_7_Unix">7th Edition
1152 UNIX</a> <code>timezone</code> function is not present in this
1153 package; it is impossible to reliably map <code>timezone</code>'s
1154 arguments (a "minutes west of <abbr>GMT</abbr>" value and a
1155 "daylight saving time in effect" flag) to a time zone
1156 abbreviation, and we refuse to guess.
1157 Programs that in the past used the <code>timezone</code> function
1158 may now examine <code>localtime(&clock)->tm_zone</code>
1159 (if <code>TM_ZONE</code> is defined) or
1160 <code>tzname[localtime(&clock)->tm_isdst]</code>
1161 (if <code>HAVE_TZNAME</code> is nonzero) to learn the correct time
1162 zone abbreviation to use.
1166 href="https://en.wikipedia.org/wiki/History_of_the_Berkeley_Software_Distribution#4.2BSD"><abbr>4.2BSD</abbr></a>
1167 <code>gettimeofday</code> function is not
1168 used in this package.
1169 This formerly let users obtain the current <abbr>UTC</abbr> offset
1170 and <abbr>DST</abbr> flag, but this functionality was removed in
1171 later versions of <abbr>BSD</abbr>.
1174 In <abbr>SVR2</abbr>, time conversion fails for near-minimum or
1175 near-maximum <code>time_t</code> values when doing conversions
1176 for places that do not use <abbr>UT</abbr>.
1177 This package takes care to do these conversions correctly.
1178 A comment in the source code tells how to get compatibly wrong
1182 The functions that are conditionally compiled
1183 if <code>STD_INSPIRED</code> is nonzero should, at this point, be
1184 looked on primarily as food for thought.
1185 They are not in any sense "standard compatible" – some are
1186 not, in fact, specified in <em>any</em> standard.
1187 They do, however, represent responses of various authors to
1188 standardization proposals.
1191 Other time conversion proposals, in particular those supported by the
1192 <a href="https://howardhinnant.github.io/date/tz.html">Time Zone
1193 Database Parser</a>, offer a wider selection of functions
1194 that provide capabilities beyond those provided here.
1195 The absence of such functions from this package is not meant to
1196 discourage the development, standardization, or use of such
1198 Rather, their absence reflects the decision to make this package
1199 contain valid extensions to POSIX, to ensure its broad
1201 If more powerful time conversion functions can be standardized, so
1208 <h2 id="stability">Interface stability</h2>
1210 The <code><abbr>tz</abbr></code> code and data supply the following interfaces:
1215 A set of timezone names as per
1216 "<a href="#naming">Timezone identifiers</a>" above.
1219 Library functions described in "<a href="#functions">Time and date
1220 functions</a>" above.
1223 The programs <code>tzselect</code>, <code>zdump</code>,
1224 and <code>zic</code>, documented in their man pages.
1227 The format of <code>zic</code> input files, documented in
1228 the <code>zic</code> man page.
1231 The format of <code>zic</code> output files, documented in
1232 the <code>tzfile</code> man page.
1235 The format of zone table files, documented in <code>zone1970.tab</code>.
1238 The format of the country code file, documented in <code>iso3166.tab</code>.
1241 The version number of the code and data, as the first line of
1242 the text file '<code>version</code>' in each release.
1247 Interface changes in a release attempt to preserve compatibility with
1249 For example, <code><abbr>tz</abbr></code> data files typically do not
1250 rely on recently added <code>zic</code> features, so that users can
1251 run older <code>zic</code> versions to process newer data files.
1252 <a href="tz-link.html#download">Downloading
1253 the <code><abbr>tz</abbr></code> database</a> describes how releases
1254 are tagged and distributed.
1258 Interfaces not listed above are less stable.
1259 For example, users should not rely on particular <abbr>UT</abbr>
1260 offsets or abbreviations for timestamps, as data entries are often
1261 based on guesswork and these guesses may be corrected or improved.
1265 Timezone boundaries are not part of the stable interface.
1266 For example, even though the <samp>Asia/Bangkok</samp> timezone
1267 currently includes Chang Mai, Hanoi, and Phnom Penh, this is not part
1268 of the stable interface and the timezone can split at any time.
1269 If a calendar application records a future event in some location other
1270 than Bangkok by putting "<samp>Asia/Bangkok</samp>" in the event's record,
1271 the application should be robust in the presence of timezone splits
1272 between now and the future time.
1277 <h2 id="leapsec">Leap seconds</h2>
1279 Leap seconds were introduced in 1972 to accommodate the
1280 difference between atomic time and the less regular rotation of the earth.
