@(#)Theory 7.15 ----- Outline ----- Time and date functions Names of time zone regions Time zone abbreviations Calendrical issues Time and time zones on Mars ----- Time and date functions ----- These time and date functions are upwards compatible with POSIX.1, an international standard for UNIX-like systems. As of this writing, the current edition of POSIX.1 is: Information technology --Portable Operating System Interface (POSIX (R)) -- Part 1: System Application Program Interface (API) [C Language] ISO/IEC 9945-1:1996 ANSI/IEEE Std 1003.1, 1996 Edition 1996-07-12 POSIX.1 has the following properties and limitations. * In POSIX.1, time display in a process is controlled by the environment variable TZ. Unfortunately, the POSIX.1 TZ string takes a form that is hard to describe and is error-prone in practice. Also, POSIX.1 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.1 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. offset is of the form `[-]hh:[mm[:ss]]' and specifies the offset west of UTC. 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. 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 Mm.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). * In POSIX.1, when a TZ value like "EST5EDT" is parsed, typically the current US DST rules are used, 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. * In POSIX.1, 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.1 requires that systems ignore leap seconds. These are the extensions that have been made to the POSIX.1 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). * To handle places where more than two time zone abbreviations are used, the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst] (where "tmp" is the value the function returns) to the time zone abbreviation to be used. This differs from POSIX.1, where the elements of tzname are only changed as a result of calls to tzset. * 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.") * The "localtime" function has been set up to deliver correct results for near-minimum or near-maximum time_t values. (A comment in the source code tells how to get compatibly wrong results). * 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.) * These functions can account for leap seconds, thanks to Bradley White (firstname.lastname@example.org). Points of interest to folks with other systems: * This package is already part of many POSIX-compliant hosts, including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun. 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 changed slightly in late 1994, and many vendors still do not support the new input format. * 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 tzname[localtime(&clock)->tm_isdst] to learn the correct time zone abbreviation to use. Alternatively, use localtime(&clock)->tm_zone if this has been enabled. * 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 UTC. This package takes care to do these conversions correctly. 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.1, to ensure its broad acceptability. If more powerful time conversion functions can be standardized, so much the better. ----- Names of time zone rule files ----- The time zone rule file naming conventions attempt to strike a balance among the following goals: * Uniquely identify every national region where clocks have all agreed since 1970. This is essential for the intended use: static clocks keeping local civil time. * Indicate to humans as to where that region is. This simplifes use. * Be robust in the presence of political changes. This reduces the number of updates and backward-compatibility hacks. 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. This promotes use of the technology. * Use a consistent naming convention over the entire world. This simplifies both use and maintenance. This naming convention is not intended for use by inexperienced users to select TZ values by themselves (though they can of course examine and reuse existing settings). Distributors should provide documentation and/or a simple selection interface that explains the names; see the 'tzselect' program supplied with this distribution for one example. 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 `/'). 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'. Include at least one location per time zone rule set per country. One such location is enough. Use ISO 3166 (see the file iso3166.tab) to help decide whether something is a country. If all the clocks in a country's 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 Jose 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 true name (which uses Greek letters). The POSIX file name restrictions encourage this rule. Use the most populous among locations in a country's time 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. The file `zone.tab' lists the geographical locations used to name time zone rule files. 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', `EET' (see the file `europe'), and `Factory' (see the file `factory'). ----- Time zone abbreviations ----- When this package is installed, it generates time zone abbreviations like `EST' to be compatible with human tradition and POSIX.1. Here are the general rules used for choosing time zone abbreviations, in decreasing order of importance: Use abbreviations that consist of three or more ASCII letters. 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 the rule has been relaxed. This rule guarantees that all abbreviations could have been specified by a POSIX.1 TZ string. POSIX.1 requires at least three characters for an abbreviation. POSIX.1-1996 says that an abbreviation cannot start with ':', and cannot contain ',', '-', '+', NUL, or a digit. Draft 7 of POSIX 1003.1-200x changes this rule to say that an abbreviation can contain only '-', '+', and alphanumeric characters in the current locale. To be portable to both sets of rules, an abbreviation must therefore use only ASCII letters, as these are the only letters that are alphabetic in all locales. 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'. 