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Obtained from: ftp://elsie.nci.nih.gov/pub/tzcode1999a.tar.gz
286 lines
13 KiB
Plaintext
286 lines
13 KiB
Plaintext
@(#)Theory 7.6
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----- Outline -----
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Time and date functions
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Names of time zone regions
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Time zone abbreviations
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----- Time and date functions -----
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These time and date functions are upwards compatible with POSIX.1,
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an international standard for Unix-like systems.
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As of this writing, the current edition of POSIX.1 is:
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Information technology --Portable Operating System Interface (POSIX (R))
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-- Part 1: System Application Program Interface (API) [C Language]
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ISO/IEC 9945-1:1996
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ANSI/IEEE Std 1003.1, 1996 Edition
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1996-07-12
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POSIX.1 has the following properties and limitations.
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* In POSIX.1, time display in a process is controlled by the
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environment variable TZ. Unfortunately, the POSIX.1 TZ string takes
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a form that is hard to describe and is error-prone in practice.
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Also, POSIX.1 TZ strings can't deal with other (for example, Israeli)
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daylight saving time rules, or situations where more than two
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time zone abbreviations are used in an area.
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The POSIX.1 TZ string takes the following form:
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stdoffset[dst[offset],date[/time],date[/time]]
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where:
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std and dst
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are 3 or more characters specifying the standard
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and daylight saving time (DST) zone names.
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offset
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is of the form `[-]hh:[mm[:ss]]' and specifies the
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offset west of UTC. The default DST offset is one hour
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ahead of standard time.
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date[/time],date[/time]
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specifies the beginning and end of DST. If this is absent,
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the system supplies its own rules for DST, and these can
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differ from year to year; typically US DST rules are used.
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time
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takes the form `hh:[mm[:ss]]' and defaults to 02:00.
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date
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takes one of the following forms:
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Jn (1<=n<=365)
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origin-1 day number not counting February 29
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n (0<=n<=365)
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origin-0 day number counting February 29 if present
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Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
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for the dth day of week n of month m of the year,
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where week 1 is the first week in which day d appears,
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and `5' stands for the last week in which day d appears
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(which may be either the 4th or 5th week).
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* In POSIX.1, when a TZ value like "EST5EDT" is parsed,
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typically the current US DST rules are used,
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but this means that the US DST rules are compiled into each program
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that does time conversion. This means that when US time conversion
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rules change (as in the United States in 1987), all programs that
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do time conversion must be recompiled to ensure proper results.
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* In POSIX.1, there's no tamper-proof way for a process to learn the
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system's best idea of local wall clock. (This is important for
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applications that an administrator wants used only at certain times--
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without regard to whether the user has fiddled the "TZ" environment
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variable. While an administrator can "do everything in UTC" to get
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around the problem, doing so is inconvenient and precludes handling
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daylight saving time shifts--as might be required to limit phone
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calls to off-peak hours.)
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* POSIX.1 requires that systems ignore leap seconds.
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These are the extensions that have been made to the POSIX.1 functions:
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* The "TZ" environment variable is used in generating the name of a file
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from which time zone information is read (or is interpreted a la
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POSIX); "TZ" is no longer constrained to be a three-letter time zone
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name followed by a number of hours and an optional three-letter
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daylight time zone name. The daylight saving time rules to be used
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for a particular time zone are encoded in the time zone file;
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the format of the file allows U.S., Australian, and other rules to be
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encoded, and allows for situations where more than two time zone
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abbreviations are used.
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It was recognized that allowing the "TZ" environment variable to
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take on values such as "America/New_York" might cause "old" programs
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(that expect "TZ" to have a certain form) to operate incorrectly;
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consideration was given to using some other environment variable
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(for example, "TIMEZONE") to hold the string used to generate the
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time zone information file name. In the end, however, it was decided
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to continue using "TZ": it is widely used for time zone purposes;
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separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
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and systems where "new" forms of "TZ" might cause problems can simply
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use TZ values such as "EST5EDT" which can be used both by
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"new" programs (a la POSIX) and "old" programs (as zone names and
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offsets).
