# Theory and pragmatics of the tz code and data

### Outline

## Scope of the tz database

The tz database attempts to record the history and predicted future of all computer-based clocks that track civil time. To represent this data, the world is partitioned into regions whose clocks all agree about timestamps that occur after the somewhat-arbitrary cutoff point of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, the database records all known clock transitions, and labels the region with a notable location. Although 1970 is a somewhat-arbitrary cutoff, there are significant challenges to moving the cutoff earlier even by a decade or two, due to the wide variety of local practices before computer timekeeping became prevalent.

Clock transitions before 1970 are recorded for each such location, because most systems support timestamps before 1970 and could misbehave if data entries were omitted for pre-1970 transitions. However, the database is not designed for and does not suffice for applications requiring accurate handling of all past times everywhere, as it would take far too much effort and guesswork to record all details of pre-1970 civil timekeeping.

As described below, reference source code for using the tz database is also available. The tz code is upwards compatible with POSIX, an international standard for UNIX-like systems. As of this writing, the current edition of POSIX is: The Open Group Base Specifications Issue 7, IEEE Std 1003.1-2008, 2016 Edition.

## Names of time zone rules

Each of the database's time zone rules has a unique name. Inexperienced users are not expected to select these names unaided. Distributors should provide documentation and/or a simple selection interface that explains the names; for one example, see the 'tzselect' program in the tz code. The Unicode Common Locale Data Repository contains data that may be useful for other selection interfaces.

The time zone rule naming conventions attempt to strike a balance among the following goals:

- Uniquely identify every region where clocks have agreed since 1970. This is essential for the intended use: static clocks keeping local civil time.
- Indicate to experts where that region is.
- Be robust in the presence of political changes. For example, names of countries are ordinarily not used, to avoid incompatibilities when countries change their name (e.g. Zaire→Congo) or when locations change countries (e.g. Hong Kong from UK colony to China).
- Be portable to a wide variety of implementations.
- Use a consistent naming conventions over the entire world.

Names normally have the form AREA`/`LOCATION, where AREA is the name of a continent or ocean, and LOCATION is the name of a specific location within that region. North and South America share the same area, '`America`'. Typical names are '`Africa/Cairo`', '`America/New_York`', and '`Pacific/Honolulu`'.

Here are the general rules used for choosing location names, in decreasing order of importance:

- Use only valid POSIX file name components (i.e., the parts of names other than '`/`'). Do not use the file name components '`.`' and '`..`'. Within a file name component, use only ASCII letters, '`.`', '`-`' and '`_`'. Do not use digits, as that might create an ambiguity with POSIX TZ strings. A file name component must not exceed 14 characters or start with '`-`'. E.g., prefer '`Brunei`' to '`Bandar_Seri_Begawan`'. Exceptions: see the discussion of legacy names below.
- A name must not be empty, or contain '`//`', or start or end with '`/`'.
- Do not use names that differ only in case. Although the reference implementation is case-sensitive, some other implementations are not, and they would mishandle names differing only in case.
- If one name A is an initial prefix of another name AB (ignoring case), then B must not start with '`/`', as a regular file cannot have the same name as a directory in POSIX. For example, '`America/New_York`' precludes '`America/New_York/Bronx`'.
- Uninhabited regions like the North Pole and Bouvet Island do not need locations, since local time is not defined there.
- There should typically be at least one name for each ISO 3166-1 officially assigned two-letter code for an inhabited country or territory.
- If all the clocks in a region have agreed since 1970, don't bother to include more than one location even if subregions' clocks disagreed before 1970. Otherwise these tables would become annoyingly large.
- If a name is ambiguous, use a less ambiguous alternative; e.g. many cities are named San José and Georgetown, so prefer '`Costa_Rica`' to '`San_Jose`' and '`Guyana`' to '`Georgetown`'.
- Keep locations compact. Use cities or small islands, not countries or regions, so that any future time zone changes do not split locations into different time zones. E.g. prefer '`Paris`' to '`France`', since France has had multiple time zones.
- Use mainstream English spelling, e.g. prefer '`Rome`' to '`Roma`', and prefer '`Athens`' to the Greek '`Αθήνα`' or the Romanized '`Athína`'. The POSIX file name restrictions encourage this rule.
- Use the most populous among locations in a zone, e.g. prefer '`Shanghai`' to '`Beijing`'. Among locations with similar populations, pick the best-known location, e.g. prefer '`Rome`' to '`Milan`'.
- Use the singular form, e.g. prefer '`Canary`' to '`Canaries`'.
- Omit common suffixes like '`_Islands`' and '`_City`', unless that would lead to ambiguity. E.g. prefer '`Cayman`' to '`Cayman_Islands`' and '`Guatemala`' to '`Guatemala_City`', but prefer '`Mexico_City`' to '`Mexico`' because the country of Mexico has several time zones.
- Use '`_`' to represent a space.
- Omit '`.`' from abbreviations in names, e.g. prefer '`St_Helena`' to '`St._Helena`'.
- Do not change established names if they only marginally violate the above rules. For example, don't change the existing name '`Rome`' to '`Milan`' merely because Milan's population has grown to be somewhat greater than Rome's.
- If a name is changed, put its old spelling in the '`backward`' file. This means old spellings will continue to work.

The file '`zone1970.tab`' lists geographical locations used to name time zone rules. It is intended to be an exhaustive list of names for geographic regions as described above; this is a subset of the names in the data. Although a '`zone1970.tab`' location's longitude corresponds to its LMT offset with one hour for every 15 degrees east longitude, this relationship is not exact.

Older versions of this package used a different naming scheme, and these older names are still supported. See the file '`backward`' for most of these older names (e.g., '`US/Eastern`' instead of '`America/New_York`'). The other old-fashioned names still supported are '`WET`', '`CET`', '`MET`', and '`EET`' (see the file '`europe`').

Older versions of this package defined legacy names that are incompatible with the first rule of location names, but which are still supported. These legacy names are mostly defined in the file '`etcetera`'. Also, the file '`backward`' defines the legacy names '`GMT0`', '`GMT-0`' and '`GMT+0`', and the file '`northamerica`' defines the legacy names '`EST5EDT`', '`CST6CDT`', '`MST7MDT`', and '`PST8PDT`'.

Excluding '`backward`' should not affect the other data. If '`backward`' is excluded, excluding '`etcetera`' should not affect the remaining data.

## Time zone abbreviations

When this package is installed, it generates time zone abbreviations like '`EST`' to be compatible with human tradition and POSIX. Here are the general rules used for choosing time zone abbreviations, in decreasing order of importance:

- Use three or more characters that are ASCII alphanumerics or '`+`' or '`-`'. Previous editions of this database also used characters like '`` ' and '`?`', but these characters have a special meaning to the shell and cause commands like '`set `date``' to have unexpected effects. Previous editions of this rule required upper-case letters, but the Congressman who introduced Chamorro Standard Time preferred "ChST", so lower-case letters are now allowed. Also, POSIX from 2001 on relaxed the rule to allow '`-`', '`+`', and alphanumeric characters from the portable character set in the current locale. In practice ASCII alphanumerics and '`+`' and '`-`' are safe in all locales. In other words, in the C locale the POSIX extended regular expression `[-+[:alnum:]]{3,}` should match the abbreviation. This guarantees that all abbreviations could have been specified by a POSIX TZ string.
- Use abbreviations that are in common use among English-speakers, e.g. 'EST' for Eastern Standard Time in North America. We assume that applications translate them to other languages as part of the normal localization process; for example, a French application might translate 'EST' to 'HNE'.
- For zones whose times are taken from a city's longitude, use the traditional xMT notation, e.g. 'PMT' for Paris Mean Time. The only name like this in current use is 'GMT'.
- Use 'LMT' for local mean time of locations before the introduction of standard time; see "Scope of the tz database".
- If there is no common English abbreviation, use numeric offsets like `-`05 and `+`0830 that are generated by zic's `%z` notation.
- Use current abbreviations for older timestamps to avoid confusion. For example, in 1910 a common English abbreviation for UT +01 in central Europe was 'MEZ' (short for both "Middle European Zone" and for "Mitteleuropäische Zeit" in German). Nowadays 'CET' ("Central European Time") is more common in English, and the database uses 'CET' even for circa-1910 timestamps as this is less confusing for modern users and avoids the need for determining when 'CET' supplanted 'MEZ' in common usage.
- Use a consistent style in a zone's history. For example, if a zone's history tends to use numeric abbreviations and a particular entry could go either way, use a numeric abbreviation.

[The remaining guidelines predate the introduction of `%z`. They are problematic as they mean tz data entries invent notation rather than record it. These guidelines are now deprecated and the plan is to gradually move to `%z` for inhabited locations and to "`-`00" for uninhabited locations.]

- If there is no common English abbreviation, abbreviate the English translation of the usual phrase used by native speakers. If this is not available or is a phrase mentioning the country (e.g. "Cape Verde Time"), then:
  - When a country is identified with a single or principal zone, append 'T' to the country's ISO code, e.g. 'CVT' for Cape Verde Time. For summer time append 'ST'; for double summer time append 'DST'; etc.
  - Otherwise, take the first three letters of an English place name identifying each zone and append 'T', 'ST', etc. as before; e.g. 'CHAST' for CHAtham Summer Time.
- Use UT (with time zone abbreviation '`-`00') for locations while uninhabited. The leading '`-`' is a flag that the time zone is in some sense undefined; this notation is derived from Internet RFC 3339.

Application writers should note that these abbreviations are ambiguous in practice: e.g. 'CST' has a different meaning in China than it does in the United States. In new applications, it's often better to use numeric UT offsets like '`-`0600' instead of time zone abbreviations like 'CST'; this avoids the ambiguity.

## Accuracy of the tz database

The tz database is not authoritative, and it surely has errors. Corrections are welcome and encouraged; see the file CONTRIBUTING. Users requiring authoritative data should consult national standards bodies and the references cited in the database's comments.

Errors in the tz database arise from many sources:

- The tz database predicts future timestamps, and current predictions will be incorrect after future governments change the rules. For example, if today someone schedules a meeting for 13:00 next October 1, Casablanca time, and tomorrow Morocco changes its daylight saving rules, software can mess up after the rule change if it blithely relies on conversions made before the change.
- The pre-1970 entries in this database cover only a tiny sliver of how clocks actually behaved; the vast majority of the necessary information was lost or never recorded. Thousands more zones would be needed if the tz database's scope were extended to cover even just the known or guessed history of standard time; for example, the current single entry for France would need to split into dozens of entries, perhaps hundreds. And in most of the world even this approach would be misleading due to widespread disagreement or indifference about what times should be observed. In her 2015 book The Global Transformation of Time, 1870-1950, Vanessa Ogle writes "Outside of Europe and North America there was no system of time zones at all, often not even a stable landscape of mean times, prior to the middle decades of the twentieth century". See: Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17.
- Most of the pre-1970 data entries come from unreliable sources, often astrology books that lack citations and whose compilers evidently invented entries when the true facts were unknown, without reporting which entries were known and which were invented. These books often contradict each other or give implausible entries, and on the rare occasions when they are checked they are typically found to be incorrect.
- For the UK the tz database relies on years of first-class work done by Joseph Myers and others; see "History of legal time in Britain". Other countries are not done nearly as well.
- Sometimes, different people in the same city would maintain clocks that differed significantly. Railway time was used by railroad companies (which did not always agree with each other), church-clock time was used for birth certificates, etc. Often this was merely common practice, but sometimes it was set by law. For example, from 1891 to 1911 the UT offset in France was legally 0:09:21 outside train stations and 0:04:21 inside.
- Although a named location in the tz database stands for the containing region, its pre-1970 data entries are often accurate for only a small subset of that region. For example, `Europe/London` stands for the United Kingdom, but its pre-1847 times are valid only for locations that have London's exact meridian, and its 1847 transition to GMT is known to be valid only for the L&NW and the Caledonian railways.
- The tz database does not record the earliest time for which a zone's data entries are thereafter valid for every location in the region. For example, `Europe/London` is valid for all locations in its region after GMT was made the standard time, but the date of standardization (1880-08-02) is not in the tz database, other than in commentary. For many zones the earliest time of validity is unknown.
- The tz database does not record a region's boundaries, and in many cases the boundaries are not known. For example, the zone `America/Kentucky/Louisville` represents a region around the city of Louisville, the boundaries of which are unclear.
- Changes that are modeled as instantaneous transitions in the tz database were often spread out over hours, days, or even decades.
- Even if the time is specified by law, locations sometimes deliberately flout the law.
- Early timekeeping practices, even assuming perfect clocks, were often not specified to the accuracy that the tz database requires.
- Sometimes historical timekeeping was specified more precisely than what the tz database can handle. For example, from 1909 to 1937 Netherlands clocks were legally UT +00:19:32.13, but the tz database cannot represent the fractional second.
- Even when all the timestamp transitions recorded by the tz database are correct, the tz rules that generate them may not faithfully reflect the historical rules. For example, from 1922 until World War II the UK moved clocks forward the day following the third Saturday in April unless that was Easter, in which case it moved clocks forward the previous Sunday. Because the tz database has no way to specify Easter, these exceptional years are entered as separate tz Rule lines, even though the legal rules did not change.
- The tz database models pre-standard time using the proleptic Gregorian calendar and local mean time (LMT), but many people used other calendars and other timescales. For example, the Roman Empire used the Julian calendar, and had 12 varying-length daytime hours with a non-hour-based system at night.
- Early clocks were less reliable, and data entries do not represent clock error.
- The tz database assumes Universal Time (UT) as an origin, even though UT is not standardized for older timestamps. In the tz database commentary, UT denotes a family of time standards that includes Coordinated Universal Time (UTC) along with other variants such as UT1 and GMT, with days starting at midnight. Although UT equals UTC for modern timestamps, UTC was not defined until 1960, so commentary uses the more-general abbreviation UT for timestamps that might predate 1960. Since UT, UT1, etc. disagree slightly, and since pre-1972 UTC seconds varied in length, interpretation of older timestamps can be problematic when subsecond accuracy is needed.
- Civil time was not based on atomic time before 1972, and we don't know the history of earth's rotation accurately enough to map SI seconds to historical solar time to more than about one-hour accuracy. See: Stephenson FR, Morrison LV, Hohenkerk CY. Measurement of the Earth's rotation: 720 BC to AD 2015. Proc Royal Soc A. 2016 Dec 7;472:20160404. Also see: Espenak F. Uncertainty in Delta T (ΔT).
- The relationship between POSIX time (that is, UTC but ignoring leap seconds) and UTC is not agreed upon after 1972. Although the POSIX clock officially stops during an inserted leap second, at least one proposed standard has it jumping back a second instead; and in practice POSIX clocks more typically either progress glacially during a leap second, or are slightly slowed while near a leap second.
- The tz database does not represent how uncertain its information is. Ideally it would contain information about when data entries are incomplete or dicey. Partial temporal knowledge is a field of active research, though, and it's not clear how to apply it here.

In short, many, perhaps most, of the tz database's pre-1970 and future timestamps are either wrong or misleading. Any attempt to pass the tz database off as the definition of time should be unacceptable to anybody who cares about the facts. In particular, the tz database's LMT offsets should not be considered meaningful, and should not prompt creation of zones merely because two locations differ in LMT or transitioned to standard time at different dates.

## Time and date functions

The tz code contains time and date functions that are upwards compatible with those of POSIX.

POSIX has the following properties and limitations.

- In POSIX, time display in a process is controlled by the environment variable TZ. Unfortunately, the POSIX TZ string takes a form that is hard to describe and is error-prone in practice. Also, POSIX TZ strings can't deal with other (for example, Israeli) daylight saving time rules, or situations where more than two time zone abbreviations are used in an area.

  The POSIX TZ string takes the following form:

  stdoffset[dst[offset][`,`date[`/`time]`,`date[`/`time]]]

  where:

  std and dst
    are 3 or more characters specifying the standard and daylight saving time (DST) zone names. Starting with POSIX.1-2001, std and dst may also be in a quoted form like '`<UTC+10>`'; this allows "`+`" and "`-`" in the names.
  offset
    is of the form '`[±]hh:[mm[:ss]]`' and specifies the offset west of UT. 'hh' may be a single digit; 0≤hh≤24. The default DST offset is one hour ahead of standard time.
  date[`/`time]`,`date[`/`time]
    specifies the beginning and end of DST. If this is absent, the system supplies its own rules for DST, and these can differ from year to year; typically US DST rules are used.
  time
    takes the form 'hh`:`[mm[`:`ss]]' and defaults to 02:00. This is the same format as the offset, except that a leading '`+`' or '`-`' is not allowed.
  date
    takes one of the following forms:
    Jn (1≤n≤365)
      origin-1 day number not counting February 29
    n (0≤n≤365)
      origin-0 day number counting February 29 if present
    `M` m`.`n`.`d (0[Sunday]≤d≤6[Saturday], 1≤n≤5, 1≤m≤12)
      for the dth day of week n of month m of the year, where week 1 is the first week in which day d appears, and '`5`' stands for the last week in which day d appears (which may be either the 4th or 5th week). Typically, this is the only useful form; the n and `J` n forms are rarely used.
  Here is an example POSIX TZ string for New Zealand after 2007. It says that standard time (NZST) is 12 hours ahead of UTC, and that daylight saving time (NZDT) is observed from September's last Sunday at 02:00 until April's first Sunday at 03:00:

      TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'

  This POSIX TZ string is hard to remember, and mishandles some timestamps before 2008. With this package you can use this instead:

      TZ='Pacific/Auckland'

- POSIX does not define the exact meaning of TZ values like "`EST5EDT`". Typically the current US DST rules are used to interpret such values, but this means that the US DST rules are compiled into each program that does time conversion. This means that when US time conversion rules change (as in the United States in 1987), all programs that do time conversion must be recompiled to ensure proper results.
- The TZ environment variable is process-global, which makes it hard to write efficient, thread-safe applications that need access to multiple time zones.
- In POSIX, there's no tamper-proof way for a process to learn the system's best idea of local wall clock. (This is important for applications that an administrator wants used only at certain times – without regard to whether the user has fiddled the TZ environment variable. While an administrator can "do everything in UTC" to get around the problem, doing so is inconvenient and precludes handling daylight saving time shifts - as might be required to limit phone calls to off-peak hours.)
- POSIX provides no convenient and efficient way to determine the UT offset and time zone abbreviation of arbitrary timestamps, particularly for time zone settings that do not fit into the POSIX model.
- POSIX requires that systems ignore leap seconds.
- The tz code attempts to support all the `time_t` implementations allowed by POSIX. The `time_t` type represents a nonnegative count of seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. In practice, `time_t` is usually a signed 64- or 32-bit integer; 32-bit signed `time_t` values stop working after 2038-01-19 03:14:07 UTC, so new implementations these days typically use a signed 64-bit integer. Unsigned 32-bit integers are used on one or two platforms, and 36-bit and 40-bit integers are also used occasionally. Although earlier POSIX versions allowed `time_t` to be a floating-point type, this was not supported by any practical systems, and POSIX.1-2013 and the tz code both require `time_t` to be an integer type.

These are the extensions that have been made to the POSIX functions:

- The TZ environment variable is used in generating the name of a file from which time zone information is read (or is interpreted a la POSIX); TZ is no longer constrained to be a three-letter time zone name followed by a number of hours and an optional three-letter daylight time zone name. The daylight saving time rules to be used for a particular time zone are encoded in the time zone file; the format of the file allows U.S., Australian, and other rules to be encoded, and allows for situations where more than two time zone abbreviations are used.

