pike.git / lib / modules / Calendar.pmod / tzdata / Theory

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pike.git/lib/modules/Calendar.pmod/tzdata/Theory:1: - This file is in the public domain, so clarified as of - 2009-05-17 by Arthur David Olson. + Theory and pragmatics of the tz code and data    -  +    ----- Outline -----    -  Time and date functions +     Scope of the tz database -  Names of time zone rule files +  Names of time zone rules    Time zone abbreviations -  +  Accuracy of the tz database +  Time and date functions    Calendrical issues    Time and time zones on Mars    - ----- Time and date functions ----- +     - These time and date functions are upwards compatible with those of POSIX, - an international standard for UNIX-like systems. - As of this writing, the current edition of POSIX is: + ----- Scope of the tz database -----    -  + The tz database attempts to record the history and predicted future of + all computer-based clocks that track civil time. To represent this + data, the world is partitioned into regions whose clocks all agree + about time stamps that occur after the somewhat-arbitrary cutoff point + of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, + the database records all known clock transitions, and labels the region + with a notable location. Although 1970 is a somewhat-arbitrary + cutoff, there are significant challenges to moving the cutoff earlier + even by a decade or two, due to the wide variety of local practices + before computer timekeeping became prevalent. +  + Clock transitions before 1970 are recorded for each such location, + because most systems support time stamps before 1970 and could + misbehave if data entries were omitted for pre-1970 transitions. + However, the database is not designed for and does not suffice for + applications requiring accurate handling of all past times everywhere, + as it would take far too much effort and guesswork to record all + details of pre-1970 civil timekeeping. +  + As described below, reference source code for using the tz database is + also available. The tz code is upwards compatible with POSIX, an + international standard for UNIX-like systems. As of this writing, the + current edition of POSIX is: +     The Open Group Base Specifications Issue 7    IEEE Std 1003.1, 2013 Edition    <http://pubs.opengroup.org/onlinepubs/9699919799/>    - POSIX has the following properties and limitations. +     - * In POSIX, time display in a process is controlled by the -  environment variable TZ. Unfortunately, the POSIX TZ string takes -  a form that is hard to describe and is error-prone in practice. -  Also, POSIX TZ strings can't deal with other (for example, Israeli) -  daylight saving time rules, or situations where more than two -  time zone abbreviations are used in an area. +     -  The POSIX TZ string takes the following form: + ----- Names of time zone rules -----    -  stdoffset[dst[offset][,date[/time],date[/time]]] + Each of the database's time zone rules has a unique name. + Inexperienced users are not expected to select these names unaided. + Distributors should provide documentation and/or a simple selection + interface that explains the names; for one example, see the 'tzselect' + program in the tz code. The Unicode Common Locale Data Repository + <http://cldr.unicode.org/> contains data that may be useful for other + selection interfaces.    -  where: + The time zone rule naming conventions attempt to strike a balance + among the following goals:    -  std and dst -  are 3 or more characters specifying the standard -  and daylight saving time (DST) zone names. -  Starting with POSIX.1-2001, std and dst may also be -  in a quoted form like "<UTC+10>"; this allows -  "+" and "-" in the names. -  offset -  is of the form '[+-]hh:[mm[:ss]]' and specifies the -  offset west of UT. 'hh' may be a single digit; 0<=hh<=24. -  The default DST offset is one hour ahead of standard time. -  date[/time],date[/time] -  specifies the beginning and end of DST. If this is absent, -  the system supplies its own rules for DST, and these can -  differ from year to year; typically US DST rules are used. -  time -  takes the form 'hh:[mm[:ss]]' and defaults to 02:00. -  This is the same format as the offset, except that a -  leading '+' or '-' is not allowed. -  date -  takes one of the following forms: -  Jn (1<=n<=365) -  origin-1 day number not counting February 29 -  n (0<=n<=365) -  origin-0 day number counting February 29 if present -  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). -  Typically, this is the only useful form; -  the n and Jn forms are rarely used. +  * Uniquely identify every region where clocks have agreed since 1970. +  This is essential for the intended use: static clocks keeping local +  civil time.    -  Here is an example POSIX TZ string, for US Pacific time using rules -  appropriate from 1987 through 2006: +  * Indicate to experts where that region is.    -  TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00' +  * 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).    -  This POSIX TZ string is hard to remember, and mishandles time stamps -  before 1987 and after 2006. With this package you can use this -  instead: +  * Be portable to a wide variety of implementations.    -  TZ='America/Los_Angeles' +  * Use a consistent naming conventions over the entire world.    - * POSIX does not define the exact meaning of TZ values like "EST5EDT". -  Typically the current US DST rules are used to interpret such values, -  but this means that the US DST rules are compiled into each program -  that does time conversion. This means that when US time conversion -  rules change (as in the United States in 1987), all programs that -  do time conversion must be recompiled to ensure proper results. + 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'.    - * 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.) + Here are the general rules used for choosing location names, + in decreasing order of importance:    - * POSIX requires that systems ignore leap seconds. +  Use only valid POSIX file name components (i.e., the parts of +  names other than '/'). Do not use the file name +  components '.' and '..'. Within a file name component, +  use only ASCII letters, '.', '-' and '_'. Do not use +  digits, as that might create an ambiguity with POSIX +  TZ strings. A file name component must not exceed 14 +  characters or start with '-'. E.g., prefer 'Brunei' +  to 'Bandar_Seri_Begawan'. Exceptions: see the discussion +  of legacy names below. +  A name must not be empty, or contain '//', or start or end with '/'. +  Do not use names that differ only in case. Although the reference +  implementation is case-sensitive, some other implementations +  are not, and they would mishandle names differing only in case. +  If one name A is an initial prefix of another name AB (ignoring case), +  then B must not start with '/', as a regular file cannot have +  the same name as a directory in POSIX. For example, +  'America/New_York' precludes 'America/New_York/Bronx'. +  Uninhabited regions like the North Pole and Bouvet Island +  do not need locations, since local time is not defined there. +  There should typically be at least one name for each ISO 3166-1 +  officially assigned two-letter code for an inhabited country +  or territory. +  If all the clocks in a region have agreed since 1970, +  don't bother to include more than one location +  even if subregions' clocks disagreed before 1970. +  Otherwise these tables would become annoyingly large. +  If a name is ambiguous, use a less ambiguous alternative; +  e.g. many cities are named San José and Georgetown, so +  prefer 'Costa_Rica' to 'San_Jose' and 'Guyana' to 'Georgetown'. +  Keep locations compact. Use cities or small islands, not countries +  or regions, so that any future time zone changes do not split +  locations into different time zones. E.g. prefer 'Paris' +  to 'France', since France has had multiple time zones. +  Use mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and +  prefer 'Athens' to the Greek 'Αθήνα' or the Romanized 'Athína'. +  The POSIX file name restrictions encourage this rule. +  Use the most populous among locations in a zone, +  e.g. prefer 'Shanghai' to 'Beijing'. Among locations with +  similar populations, pick the best-known location, +  e.g. prefer 'Rome' to 'Milan'. +  Use the singular form, e.g. prefer 'Canary' to 'Canaries'. +  Omit common suffixes like '_Islands' and '_City', unless that +  would lead to ambiguity. E.g. prefer 'Cayman' to +  'Cayman_Islands' and 'Guatemala' to 'Guatemala_City', +  but prefer 'Mexico_City' to 'Mexico' because the country +  of Mexico has several time zones. +  Use '_' to represent a space. +  Omit '.' from abbreviations in names, e.g. prefer 'St_Helena' +  to 'St._Helena'. +  Do not change established names if they only marginally +  violate the above rules. For example, don't change +  the existing name 'Rome' to 'Milan' merely because +  Milan's population has grown to be somewhat greater +  than Rome's. +  If a name is changed, put its old spelling in the 'backward' file. +  This means old spellings will continue to work.    - * The tz code attempts to support all the time_t implementations -  allowed by POSIX. The time_t type represents a nonnegative count of -  seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. -  In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit -  signed time_t values stop working after 2038-01-19 03:14:07 UTC, so -  new implementations these days typically use a signed 64-bit integer. -  Unsigned 32-bit integers are used on one or two platforms, -  and 36-bit and 40-bit integers are also used occasionally. -  Although earlier POSIX versions allowed time_t to be a -  floating-point type, this was not supported by any practical -  systems, and POSIX.1-2013 and the tz code both require time_t -  to be an integer type. + The file 'zone1970.tab' lists geographical locations used to name time + zone rules. It is intended to be an exhaustive list of names for + geographic regions as described above; this is a subset of the names + in the data. Although a 'zone1970.tab' location's longitude + corresponds to its LMT offset with one hour for every 15 degrees east + longitude, this relationship is not exact.    - These are the extensions that have been made to the POSIX functions: + Older versions of this package used a different naming scheme, + and these older names are still supported. + See the file 'backward' for most of these older names + (e.g., 'US/Eastern' instead of 'America/New_York'). + The other old-fashioned names still supported are + 'WET', 'CET', 'MET', and 'EET' (see the file 'europe').    - * 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. + Older versions of this package defined legacy names that are + incompatible with the first rule of location names, but which are + still supported. These legacy names are mostly defined in the file + 'etcetera'. Also, the file 'backward' defines the legacy names + 'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file + 'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT', + 'MST7MDT', and 'PST8PDT'.    -  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). + Excluding 'backward' should not affect the other data. If + 'backward' is excluded, excluding 'etcetera' should not affect the + remaining data.    - * 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, 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.") + ----- Time zone abbreviations -----    - * 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). + When this package is installed, it generates time zone abbreviations + like 'EST' to be compatible with human tradition and POSIX. + Here are the general rules used for choosing time zone abbreviations, + in decreasing order of importance:    - * 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.) +  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.    - * Negative time_t values are supported, on systems where time_t is signed. +  This rule guarantees that all abbreviations could have +  been specified by a POSIX TZ string. POSIX +  requires at least three characters for an +  abbreviation. POSIX through 2000 says that an abbreviation +  cannot start with ':', and cannot contain ',', '-', +  '+', NUL, or a digit. POSIX from 2001 on changes this +  rule to say that an abbreviation can contain only '-', '+', +  and alphanumeric characters from the portable character set +  in the current locale. To be portable to both sets of +  rules, an abbreviation must therefore use only ASCII +  letters.    - * These functions can account for leap seconds, thanks to Bradley White. +  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'.    - Points of interest to folks with other systems: +  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'.    - * 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. +  Use 'LMT' for local mean time of locations before the introduction +  of standard time; see "Scope of the tz database".    - * 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. +  If there is no common English abbreviation, use numeric offsets like +  -05 and +0830 that are generated by zic's %z notation.    - * 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. +  [The remaining guidelines predate the introduction of %z. +  They are problematic as they mean tz data entries invent +  notation rather than record it. These guidelines are now +  deprecated and the plan is to gradually move to %z for +  inhabited locations and to "-00" for uninhabited locations.]    - * In SVR2, time conversion fails for near-minimum or near-maximum -  time_t values when doing conversions for places that don't use UT. -  This package takes care to do these conversions correctly. +  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:    - 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. +  When a country is identified with a single or principal zone, +  append 'T' to the country's ISO code, e.g. 'CVT' for +  Cape Verde Time. For summer time append 'ST'; +  for double summer time append 'DST'; etc. +  Otherwise, take the first three letters of an English place +  name identifying each zone and append 'T', 'ST', etc. +  as before; e.g. 'VLAST' for VLAdivostok Summer Time.    - 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. +  Use UT (with time zone abbreviation 'zzz') for locations while +  uninhabited. The 'zzz' mnemonic is that these locations are, +  in some sense, asleep.    -  + Application writers should note that these abbreviations are ambiguous + in practice: e.g. 'CST' has a different meaning in China than + it does in the United States. In new applications, it's often better + to use numeric UT offsets like '-0600' instead of time zone + abbreviations like 'CST'; this avoids the ambiguity.    - ----- Scope of the tz database ----- +     - The tz database attempts to record the history and predicted future of - all computer-based clocks that track civil time. To represent this - data, the world is partitioned into regions whose clocks all agree - about time stamps that occur after the somewhat-arbitrary cutoff point - of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region, - the database records all known clock transitions, and labels the region - with a notable location. Although 1970 is a somewhat-arbitrary - cutoff, there are significant challenges to moving the cutoff earlier - even by a decade or two, due to the wide variety of local practices - before computer timekeeping became prevalent. -  - Clock transitions before 1970 are recorded for each such location, - because most POSIX-compatible systems support negative time stamps 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. -  -  +    ----- Accuracy of the tz database -----      The tz database is not authoritative, and it surely has errors.   Corrections are welcome and encouraged; see the file CONTRIBUTING.   Users requiring authoritative data should consult national standards   bodies and the references cited in the database's comments.      Errors in the tz database arise from many sources:       * The tz database predicts future time stamps, and current predictions
pike.git/lib/modules/Calendar.pmod/tzdata/Theory:351:      In short, many, perhaps most, of the tz database's pre-1970 and future   time stamps 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.       - ----- Names of time zone rule files ----- + ----- Time and date functions -----    - The time zone rule file naming conventions attempt to strike a balance - among the following goals: + The tz code contains time and date functions that are upwards + compatible with those of POSIX.    -  * 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. + POSIX has the following properties and limitations.    -  * Indicate to humans as to where that region is. This simplifies use. + * 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.    -  * 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). +  The POSIX TZ string takes the following form:    -  * Be portable to a wide variety of implementations. -  This promotes use of the technology. +  stdoffset[dst[offset][,date[/time],date[/time]]]    -  * Use a consistent naming convention over the entire world. -  This simplifies both use and maintenance. +  where:    - 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. +  std and dst +  are 3 or more characters specifying the standard +  and daylight saving time (DST) zone names. +  Starting with POSIX.1-2001, std and dst may also be +  in a quoted form like "<UTC+10>"; this allows +  "+" and "-" in the names. +  offset +  is of the form '[+-]hh:[mm[:ss]]' and specifies the +  offset west of UT. 'hh' may be a single digit; 0<=hh<=24. +  The default DST offset is one hour ahead of standard time. +  date[/time],date[/time] +  specifies the beginning and end of DST. If this is absent, +  the system supplies its own rules for DST, and these can +  differ from year to year; typically US DST rules are used. +  time +  takes the form 'hh:[mm[:ss]]' and defaults to 02:00. +  This is the same format as the offset, except that a +  leading '+' or '-' is not allowed. +  date +  takes one of the following forms: +  Jn (1<=n<=365) +  origin-1 day number not counting February 29 +  n (0<=n<=365) +  origin-0 day number counting February 29 if present +  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). +  Typically, this is the only useful form; +  the n and Jn forms are rarely used.    - 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 is an example POSIX TZ string, for US Pacific time using rules +  appropriate from 1987 through 2006:    - Here are the general rules used for choosing location names, - in decreasing order of importance: +  TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'    -  Use only valid POSIX file name components (i.e., the parts of -  names other than '/'). Do not use the file name -  components '.' and '..'