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* To handle places where more than two time zone abbreviations are used,
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the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
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(where "tmp" is the value the function returns) to the time zone
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abbreviation to be used. This differs from POSIX.1, where the elements
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of tzname are only changed as a result of calls to tzset.
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* Since the "TZ" environment variable can now be used to control time
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conversion, the "daylight" and "timezone" variables are no longer
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needed. (These variables are defined and set by "tzset"; however, their
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values will not be used by "localtime.")
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* The "localtime" function has been set up to deliver correct results
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for near-minimum or near-maximum time_t values. (A comment in the
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source code tells how to get compatibly wrong results).
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* A function "tzsetwall" has been added to arrange for the system's
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best approximation to local wall clock time to be delivered by
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subsequent calls to "localtime." Source code for portable
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applications that "must" run on local wall clock time should call
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"tzsetwall();" if such code is moved to "old" systems that don't
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provide tzsetwall, you won't be able to generate an executable program.
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(These time zone functions also arrange for local wall clock time to be
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used if tzset is called--directly or indirectly--and there's no "TZ"
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environment variable; portable applications should not, however, rely
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on this behavior since it's not the way SVR2 systems behave.)
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* These functions can account for leap seconds, thanks to Bradley White
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(bww@k.cs.cmu.edu).
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Points of interest to folks with other systems:
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* This package is already part of many POSIX-compliant hosts,
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including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
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On such hosts, the primary use of this package
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is to update obsolete time zone rule tables.
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To do this, you may need to compile the time zone compiler
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`zic' supplied with this package instead of using the system `zic',
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since the format of zic's input changed slightly in late 1994,
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and many vendors still do not support the new input format.
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* The Unix Version 7 "timezone" function is not present in this package;
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it's impossible to reliably map timezone's arguments (a "minutes west
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of GMT" value and a "daylight saving time in effect" flag) to a
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time zone abbreviation, and we refuse to guess.
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Programs that in the past used the timezone function may now examine
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tzname[localtime(&clock)->tm_isdst] to learn the correct time
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zone abbreviation to use. Alternatively, use
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localtime(&clock)->tm_zone if this has been enabled.
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* The 4.2BSD gettimeofday function is not used in this package.
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This formerly let users obtain the current UTC offset and DST flag,
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but this functionality was removed in later versions of BSD.
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* In SVR2, time conversion fails for near-minimum or near-maximum
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time_t values when doing conversions for places that don't use UTC.
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This package takes care to do these conversions correctly.
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The functions that are conditionally compiled if STD_INSPIRED is defined
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should, at this point, be looked on primarily as food for thought. They are
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not in any sense "standard compatible"--some are not, in fact, specified in
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*any* standard. They do, however, represent responses of various authors to
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standardization proposals.
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Other time conversion proposals, in particular the one developed by folks at
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Hewlett Packard, offer a wider selection of functions that provide capabilities
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beyond those provided here. The absence of such functions from this package
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is not meant to discourage the development, standardization, or use of such
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functions. Rather, their absence reflects the decision to make this package
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contain valid extensions to POSIX.1, to ensure its broad
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acceptability. If more powerful time conversion functions can be standardized,
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so much the better.
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----- Names of time zone rule files -----
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The names of this package's installed time zone rule files are chosen to
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help minimize possible future incompatibilities due to political events.
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Ordinarily, names of countries are not used, to avoid incompatibilities
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when countries change their name (e.g. Zaire->Congo) or
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when locations change countries (e.g. Hong Kong from UK colony to China).
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Names normally have the form AREA/LOCATION, where AREA is the name
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of a continent or ocean, and LOCATION is the name of a specific
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location within that region. North and South America share the same
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area, `America'. Typical names are `Africa/Cairo', `America/New_York',
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and `Pacific/Honolulu'.
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Here are the general rules used for choosing location names,
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in decreasing order of importance:
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Use only valid Posix file names. Use only Ascii letters, digits, `.',
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`-' and `_'. Do not exceed 14 characters or start with `-'.
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E.g. prefer `Brunei' to `Bandar_Seri_Begawan'.
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Include at least one location per time zone rule set per country.
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One such location is enough.
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If all the clocks in a country's region have agreed since 1970,
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don't bother to include more than one location
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even if subregions' clocks disagreed before 1970.
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Otherwise these tables would become annoyingly large.
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If a name is ambiguous, use a less ambiguous alternative;
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e.g. many cities are named San Jose and Georgetown, so
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prefer `Costa_Rica' to `San_Jose' and `Guyana' to `Georgetown'.
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Keep locations compact. Use cities or small islands, not countries
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or regions, so that any future time zone changes do not split
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locations into different time zones. E.g. prefer `Paris'
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to `France', since France has had multiple time zones.
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Use traditional English spelling, e.g. prefer `Rome' to `Roma', and
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prefer `Athens' to the true name (which uses Greek letters).
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The Posix file name restrictions encourage this rule.
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Use the most populous among locations in a country's time zone,
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e.g. prefer `Shanghai' to `Beijing'. Among locations with
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similar populations, pick the best-known location,
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e.g. prefer `Rome' to `Milan'.
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Use the singular form, e.g. prefer `Canary' to `Canaries'.
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Omit common suffixes like `_Islands' and `_City', unless that
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would lead to ambiguity. E.g. prefer `Cayman' to
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`Cayman_Islands' and `Guatemala' to `Guatemala_City',
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but prefer `Mexico_City' to `Mexico' because the country
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of Mexico has several time zones.
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Use `_' to represent a space.
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Omit `.' from abbreviations in names, e.g. prefer `St_Helena'
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to `St._Helena'.
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The file `zone.tab' lists the geographical locations used to name
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time zone rule files.
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Older versions of this package used a different naming scheme,
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and these older names are still supported.
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See the file `backwards' for most of these older names
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(e.g. `US/Eastern' instead of `America/New_York').
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The other old-fashioned names still supported are
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`WET', `CET', `MET', `EET' (see the file `europe'),
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and `Factory' (see the file `factory').
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----- Time zone abbreviations -----
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When this package is installed, it generates time zone abbreviations
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like `EST' to be compatible with human tradition and POSIX.1.
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Here are the general rules used for choosing time zone abbreviations,
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in decreasing order of importance:
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Use abbreviations that consist of 3 or more upper-case Ascii letters,
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except use "___" for locations while uninhabited.
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Posix.1 requires at least 3 characters, and the restriction to
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upper-case Ascii letters follows most traditions.
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Previous editions of this database also used characters like
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' ' and '?', but these characters have a special meaning to
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the shell and cause commands like
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set `date`
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to have unexpected effects. In theory, the character set could
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be !%./@A-Z^_a-z{}, but these tables use only upper-case
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Ascii letters (and "___").
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Use abbreviations that are in common use among English-speakers,
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e.g. `EST' for Eastern Standard Time in North America.
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We assume that applications translate them to other languages
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as part of the normal localization process; for example,
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a French application might translate `EST' to `HNE'.
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For zones whose times are taken from a city's longitude, use the
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traditional xMT notation, e.g. `PMT' for Paris Mean Time.
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The only name like this in current use is `GMT'.
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If there is no common English abbreviation, abbreviate the English
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translation of the usual phrase used by native speakers.
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If this is not available or is a phrase mentioning the country
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(e.g. ``Cape Verde Time''), then:
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When a country has a single or principal time zone region,
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append `T' to the country's ISO code, e.g. `CVT' for
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Cape Verde Time. For summer time append `ST';
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for double summer time append `DST'; etc.
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When a country has multiple time zones, take the first three
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letters of an English place name identifying each zone
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and then append `T', `ST', etc. as before;
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e.g. `VLAST' for VLAdivostok Summer Time.
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Application writers should note that these abbreviations are ambiguous
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in practice: e.g. `EST' has a different meaning in Australia than
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it does in the United States. In new applications, it's often better
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to use numeric UTC offsets like `-0500' instead of time zone
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abbreviations like `EST'; this avoids the ambiguity.
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