  It was recognized that allowing the TZ environment variable to take on values such as '`America/New_York`' might cause "old" programs (that expect TZ to have a certain form) to operate incorrectly; consideration was given to using some other environment variable (for example, TIMEZONE) to hold the string used to generate the time zone information file name. In the end, however, it was decided to continue using TZ: it is widely used for time zone purposes; separately maintaining both TZ and TIMEZONE seemed a nuisance; and systems where "new" forms of TZ might cause problems can simply use TZ values such as "`EST5EDT`" which can be used both by "new" programs (a la POSIX) and "old" programs (as zone names and offsets).

- The code supports platforms with a UT offset member in `struct tm`, e.g., `tm_gmtoff`.
- The code supports platforms with a time zone abbreviation member in `struct tm`, e.g., `tm_zone`.
- Since the TZ environment variable can now be used to control time conversion, the `daylight` and `timezone` variables are no longer needed. (These variables are defined and set by `tzset`; however, their values will not be used by `localtime`.)
- Functions `tzalloc`, `tzfree`, `localtime_rz`, and `mktime_z` for more-efficient thread-safe applications that need to use multiple time zones. The `tzalloc` and `tzfree` functions allocate and free objects of type `timezone_t`, and `localtime_rz` and `mktime_z` are like `localtime_r` and `mktime` with an extra `timezone_t` argument. The functions were inspired by NetBSD.
- A function `tzsetwall` has been added to arrange for the system's best approximation to local wall clock time to be delivered by subsequent calls to `localtime`. Source code for portable applications that "must" run on local wall clock time should call `tzsetwall`; if such code is moved to "old" systems that don't provide tzsetwall, you won't be able to generate an executable program. (These time zone functions also arrange for local wall clock time to be used if tzset is called – directly or indirectly – and there's no TZ environment variable; portable applications should not, however, rely on this behavior since it's not the way SVR2 systems behave.)
- Negative `time_t` values are supported, on systems where `time_t` is signed.
- These functions can account for leap seconds, thanks to Bradley White.

Points of interest to folks with other systems:

- Code compatible with this package is already part of many platforms, including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS, BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris. On such hosts, the primary use of this package is to update obsolete time zone rule tables. To do this, you may need to compile the time zone compiler '`zic`' supplied with this package instead of using the system '`zic`', since the format of `zic`'s input is occasionally extended, and a platform may still be shipping an older `zic`.
- The UNIX Version 7 `timezone` function is not present in this package; it's impossible to reliably map timezone's arguments (a "minutes west of GMT" value and a "daylight saving time in effect" flag) to a time zone abbreviation, and we refuse to guess. Programs that in the past used the timezone function may now examine `localtime(&clock)->tm_zone` (if `TM_ZONE` is defined) or `tzname[localtime(&clock)->tm_isdst]` (if `HAVE_TZNAME` is defined) to learn the correct time zone abbreviation to use.
- The 4.2BSD `gettimeofday` function is not used in this package. This formerly let users obtain the current UTC offset and DST flag, but this functionality was removed in later versions of BSD.
- In SVR2, time conversion fails for near-minimum or near-maximum `time_t` values when doing conversions for places that don't use UT. This package takes care to do these conversions correctly. A comment in the source code tells how to get compatibly wrong results.

The functions that are conditionally compiled if `STD_INSPIRED` is defined should, at this point, be looked on primarily as food for thought. They are not in any sense "standard compatible" – some are not, in fact, specified in any standard. They do, however, represent responses of various authors to standardization proposals.

Other time conversion proposals, in particular the one developed by folks at Hewlett Packard, offer a wider selection of functions that provide capabilities beyond those provided here. The absence of such functions from this package is not meant to discourage the development, standardization, or use of such functions. Rather, their absence reflects the decision to make this package contain valid extensions to POSIX, to ensure its broad acceptability. If more powerful time conversion functions can be standardized, so much the better.