. Within a file name component, -  use only ASCII letters, '.', '-' and '_'. Do not use -  digits, as that might create an ambiguity with POSIX -  TZ strings. A file name component must not exceed 14 -  characters or start with '-'. E.g., prefer 'Brunei' -  to 'Bandar_Seri_Begawan'. Exceptions: see the discussion -  of legacy names below. -  A name must not be empty, or contain '//', or start or end with '/'. -  Do not use names that differ only in case. Although the reference -  implementation is case-sensitive, some other implementations -  are not, and they would mishandle names differing only in case. -  If one name A is an initial prefix of another name AB (ignoring case), -  then B must not start with '/', as a regular file cannot have -  the same name as a directory in POSIX. For example, -  'America/New_York' precludes 'America/New_York/Bronx'. -  Uninhabited regions like the North Pole and Bouvet Island -  do not need locations, since local time is not defined there. -  There should typically be at least one name for each ISO 3166-1 -  officially assigned two-letter code for an inhabited country -  or territory. -  If all the clocks in a region have agreed since 1970, -  don't bother to include more than one location -  even if subregions' clocks disagreed before 1970. -  Otherwise these tables would become annoyingly large. -  If a name is ambiguous, use a less ambiguous alternative; -  e.g. many cities are named San José and Georgetown, so -  prefer 'Costa_Rica' to 'San_Jose' and 'Guyana' to 'Georgetown'. -  Keep locations compact. Use cities or small islands, not countries -  or regions, so that any future time zone changes do not split -  locations into different time zones. E.g. prefer 'Paris' -  to 'France', since France has had multiple time zones. -  Use mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and -  prefer 'Athens' to the Greek 'Αθήνα' or the Romanized 'Athína'. -  The POSIX file name restrictions encourage this rule. -  Use the most populous among locations in a zone, -  e.g. prefer 'Shanghai' to 'Beijing'. Among locations with -  similar populations, pick the best-known location, -  e.g. prefer 'Rome' to 'Milan'. -  Use the singular form, e.g. prefer 'Canary' to 'Canaries'. -  Omit common suffixes like '_Islands' and '_City', unless that -  would lead to ambiguity. E.g. prefer 'Cayman' to -  'Cayman_Islands' and 'Guatemala' to 'Guatemala_City', -  but prefer 'Mexico_City' to 'Mexico' because the country -  of Mexico has several time zones. -  Use '_' to represent a space. -  Omit '.' from abbreviations in names, e.g. prefer 'St_Helena' -  to 'St._Helena'. -  Do not change established names if they only marginally -  violate the above rules. For example, don't change -  the existing name 'Rome' to 'Milan' merely because -  Milan's population has grown to be somewhat greater -  than Rome's. -  If a name is changed, put its old spelling in the 'backward' file. -  This means old spellings will continue to work. +  This POSIX TZ string is hard to remember, and mishandles time stamps +  before 1987 and after 2006. With this package you can use this +  instead:    - The file 'zone1970.tab' lists geographical locations used to name time - zone rule files. It is intended to be an exhaustive list of names - for geographic regions as described above; this is a subset of the - names in the data. Although a 'zone1970.tab' location's longitude - corresponds to its LMT offset with one hour for every 15 degrees east - longitude, this relationship is not exact. +  TZ='America/Los_Angeles'    - Older versions of this package used a different naming scheme, - and these older names are still supported. - See the file 'backward' for most of these older names - (e.g., 'US/Eastern' instead of 'America/New_York'). - The other old-fashioned names still supported are - 'WET', 'CET', 'MET', and 'EET' (see the file 'europe'). + * POSIX does not define the exact meaning of TZ values like "EST5EDT". +  Typically the current US DST rules are used to interpret such values, +  but this means that the US DST rules are compiled into each program +  that does time conversion. This means that when US time conversion +  rules change (as in the United States in 1987), all programs that +  do time conversion must be recompiled to ensure proper results.    - Older versions of this package defined legacy names that are - incompatible with the first rule of location names, but which are - still supported. These legacy names are mostly defined in the file - 'etcetera'. Also, the file 'backward' defines the legacy names - 'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file - 'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT', - 'MST7MDT', and 'PST8PDT'. + * 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.)    - Excluding 'backward' should not affect the other data. If - 'backward' is excluded, excluding 'etcetera' should not affect the - remaining data. + * POSIX requires that systems ignore leap seconds.    -  + * The tz code attempts to support all the time_t implementations +  allowed by POSIX. The time_t type represents a nonnegative count of +  seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds. +  In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit +  signed time_t values stop working after 2038-01-19 03:14:07 UTC, so +  new implementations these days typically use a signed 64-bit integer. +  Unsigned 32-bit integers are used on one or two platforms, +  and 36-bit and 40-bit integers are also used occasionally. +  Although earlier POSIX versions allowed time_t to be a +  floating-point type, this was not supported by any practical +  systems, and POSIX.1-2013 and the tz code both require time_t +  to be an integer type.    - ----- Time zone abbreviations ----- + These are the extensions that have been made to the POSIX functions:    - When this package is installed, it generates time zone abbreviations - like 'EST' to be compatible with human tradition and POSIX. - Here are the general rules used for choosing time zone abbreviations, - in decreasing order of importance: + * 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.    -  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. +  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).    -  This rule guarantees that all abbreviations could have -  been specified by a POSIX TZ string. POSIX -  requires at least three characters for an -  abbreviation. POSIX through 2000 says that an abbreviation -  cannot start with ':', and cannot contain ',', '-', -  '+', NUL, or a digit. POSIX from 2001 on changes this -  rule to say that an abbreviation can contain only '-', '+', -  and alphanumeric characters from the portable character set -  in the current locale. To be portable to both sets of -  rules, an abbreviation must therefore use only ASCII -  letters. + * 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, where the elements +  of tzname are only changed as a result of calls to tzset.    -  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'. + * 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.")    -  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'. + * 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).    -  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: + * 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.)    -  When a country is identified with a single or principal zone, -  append 'T' to the country's ISO code, e.g. 'CVT' for -  Cape Verde Time. For summer time append 'ST'; -  for double summer time append 'DST'; etc. -  Otherwise, take the first three letters of an English place -  name identifying each zone and append 'T', 'ST', etc. -  as before; e.g. 'VLAST' for VLAdivostok Summer Time. + * Negative time_t values are supported, on systems where time_t is signed.    -  Use 'LMT' for local mean time of locations before the introduction -  of standard time; see "Scope of the tz database". + * These functions can account for leap seconds, thanks to Bradley White.    -  Use UT (with time zone abbreviation 'zzz') for locations while -  uninhabited. The 'zzz' mnemonic is that these locations are, -  in some sense, asleep. + Points of interest to folks with other systems:    - Application writers should note that these abbreviations are ambiguous - in practice: e.g. 'CST' has a different meaning in China than - it does in the United States. In new applications, it's often better - to use numeric UT offsets like '-0600' instead of time zone - abbreviations like 'CST'; this avoids the ambiguity. + * 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 UT. +  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, to ensure its broad acceptability. If + more powerful time conversion functions can be standardized, so much the + better. +  +    ----- Calendrical issues -----      Calendrical issues are a bit out of scope for a time zone database,   but they indicate the sort of problems that we would run into if we   extended the time zone database further into the past. An excellent   resource in this area is Nachum Dershowitz and Edward M. Reingold,   Calendrical Calculations: Third Edition, Cambridge University Press (2008)   <http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>.   Other information and sources are given below. They sometimes disagree.   
pike.git/lib/modules/Calendar.pmod/tzdata/Theory:759:   <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08).      Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times   <http://articles.latimes.com/2004/jan/14/science/sci-marstime14>   (2004-01-14), pp A1, A20-A21.      Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26)   <http://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/>      ----- +  + This file is in the public domain, so clarified as of 2009-05-17 by + Arthur David Olson. +  + -----   Local Variables:   coding: utf-8   End: