|author||Garrett Wollman <wollman@FreeBSD.org>||2001-11-09 19:19:42 +0000|
|committer||Garrett Wollman <wollman@FreeBSD.org>||2001-11-09 19:19:42 +0000|
Add `Theory' file from tzcode which explains how and why zones are named.
Notes: svn path=/vendor/tzdata/dist/; revision=86223
1 files changed, 506 insertions, 0 deletions
diff --git a/share/zoneinfo/Theory b/share/zoneinfo/Theory
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+----- Outline -----
+ Time and date functions
+ Names of time zone regions
+ Time zone abbreviations
+ Calendrical issues
+----- 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
+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:
+ std and dst
+ are 3 or more characters specifying the standard
+ and daylight saving time (DST) zone names.
+ 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.
+ 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.
+ takes the form `hh:[mm[:ss]]' and defaults to 02:00.
+ 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
+* 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
+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
+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
+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',
+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>, Cambridge University Press (1997). Other information and
+sources are given below. They sometimes disagree.
+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.
+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
+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)
+Subject: Re: Gregorian reform -- a part of locale?
+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
+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).)
+From: "Michael Palmer" <email@example.com> [with one obvious typo fixed]
+Subject: Re: Gregorian Calendar (was Re: Another FHC related question
+Date: Tue, 9 Feb 1999 02:32:48 -800
+The following is a(n incomplete) listing, arranged chronologically, of
+European states, with the date they converted from the Julian to the
+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
+ 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
+ 02 Sep 1612 - duchy of Prussia
+13/24 Dec 1614 - Pfalz-Neuburg
+ 1617 - duchy of Kurland (reverted to the Julian calendar in
+ 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
+ 01 Mar 1700 - Protestant Germany (including Swedish possessions in
+ Germany), Denmark, Norway
+ 12 Jul 1700 - Gelderland, Zutphen
+ 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
+ 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.