## Interface stability

The tz code and data supply the following interfaces:

- A set of zone names as per "Names of time zone rules" above.
- Library functions described in "Time and date functions" above.
- The programs `tzselect`, `zdump`, and `zic`, documented in their man pages.
- The format of `zic` input files, documented in the `zic` man page.
- The format of `zic` output files, documented in the `tzfile` man page.
- The format of zone table files, documented in `zone1970.tab`.
- The format of the country code file, documented in `iso3166.tab`.
- The version number of the code and data, as the first line of the text file '`version`' in each release.

Interface changes in a release attempt to preserve compatibility with recent releases. For example, tz data files typically do not rely on recently-added `zic` features, so that users can run older `zic` versions to process newer data files. Sources for time zone and daylight saving time data describes how releases are tagged and distributed.

Interfaces not listed above are less stable. For example, users should not rely on particular UT offsets or abbreviations for timestamps, as data entries are often based on guesswork and these guesses may be corrected or improved.

## Calendrical issues

Calendrical issues are a bit out of scope for a time zone database, but they indicate the sort of problems that we would run into if we extended the time zone database further into the past. An excellent resource in this area is Nachum Dershowitz and Edward M. Reingold, Calendrical Calculations: Third Edition, Cambridge University Press (2008). Other information and sources are given in the file '`calendars`' in the tz distribution. They sometimes disagree.

## Time and time zones on other planets

Some people's work schedules use Mars time. Jet Propulsion Laboratory (JPL) coordinators have kept Mars time on and off at least since 1997 for the Mars Pathfinder mission. Some of their family members have also adapted to Mars time. Dozens of special Mars watches were built for JPL workers who kept Mars time during the Mars Exploration Rovers mission (2004). These timepieces look like normal Seikos and Citizens but use Mars seconds rather than terrestrial seconds.

A Mars solar day is called a "sol" and has a mean period equal to about 24 hours 39 minutes 35.244 seconds in terrestrial time. It is divided into a conventional 24-hour clock, so each Mars second equals about 1.02749125 terrestrial seconds.

The prime meridian of Mars goes through the center of the crater Airy-0, named in honor of the British astronomer who built the Greenwich telescope that defines Earth's prime meridian. Mean solar time on the Mars prime meridian is called Mars Coordinated Time (MTC).

Each landed mission on Mars has adopted a different reference for solar time keeping, so there is no real standard for Mars time zones. For example, the Mars Exploration Rover project (2004) defined two time zones "Local Solar Time A" and "Local Solar Time B" for its two missions, each zone designed so that its time equals local true solar time at approximately the middle of the nominal mission. Such a "time zone" is not particularly suited for any application other than the mission itself.

Many calendars have been proposed for Mars, but none have achieved wide acceptance. Astronomers often use Mars Sol Date (MSD) which is a sequential count of Mars solar days elapsed since about 1873-12-29 12:00 GMT.

In our solar system, Mars is the planet with time and calendar most like Earth's. On other planets, Sun-based time and calendars would work quite differently. For example, although Mercury's sidereal rotation period is 58.646 Earth days, Mercury revolves around the Sun so rapidly that an observer on Mercury's equator would see a sunrise only every 175.97 Earth days, i.e., a Mercury year is 0.5 of a Mercury day. Venus is more complicated, partly because its rotation is slightly retrograde: its year is 1.92 of its days. Gas giants like Jupiter are trickier still, as their polar and equatorial regions rotate at different rates, so that the length of a day depends on latitude. This effect is most pronounced on Neptune, where the day is about 12 hours at the poles and 18 hours at the equator.

Although the tz database does not support time on other planets, it is documented here in the hopes that support will be added eventually.

Sources:

- Michael Allison and Robert Schmunk, "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock" (2012-08-08).
- Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times (2004-01-14), pp A1, A20-A21.
- Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26)
- Matt Williams, "How long is a day on the other planets of the solar system?" (2017-04-27).