diff options
| author | gordon <gordon@FreeBSD.org> | 2017-11-02 15:40:19 +0000 |
|---|---|---|
| committer | gordon <gordon@FreeBSD.org> | 2017-11-02 15:40:19 +0000 |
| commit | eb519e5c3cd4ca13794cb43d3b37ae52c0f86cdf (patch) | |
| tree | 5c8da6b8cc88ef59f230ee83251c3b90a7954868 /contrib/tzdata/theory.html | |
| parent | 304196ebc46a7aa18b7e1db641c39d8e0eaa4d84 (diff) | |
| download | FreeBSD-src-eb519e5c3cd4ca13794cb43d3b37ae52c0f86cdf.zip FreeBSD-src-eb519e5c3cd4ca13794cb43d3b37ae52c0f86cdf.tar.gz | |
Update timezone database information. [EN-17:09]
Approved by: so
Diffstat (limited to 'contrib/tzdata/theory.html')
| -rw-r--r-- | contrib/tzdata/theory.html | 1034 |
1 files changed, 1034 insertions, 0 deletions
diff --git a/contrib/tzdata/theory.html b/contrib/tzdata/theory.html new file mode 100644 index 0000000..965135d --- /dev/null +++ b/contrib/tzdata/theory.html @@ -0,0 +1,1034 @@ +<!DOCTYPE html> +<html lang="en"> +<head> + <title>Theory and pragmatics of the tz code and data</title> + <meta charset="UTF-8"> +</head> + +<!-- The somewhat-unusal indenting style in this file is intended to + shrink the output of the shell command 'diff Theory Theory.html', + where 'Theory' was the plain text file that this file is derived + from. The 'Theory' file used leading white space to indent, and + when possible that indentation is preserved here. Eventually we + may stop doing this and remove this comment. --> + +<body> + <h1>Theory and pragmatics of the tz code and data</h1> + <h3>Outline</h3> + <nav> + <ul> + <li><a href="#scope">Scope of the tz database</a></li> + <li><a href="#naming">Names of time zone rules</a></li> + <li><a href="#abbreviations">Time zone abbreviations</a></li> + <li><a href="#accuracy">Accuracy of the tz database</a></li> + <li><a href="#functions">Time and date functions</a></li> + <li><a href="#stability">Interface stability</a></li> + <li><a href="#calendar">Calendrical issues</a></li> + <li><a href="#planets">Time and time zones on other planets</a></li> + </ul> + </nav> + + + <section> + <h2 id="scope">Scope of the tz database</h2> +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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: + <a href="http://pubs.opengroup.org/onlinepubs/9699919799/"> + The Open Group Base Specifications Issue 7</a>, + IEEE Std 1003.1-2008, 2016 Edition. +</p> + </section> + + + + <section> + <h2 id="naming">Names of time zone rules</h2> +<p> +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 +<a href="http://cldr.unicode.org/">Unicode Common Locale Data +Repository</a> contains data that may be useful for other +selection interfaces. +</p> + +<p> +The time zone rule naming conventions attempt to strike a balance +among the following goals: +</p> +<ul> + <li> + Uniquely identify every region where clocks have agreed since 1970. + This is essential for the intended use: static clocks keeping local + civil time. + </li> + <li> + Indicate to experts where that region is. + </li> + <li> + 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). + </li> + <li> + Be portable to a wide variety of implementations. + </li> + <li> + Use a consistent naming conventions over the entire world. + </li> +</ul> +<p> +Names normally have the +form <var>AREA</var><code>/</code><var>LOCATION</var>, +where <var>AREA</var> is the name of a continent or ocean, +and <var>LOCATION</var> is the name of a specific +location within that region. North and South America share the same +area, '<code>America</code>'. Typical names are +'<code>Africa/Cairo</code>', '<code>America/New_York</code>', and +'<code>Pacific/Honolulu</code>'. +</p> + +<p> +Here are the general rules used for choosing location names, +in decreasing order of importance: +</p> +<ul> + <li> + Use only valid POSIX file name components (i.e., the parts of + names other than '<code>/</code>'). Do not use the file name + components '<code>.</code>' and '<code>..</code>'. + Within a file name component, + use only ASCII letters, '<code>.</code>', + '<code>-</code>' and '<code>_</code>'. 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 '<code>-</code>'. E.g., + prefer '<code>Brunei</code>' to + '<code>Bandar_Seri_Begawan</code>'. Exceptions: see + the discussion + of legacy names below. + </li> + <li> + A name must not be empty, or contain '<code>//</code>', or + start or end with '<code>/</code>'. + </li> + <li> + 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. + </li> + <li> + If one name <var>A</var> is an initial prefix of another + name <var>AB</var> (ignoring case), then <var>B</var> + must not start with '<code>/</code>', as a + regular file cannot have + the same name as a directory in POSIX. For example, + '<code>America/New_York</code>' precludes + '<code>America/New_York/Bronx</code>'. + </li> + <li> + Uninhabited regions like the North Pole and Bouvet Island + do not need locations, since local time is not defined there. + </li> + <li> + There should typically be at least one name for each ISO 3166-1 + officially assigned two-letter code for an inhabited country + or territory. + </li> + <li> + 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. + </li> + <li> + If a name is ambiguous, use a less ambiguous alternative; + e.g. many cities are named San José and Georgetown, so + prefer '<code>Costa_Rica</code>' to '<code>San_Jose</code>' and '<code>Guyana</code>' to '<code>Georgetown</code>'. + </li> + <li> + 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 + '<code>Paris</code>' to '<code>France</code>', since + France has had multiple time zones. + </li> + <li> + Use mainstream English spelling, e.g. prefer + '<code>Rome</code>' to '<code>Roma</code>', and prefer + '<code>Athens</code>' to the Greek + '<code>Αθήνα</code>' or the Romanized + '<code>Athína</code>'. + The POSIX file name restrictions encourage this rule. + </li> + <li> + Use the most populous among locations in a zone, + e.g. prefer '<code>Shanghai</code>' to + '<code>Beijing</code>'. Among locations with + similar populations, pick the best-known location, + e.g. prefer '<code>Rome</code>' to '<code>Milan</code>'. + </li> + <li> + Use the singular form, e.g. prefer '<code>Canary</code>' to '<code>Canaries</code>'. + </li> + <li> + Omit common suffixes like '<code>_Islands</code>' and + '<code>_City</code>', unless that would lead to + ambiguity. E.g. prefer '<code>Cayman</code>' to + '<code>Cayman_Islands</code>' and + '<code>Guatemala</code>' to + '<code>Guatemala_City</code>', but prefer + '<code>Mexico_City</code>' to '<code>Mexico</code>' + because the country + of Mexico has several time zones. + </li> + <li> + Use '<code>_</code>' to represent a space. + </li> + <li> + Omit '<code>.</code>' from abbreviations in names, e.g. prefer + '<code>St_Helena</code>' to '<code>St._Helena</code>'. + </li> + <li> + Do not change established names if they only marginally + violate the above rules. For example, don't change + the existing name '<code>Rome</code>' to + '<code>Milan</code>' merely because + Milan's population has grown to be somewhat greater + than Rome's. + </li> + <li> + If a name is changed, put its old spelling in the + '<code>backward</code>' file. + This means old spellings will continue to work. + </li> +</ul> + +<p> +The file '<code>zone1970.tab</code>' 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 '<code>zone1970.tab</code>' location's longitude +corresponds to its LMT offset with one hour for every 15 degrees east +longitude, this relationship is not exact. +</p> + +<p> +Older versions of this package used a different naming scheme, +and these older names are still supported. +See the file '<code>backward</code>' for most of these older names +(e.g., '<code>US/Eastern</code>' instead of '<code>America/New_York</code>'). +The other old-fashioned names still supported are +'<code>WET</code>', '<code>CET</code>', '<code>MET</code>', and '<code>EET</code>' (see the file '<code>europe</code>'). +</p> + +<p> +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 +'<code>etcetera</code>'. Also, the file '<code>backward</code>' defines the legacy names +'<code>GMT0</code>', '<code>GMT-0</code>' and '<code>GMT+0</code>', and the file '<code>northamerica</code>' defines the +legacy names '<code>EST5EDT</code>', '<code>CST6CDT</code>', '<code>MST7MDT</code>', and '<code>PST8PDT</code>'. +</p> + +<p> +Excluding '<code>backward</code>' should not affect the other data. If +'<code>backward</code>' is excluded, excluding '<code>etcetera</code>' should not affect the +remaining data. +</p> + + + </section> + <section> + <h2 id="abbreviations">Time zone abbreviations</h2> +<p> +When this package is installed, it generates time zone abbreviations +like '<code>EST</code>' to be compatible with human tradition and POSIX. +Here are the general rules used for choosing time zone abbreviations, +in decreasing order of importance: +<ul> + <li> + Use three or more characters that are ASCII alphanumerics or + '<code>+</code>' or '<code>-</code>'. + Previous editions of this database also used characters like + '<code> </code>' and '<code>?</code>', but these + characters have a special meaning to + the shell and cause commands like + '<code>set `date`</code>' + 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 + '<code>-</code>', '<code>+</code>', + and alphanumeric characters from the portable character set + in the current locale. In practice ASCII alphanumerics and + '<code>+</code>' and '<code>-</code>' are safe in all locales. + + In other words, in the C locale the POSIX extended regular + expression <code>[-+[:alnum:]]{3,}</code> should match + the abbreviation. + This guarantees that all abbreviations could have been + specified by a POSIX TZ string. + </li> + <li> + Use abbreviations that are in common use among English-speakers, + e.g. 'EST' for Eastern Standard Time in North America. + We assume that applications translate them to other languages + as part of the normal localization process; for example, + a French application might translate 'EST' to 'HNE'. + </li> + <li> + For zones whose times are taken from a city's longitude, use the + traditional <var>x</var>MT notation, e.g. 'PMT' for + Paris Mean Time. + The only name like this in current use is 'GMT'. + </li> + <li> + Use 'LMT' for local mean time of locations before the introduction + of standard time; see "<a href="#scope">Scope of the + tz database</a>". + </li> + <li> + If there is no common English abbreviation, use numeric offsets like + <code>-</code>05 and <code>+</code>0830 that are + generated by zic's <code>%z</code> notation. + </li> + <li> + 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. + </li> + <li> + 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. + </li> +</ul> + [The remaining guidelines predate the introduction of <code>%z</code>. + 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 <code>%z</code> for + inhabited locations and to "<code>-</code>00" for uninhabited locations.] +<ul> + <li> + 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: + <ul> + <li> + 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. + </li> + <li> + 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. + </li> + </ul> + </li> + <li> + Use UT (with time zone abbreviation '<code>-</code>00') for + locations while uninhabited. The leading + '<code>-</code>' is a flag that the time + zone is in some sense undefined; this notation is + derived from Internet RFC 3339. + </li> +</ul> +<p> +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 '<code>-</code>0600' instead of time zone +abbreviations like 'CST'; this avoids the ambiguity. +</p> + </section> + + + <section> + <h2 id="accuracy">Accuracy of the tz database</h2> +<p> +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. +</p> + +<p> +Errors in the tz database arise from many sources: +</p> +<ul> + <li> + 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. + </li> + <li> + The pre-1970 entries in this database cover only a tiny sliver of how + clocks actually behaved; the vast majority of the necessary + information was lost or never recorded. 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 + <cite>The Global Transformation of Time, 1870-1950</cite>, Vanessa Ogle writes + "Outside of Europe and North America there was no system of time + zones at all, often not even a stable landscape of mean times, + prior to the middle decades of the twentieth century". See: + Timothy Shenk, <a + href="https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle">Booked: + A Global History of Time</a>. <cite>Dissent</cite> 2015-12-17. + </li> + <li> + Most of the pre-1970 data entries come from unreliable sources, often + astrology books that lack citations and whose compilers evidently + invented entries when the true facts were unknown, without + reporting which entries were known and which were invented. + These books often contradict each other or give implausible entries, + and on the rare occasions when they are checked they are + typically found to be incorrect. + </li> + <li> + For the UK the tz database relies on years of first-class work done by + Joseph Myers and others; see + "<a href="https://www.polyomino.org.uk/british-time/">History of + legal time in Britain</a>". + Other countries are not done nearly as well. + </li> + <li> + Sometimes, different people in the same city 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. + </li> + <li> + 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, <code>Europe/London</code> + stands for the United Kingdom, but its pre-1847 times are valid + only for locations that have London's exact meridian, and its 1847 + transition to GMT is known to be valid only for the L&NW and the + Caledonian railways. + </li> + <li> + 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, <code>Europe/London</code> 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. + </li> + <li> + The tz database does not record a region's boundaries, and in many + cases the boundaries are not known. For example, the zone + <code>America/Kentucky/Louisville</code> represents a region around + the city of + Louisville, the boundaries of which are unclear. + </li> + <li> + Changes that are modeled as instantaneous transitions in the tz + database were often spread out over hours, days, or even decades. + </li> + <li> + Even if the time is specified by law, locations sometimes + deliberately flout the law. + </li> + <li> + Early timekeeping practices, even assuming perfect clocks, were + often not specified to the accuracy that the tz database requires. + </li> + <li> + 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. + </li> + <li> + 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. + </li> + <li> + 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. + </li> + <li> + Early clocks were less reliable, and data entries do not represent + clock error. + </li> + <li> + 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. + </li> + <li> + 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. + <a href="http://dx.doi.org/10.1098/rspa.2016.0404">Measurement + of the Earth's rotation: 720 BC to AD 2015</a>. + <cite>Proc Royal Soc A</cite>. 2016 Dec 7;472:20160404. + Also see: Espenak F. <a + href="https://eclipse.gsfc.nasa.gov/SEhelp/uncertainty2004.html">Uncertainty + in Delta T (ΔT)</a>. + </li> + <li> + The relationship between POSIX time (that is, 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. + </li> + <li> + 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. + </li> +</ul> +<p> +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. +</p> + </section> + + + <section> + <h2 id="functions">Time and date functions</h2> +<p> +The tz code contains time and date functions that are upwards +compatible with those of POSIX. +</p> + +<p> +POSIX has the following properties and limitations. +</p> +<ul> + <li> + <p> + 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. + </p> + <p> + The POSIX TZ string takes the following form: + </p> + <p> + <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>]]] + </p> + <p> + where: + <dl> + <dt><var>std</var> and <var>dst</var></dt><dd> + are 3 or more characters specifying the standard + and daylight saving time (DST) zone names. + Starting with POSIX.1-2001, <var>std</var> + and <var>dst</var> may also be + in a quoted form like '<code><UTC+10></code>'; this allows + "<code>+</code>" and "<code>-</code>" in the names. + </dd> + <dt><var>offset</var></dt><dd> + is of the form + '<code>[±]<var>hh</var>:[<var>mm</var>[:<var>ss</var>]]</code>' + and specifies the offset west of UT. '<var>hh</var>' + may be a single digit; 0≤<var>hh</var>≤24. + The default DST offset is one hour ahead of standard time. + </dd> + <dt><var>date</var>[<code>/</code><var>time</var>]<code>,</code><var>date</var>[<code>/</code><var>time</var>]</dt><dd> + 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. + </dd> + <dt><var>time</var></dt><dd> + takes the form + '<var>hh</var><code>:</code>[<var>mm</var>[<code>:</code><var>ss</var>]]' + and defaults to 02:00. + This is the same format as the offset, except that a + leading '<code>+</code>' or '<code>-</code>' is not allowed. + </dd> + <dt><var>date</var></dt><dd> + takes one of the following forms: + <dl> + <dt>J<var>n</var> (1≤<var>n</var>≤365)</dt><dd> + origin-1 day number not counting February 29 + </dd> + <dt><var>n</var> (0≤<var>n</var>≤365)</dt><dd> + origin-0 day number counting February 29 if present + </dd> + <dt><code>M</code><var>m</var><code>.</code><var>n</var><code>.</code><var>d</var> (0[Sunday]≤<var>d</var>≤6[Saturday], 1≤<var>n</var>≤5, 1≤<var>m</var>≤12)</dt><dd> + for the <var>d</var>th day of + week <var>n</var> of month <var>m</var> of the + year, where week 1 is the first week in which + day <var>d</var> appears, and '<code>5</code>' + stands for the last week in which + day <var>d</var> appears + (which may be either the 4th or 5th week). + Typically, this is the only useful form; + the <var>n</var> + and <code>J</code><var>n</var> forms are + rarely used. + </dd> +</dl> +</dd> +</dl> + 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: + + <pre><code>TZ='NZST-12NZDT,M9.5.0,M4.1.0/3'</code></pre> + + This POSIX TZ string is hard to remember, and mishandles some + timestamps before 2008. With this package you can use this + instead: + + <pre><code>TZ='Pacific/Auckland'</code></pre> + </li> + <li> + POSIX does not define the exact meaning of TZ values like + "<code>EST5EDT</code>". + 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. + </li> + <li> + The TZ environment variable is process-global, which makes it hard + to write efficient, thread-safe applications that need access + to multiple time zones. + </li> + <li> + 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.) + </li> + <li> + 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. + </li> + <li> + POSIX requires that systems ignore leap seconds. + </li> + <li> + The tz code attempts to support all the <code>time_t</code> + implementations allowed by POSIX. The <code>time_t</code> + type represents a nonnegative count of + seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. + In practice, <code>time_t</code> is usually a signed 64- or + 32-bit integer; 32-bit signed <code>time_t</code> values stop + working after 2038-01-19 03:14:07 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 <code>time_t</code> to be a + floating-point type, this was not supported by any practical + systems, and POSIX.1-2013 and the tz code both + require <code>time_t</code> + to be an integer type. + </li> +</ul> +<p> +These are the extensions that have been made to the POSIX functions: +</p> +<ul> + <li> + <p> + 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. + </p> + <p> + It was recognized that allowing the TZ environment variable to + take on values such as '<code>America/New_York</code>' 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 "<code>EST5EDT</code>" which can be used both by + "new" programs (a la POSIX) and "old" programs (as zone names and + offsets). + </p> +</li> +<li> + The code supports platforms with a UT offset member + in <code>struct tm</code>, + e.g., <code>tm_gmtoff</code>. +</li> +<li> + The code supports platforms with a time zone abbreviation member in + <code>struct tm</code>, e.g., <code>tm_zone</code>. +</li> +<li> + Since the TZ environment variable can now be used to control time + conversion, the <code>daylight</code> + and <code>timezone</code> variables are no longer needed. + (These variables are defined and set by <code>tzset</code>; + however, their values will not be used + by <code>localtime</code>.) +</li> +<li> + Functions <code>tzalloc</code>, <code>tzfree</code>, + <code>localtime_rz</code>, and <code>mktime_z</code> for + more-efficient thread-safe applications that need to use + multiple time zones. The <code>tzalloc</code> + and <code>tzfree</code> functions allocate and free objects of + type <code>timezone_t</code>, and <code>localtime_rz</code> + and <code>mktime_z</code> are like <code>localtime_r</code> + and <code>mktime</code> with an extra + <code>timezone_t</code> argument. The functions were inspired + by NetBSD. +</li> +<li> + A function <code>tzsetwall</code> has been added to arrange + for the system's + best approximation to local wall clock time to be delivered by + subsequent calls to <code>localtime</code>. Source code for portable + applications that "must" run on local wall clock time should call + <code>tzsetwall</code>; 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.) +</li> +<li> + Negative <code>time_t</code> values are supported, on systems + where <code>time_t</code> is signed. +</li> +<li> + These functions can account for leap seconds, thanks to Bradley White. +</li> +</ul> +<p> +Points of interest to folks with other systems: +</p> +<ul> + <li> + 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 + '<code>zic</code>' supplied with this package instead of using + the system '<code>zic</code>', since the format + of <code>zic</code>'s input is occasionally extended, and a + platform may still be shipping an older <code>zic</code>. + </li> + <li> + The UNIX Version 7 <code>timezone</code> 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 + <code>localtime(&clock)->tm_zone</code> + (if <code>TM_ZONE</code> is defined) or + <code>tzname[localtime(&clock)->tm_isdst]</code> + (if <code>HAVE_TZNAME</code> is defined) + to learn the correct time zone abbreviation to use. + </li> + <li> + The 4.2BSD <code>gettimeofday</code> 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. + </li> + <li> + In SVR2, time conversion fails for near-minimum or near-maximum + <code>time_t</code> 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. + </li> +</ul> +<p> +The functions that are conditionally compiled +if <code>STD_INSPIRED</code> is defined +should, at this point, be looked on primarily as food for thought. They are +not in any sense "standard compatible" – some are not, in fact, +specified in <em>any</em> standard. They do, however, represent responses of +various authors to +standardization proposals. +</p> + +<p> +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. +</p> + </section> + + + <section> + <h2 id="stability">Interface stability</h2> +<p> +The tz code and data supply the following interfaces: +</p> +<ul> + <li> + A set of zone names as per "<a href="#naming">Names of time zone + rules</a>" above. + </li> + <li> + Library functions described in "<a href="#functions">Time and date + functions</a>" above. + </li> + <li> + The programs <code>tzselect</code>, <code>zdump</code>, + and <code>zic</code>, documented in their man pages. + </li> + <li> + The format of <code>zic</code> input files, documented in + the <code>zic</code> man page. + </li> + <li> + The format of <code>zic</code> output files, documented in + the <code>tzfile</code> man page. + </li> + <li> + The format of zone table files, documented in <code>zone1970.tab</code>. + </li> + <li> + The format of the country code file, documented in <code>iso3166.tab</code>. + </li> + <li> + The version number of the code and data, as the first line of + the text file '<code>version</code>' in each release. + </li> +</ul> +<p> +Interface changes in a release attempt to preserve compatibility with +recent releases. For example, tz data files typically do not rely on +recently-added <code>zic</code> features, so that users can run +older <code>zic</code> versions to process newer data +files. <a href="tz-link.htm">Sources for time zone and daylight +saving time data</a> describes how +releases are tagged and distributed. +</p> + +<p> +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. +</p> + </section> + + + <section> + <h2 id="calendar">Calendrical issues</h2> +<p> +Calendrical issues are a bit out of scope for a time zone database, +but they indicate the sort of problems that we would run into if we +extended the time zone database further into the past. An excellent +resource in this area is Nachum Dershowitz and Edward M. Reingold, +<cite><a href="https://www.cs.tau.ac.il/~nachum/calendar-book/third-edition/">Calendrical +Calculations: Third Edition</a></cite>, Cambridge University Press (2008). +Other information and sources are given in the file '<samp>calendars</samp>' +in the tz distribution. They sometimes disagree. +</p> + </section> + + + <section> + <h2 id="planets">Time and time zones on other planets</h2> +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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). +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Although the tz database does not support time on other planets, it is +documented here in the hopes that support will be added eventually. +</p> + +<p> +Sources: +</p> +<ul> + <li> +Michael Allison and Robert Schmunk, +"<a href="https://www.giss.nasa.gov/tools/mars24/help/notes.html">Technical +Notes on Mars Solar Time as Adopted by the Mars24 Sunclock</a>" +(2012-08-08). + </li> + <li> +Jia-Rui Chong, +"<a href="http://articles.latimes.com/2004/jan/14/science/sci-marstime14">Workdays +Fit for a Martian</a>", Los Angeles Times +(2004-01-14), pp A1, A20-A21. + </li> + <li> +Tom Chmielewski, +"<a href="https://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/">Jet +Lag Is Worse on Mars</a>", The Atlantic (2015-02-26) + </li> + <li> +Matt Williams, +"<a href="https://www.universetoday.com/37481/days-of-the-planets/">How +long is a day on the other planets of the solar system?</a>" +(2017-04-27). + </li> +</ul> + </section> + + <footer> + <hr> +This file is in the public domain, so clarified as of 2009-05-17 by +Arthur David Olson. + </footer> +</body> +</html> |