1281 Unfortunately they caused so many problems with civil
1282 timekeeping that they
1283 are <a href="https://www.bipm.org/en/cgpm-2022/resolution-4">planned
1284 to be discontinued by 2035</a>, with some as-yet-undetermined
1285 mechanism replacing them, perhaps after the year 2135.
1286 Despite their impending obsolescence, a record of leap seconds is still
1287 needed to resolve timestamps from 1972 through 2035.
1291 The <code><abbr>tz</abbr></code> code and data can account for leap seconds,
1292 thanks to code contributed by Bradley White.
1293 However, the leap second support of this package is rarely used directly
1294 because POSIX requires leap seconds to be excluded and many
1295 software packages would mishandle leap seconds if they were present.
1296 Instead, leap seconds are more commonly handled by occasionally adjusting
1297 the operating system kernel clock as described in
1298 <a href="tz-link.html#precision">Precision timekeeping</a>,
1299 and this package by default installs a <samp>leapseconds</samp> file
1301 <a href="https://www.ntp.org"><abbr title="Network Time Protocol">NTP</abbr></a>
1302 software that adjusts the kernel clock.
1303 However, kernel-clock twiddling approximates UTC only roughly,
1304 and systems needing more precise UTC can use this package's leap
1305 second support directly.
1309 The directly supported mechanism assumes that <code>time_t</code>
1310 counts of seconds since the POSIX epoch normally include leap seconds,
1311 as opposed to POSIX <code>time_t</code> counts which exclude leap seconds.
1312 This modified timescale is converted to <abbr>UTC</abbr>
1313 at the same point that time zone and <abbr>DST</abbr>
1314 adjustments are applied –
1315 namely, at calls to <code>localtime</code> and analogous functions –
1316 and the process is driven by leap second information
1317 stored in alternate versions of the <abbr>TZif</abbr> files.
1318 Because a leap second adjustment may be needed even
1319 if no time zone correction is desired,
1320 calls to <code>gmtime</code>-like functions
1321 also need to consult a <abbr>TZif</abbr> file,
1322 conventionally named <samp><abbr>Etc/UTC</abbr></samp>
1323 (<samp><abbr>GMT</abbr></samp> in previous versions),
1324 to see whether leap second corrections are needed.
1325 To convert an application's <code>time_t</code> timestamps to or from
1326 POSIX <code>time_t</code> timestamps (for use when, say,
1327 embedding or interpreting timestamps in portable
1328 <a href="https://en.wikipedia.org/wiki/Tar_(computing)"><code>tar</code></a>
1330 the application can call the utility functions
1331 <code>time2posix</code> and <code>posix2time</code>
1332 included with this package.
1336 If the POSIX-compatible <abbr>TZif</abbr> file set is installed
1337 in a directory whose basename is <samp>zoneinfo</samp>, the
1338 leap-second-aware file set is by default installed in a separate
1339 directory <samp>zoneinfo-leaps</samp>.
1340 Although each process can have its own time zone by setting
1341 its <code>TZ</code> environment variable, there is no support for some
1342 processes being leap-second aware while other processes are
1343 POSIX-compatible; the leap-second choice is system-wide.
1344 So if you configure your kernel to count leap seconds, you should also
1345 discard <samp>zoneinfo</samp> and rename <samp>zoneinfo-leaps</samp>
1346 to <samp>zoneinfo</samp>.
1347 Alternatively, you can install just one set of <abbr>TZif</abbr> files
1348 in the first place; see the <code>REDO</code> variable in this package's
1349 <a href="https://en.wikipedia.org/wiki/Makefile">makefile</a>.
1354 <h2 id="calendar">Calendrical issues</h2>
1356 Calendrical issues are a bit out of scope for a time zone database,
1357 but they indicate the sort of problems that we would run into if we
1358 extended the time zone database further into the past.
1359 An excellent resource in this area is Edward M. Reingold
1360 and Nachum Dershowitz, <cite><a
1361 href="https://www.cambridge.org/fr/academic/subjects/computer-science/computing-general-interest/calendrical-calculations-ultimate-edition-4th-edition">Calendrical
1362 Calculations: The Ultimate Edition</a></cite>, Cambridge University Press (2018).
1363 Other information and sources are given in the file '<code>calendars</code>'
1364 in the <code><abbr>tz</abbr></code> distribution.
1365 They sometimes disagree.
1370 <h2 id="planets">Time and time zones off Earth</h2>
1372 The European Space Agency is <a
1373 href='https://www.esa.int/Applications/Navigation/Telling_time_on_the_Moon'>considering</a>
1374 the establishment of a reference timescale for the Moon, which has
1375 days roughly equivalent to 29.5 Earth days, and where relativistic
1376 effects cause clocks to tick slightly faster than on Earth.
1380 Some people's work schedules have used
1381 <a href="https://en.wikipedia.org/wiki/Timekeeping_on_Mars">Mars time</a>.
1382 Jet Propulsion Laboratory (JPL) coordinators kept Mars time on
1384 <a href="https://en.wikipedia.org/wiki/Mars_Pathfinder">Mars
1385 Pathfinder</a> mission (1997).
1386 Some of their family members also adapted to Mars time.
1387 Dozens of special Mars watches were built for JPL workers who kept
1388 Mars time during the
1389 <a href="https://en.wikipedia.org/wiki/Mars_Exploration_Rover">Mars
1390 Exploration Rovers (MER)</a> mission (2004–2018).
1391 These timepieces looked like normal Seikos and Citizens but were adjusted
1392 to use Mars seconds rather than terrestrial seconds, although
1393 unfortunately the adjusted watches were unreliable and appear to have
1394 had only limited use.
1398 A Mars solar day is called a "sol" and has a mean period equal to
1399 about 24 hours 39 minutes 35.244 seconds in terrestrial time.
1400 It is divided into a conventional 24-hour clock, so each Mars second
1401 equals about 1.02749125 terrestrial seconds.
1402 (One MER worker noted, "If I am working Mars hours, and Mars hours are
1403 2.5% more than Earth hours, shouldn't I get an extra 2.5% pay raise?")
1407 The <a href="https://en.wikipedia.org/wiki/Prime_meridian">prime
1408 meridian</a> of Mars goes through the center of the crater
1409 <a href="https://en.wikipedia.org/wiki/Airy-0">Airy-0</a>, named in
1410 honor of the British astronomer who built the Greenwich telescope that
1411 defines Earth's prime meridian.
1412 Mean solar time on the Mars prime meridian is
1413 called Mars Coordinated Time (<abbr>MTC</abbr>).
1417 Each landed mission on Mars has adopted a different reference for
1418 solar timekeeping, so there is no real standard for Mars time zones.
1419 For example, the MER mission defined two time zones "Local
1420 Solar Time A" and "Local Solar Time B" for its two missions, each zone
1421 designed so that its time equals local true solar time at
1422 approximately the middle of the nominal mission.
1423 The A and B zones differ enough so that an MER worker assigned to
1424 the A zone might suffer "Mars lag" when switching to work in the B zone.
1425 Such a "time zone" is not particularly suited for any application
1426 other than the mission itself.
1430 Many calendars have been proposed for Mars, but none have achieved
1432 Astronomers often use Mars Sol Date (<abbr>MSD</abbr>) which is a
1433 sequential count of Mars solar days elapsed since about 1873-12-29
1434 12:00 <abbr>GMT</abbr>.
1438 In our solar system, Mars is the planet with time and calendar most
1440 On other planets, Sun-based time and calendars would work quite
1442 For example, although Mercury's
1443 <a href="https://en.wikipedia.org/wiki/Rotation_period">sidereal
1444 rotation period</a> is 58.646 Earth days, Mercury revolves around the
1445 Sun so rapidly that an observer on Mercury's equator would see a
1446 sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a
1448 Venus is more complicated, partly because its rotation is slightly
1449 <a href="https://en.wikipedia.org/wiki/Retrograde_motion">retrograde</a>:
1450 its year is 1.92 of its days.
1451 Gas giants like Jupiter are trickier still, as their polar and
1452 equatorial regions rotate at different rates, so that the length of a
1453 day depends on latitude.
1454 This effect is most pronounced on Neptune, where the day is about 12
1455 hours at the poles and 18 hours at the equator.
1459 Although the <code><abbr>tz</abbr></code> database does not support
1460 time on other planets, it is documented here in the hopes that support
1461 will be added eventually.
1465 Sources for time on other planets:
1470 Michael Allison and Robert Schmunk,
1471 "<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical
1472 Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>"
1477 <em><a href="https://mitpress.mit.edu/books/making-time-mars">Making
1478 Time on Mars</a></em>, MIT Press (March 2020), ISBN 978-0262043854.
1482 "<a href="https://www.latimes.com/archives/la-xpm-2004-jan-14-sci-marstime14-story.html">Workdays
1483 Fit for a Martian</a>", <cite>Los Angeles Times</cite>
1484 (2004-01-14), pp A1, A20–A21.
1488 "<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet
1489 Lag Is Worse on Mars</a>", <cite>The Atlantic</cite> (2015-02-26)
1493 "<a href="https://www.universetoday.com/37481/days-of-the-planets/">How
1494 long is a day on the other planets of the solar system?</a>"
1502 This file is in the public domain, so clarified as of 2009-05-17 by