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 has a single or principal time zone region, 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. When a country has multiple time zones, take the first three letters of an English place name identifying each zone and then append `T', `ST', etc. as before; e.g. `VLAST' for VLAdivostok Summer Time. Use "zzz" for locations while uninhabited. The mnemonic is that these locations are, in some sense, asleep. Application writers should note that these abbreviations are ambiguous in practice: e.g. `EST' has a different meaning in Australia than it does in the United States. In new applications, it's often better to use numeric UTC offsets like `-0500' instead of time zone abbreviations like `EST'; this avoids the ambiguity. ----- 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, <a href="http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml"> Calendrical Calculations </a>, Cambridge University Press (1997). Other information and sources are given below. They sometimes disagree. France Gregorian calendar adopted 1582-12-20. French Revolutionary calendar used 1793-11-24 through 1805-12-31, and (in Paris only) 1871-05-06 through 1871-05-23. Russia From Chris Carrier <72157.3334@CompuServe.COM> (1996-12-02): On 1929-10-01 the Soviet Union instituted an ``Eternal Calendar'' with 30-day months plus 5 holidays, with a 5-day week. On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the Gregorian calendar while retaining the 6-day week; on 1940-06-27 it reverted to the 7-day week. With the 6-day week the usual days off were the 6th, 12th, 18th, 24th and 30th of the month. (Source: Evitiar Zerubavel, _The Seven Day Circle_) Mark Brader reported a similar story in "The Book of Calendars", edited by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377. But: From: Petteri Sulonen (via Usenet) Date: 14 Jan 1999 00:00:00 GMT Message-ID: <Petteri.Sulonenemail@example.com> If your source is correct, how come documents between 1929 -- 1940 were still dated using the conventional, Gregorian calendar? I can post a scan of a document dated December 1, 1934, signed by Yenukidze, the secretary, on behalf of Kalinin, the President of the Executive Committee of the Supreme Soviet, if you like. Sweden (and Finland) From: firstname.lastname@example.org (Mark Brader) <a href="news:1996Jul6.email@example.com"> Subject: Re: Gregorian reform -- a part of locale? </a> Date: 1996-07-06 In 1700, Denmark made the transition from Julian to Gregorian. Sweden decided to *start* a transition in 1700 as well, but rather than have one of those unsightly calendar gaps :-), they simply decreed that the next leap year after 1696 would be in 1744 -- putting the whole country on a calendar different from both Julian and Gregorian for a period of 40 years. However, in 1704 something went wrong and the plan was not carried through; they did, after all, have a leap year that year. And one in 1708. In 1712 they gave it up and went back to Julian, putting 30 days in February that year!... Then in 1753, Sweden made the transition to Gregorian in the usual manner, getting there only 13 years behind the original schedule. (A previous posting of this story was challenged, and Swedish readers produced the following references to support it: "Tiderakning och historia" by Natanael Beckman (1924) and "Tid, en bok om tiderakning och kalendervasen" by Lars-Olof Lode'n (no date was given).) Grotefend's data From: "Michael Palmer" <firstname.lastname@example.org> [with one obvious typo fixed] Subject: Re: Gregorian Calendar (was Re: Another FHC related question Newsgroups: soc.genealogy.german Date: Tue, 9 Feb 1999 02:32:48 -800 Message-ID: <199902091032.CAA09644@netcom10.netcom.com> The following is a(n incomplete) listing, arranged chronologically, of European states, with the date they converted from the Julian to the Gregorian calendar: 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman Catholics and Danzig only) 09/20 Dec 1582 - France, Lorraine 21 Dec 1582/ 01 Jan 1583 - Holland, Brabant, Flanders, Hennegau 10/21 Feb 1583 - bishopric of Liege (L"uttich) 13/24 Feb 1583 - bishopric of Augsburg 04/15 Oct 1583 - electorate of Trier 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg, Salzburg, Brixen 13/24 Oct 1583 - Austrian Oberelsass and Breisgau 20/31 Oct 1583 - bishopric of Basel 02/13 Nov 1583 - duchy of J"ulich-Berg 02/13 Nov 1583 - electorate and city of K"oln 04/15 Nov 1583 - bishopric of W"urzburg 11/22 Nov 1583 - electorate of Mainz 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden 17/28 Nov 1583 - bishopric of M"unster and duchy of Cleve 14/25 Dec 1583 - Steiermark 06/17 Jan 1584 - Austria and Bohemia 11/22 Jan 1584 - Luzern, Uri, Schwyz, Zug, Freiburg, Solothurn 12/23 Jan 1584 - Silesia and the Lausitz 22 Jan/ 02 Feb 1584 - Hungary (legally on 21 Oct 1587) Jun 1584 - Unterwalden 01/12 Jul 1584 - duchy of Westfalen 16/27 Jun 1585 - bishopric of Paderborn 14/25 Dec 1590 - Transylvania 22 Aug/ 02 Sep 1612 - duchy of Prussia 13/24 Dec 1614 - Pfalz-Neuburg 1617 - duchy of Kurland (reverted to the Julian calendar in 1796) 1624 - bishopric of Osnabr"uck 1630 - bishopric of Minden 15/26 Mar 1631 - bishopric of Hildesheim 1655 - Kanton Wallis 05/16 Feb 1682 - city of Strassburg 18 Feb/ 01 Mar 1700 - Protestant Germany (including Swedish possessions in Germany), Denmark, Norway 30 Jun/ 12 Jul 1700 - Gelderland, Zutphen 10 Nov/ 12 Dec 1700 - Utrecht, Overijssel 31 Dec 1700/ 12 Jan 1701 - Friesland, Groningen, Z"urich, Bern, Basel, Geneva, Turgau, and Schaffhausen 1724 - Glarus, Appenzell, and the city of St. Gallen 01 Jan 1750 - Pisa and Florence 02/14 Sep 1752 - Great Britain 17 Feb/ 01 Mar 1753 - Sweden 1760-1812 - Graub"unden The Russian empire (including Finland and the Baltic states) did not convert to the Gregorian calendar until the Soviet revolution of 1917. Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28. ----- Time and time zones on Mars ----- Some people have adjusted their work schedules to fit Mars time. Dozens of special Mars watches were built for Jet Propulsion Laboratory 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. The tz database does not currently support Mars time, but 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" <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2004-03-15). Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times (2004-01-14), pp A1, A20-A21.