858 lines
37 KiB
Plaintext
858 lines
37 KiB
Plaintext
Theory and pragmatics of the tz code and data
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----- Outline -----
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Scope of the tz database
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Names of time zone rules
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Time zone abbreviations
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Accuracy of the tz database
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Time and date functions
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Interface stability
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Calendrical issues
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Time and time zones on Mars
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----- Scope of the tz database -----
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The tz database attempts to record the history and predicted future of
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all computer-based clocks that track civil time. To represent this
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data, the world is partitioned into regions whose clocks all agree
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about time stamps that occur after the somewhat-arbitrary cutoff point
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of the POSIX Epoch (1970-01-01 00:00:00 UTC). For each such region,
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the database records all known clock transitions, and labels the region
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with a notable location. Although 1970 is a somewhat-arbitrary
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cutoff, there are significant challenges to moving the cutoff earlier
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even by a decade or two, due to the wide variety of local practices
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before computer timekeeping became prevalent.
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Clock transitions before 1970 are recorded for each such location,
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because most systems support time stamps before 1970 and could
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misbehave if data entries were omitted for pre-1970 transitions.
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However, the database is not designed for and does not suffice for
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applications requiring accurate handling of all past times everywhere,
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as it would take far too much effort and guesswork to record all
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details of pre-1970 civil timekeeping.
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As described below, reference source code for using the tz database is
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also available. The tz code is upwards compatible with POSIX, an
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international standard for UNIX-like systems. As of this writing, the
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current edition of POSIX is:
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The Open Group Base Specifications Issue 7
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IEEE Std 1003.1, 2013 Edition
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<http://pubs.opengroup.org/onlinepubs/9699919799/>
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----- Names of time zone rules -----
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Each of the database's time zone rules has a unique name.
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Inexperienced users are not expected to select these names unaided.
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Distributors should provide documentation and/or a simple selection
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interface that explains the names; for one example, see the 'tzselect'
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program in the tz code. The Unicode Common Locale Data Repository
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<http://cldr.unicode.org/> contains data that may be useful for other
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selection interfaces.
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The time zone rule naming conventions attempt to strike a balance
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among the following goals:
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* Uniquely identify every region where clocks have agreed since 1970.
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This is essential for the intended use: static clocks keeping local
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civil time.
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* Indicate to experts where that region is.
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* Be robust in the presence of political changes. For example, names
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of countries are ordinarily not used, to avoid incompatibilities
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when countries change their name (e.g. Zaire->Congo) or when
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locations change countries (e.g. Hong Kong from UK colony to
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China).
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* Be portable to a wide variety of implementations.
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* Use a consistent naming conventions over the entire world.
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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 name components (i.e., the parts of
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names other than '/'). Do not use the file name
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components '.' and '..'. Within a file name component,
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use only ASCII letters, '.', '-' and '_'. Do not use
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digits, as that might create an ambiguity with POSIX
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TZ strings. A file name component must not exceed 14
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characters or start with '-'. E.g., prefer 'Brunei'
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to 'Bandar_Seri_Begawan'. Exceptions: see the discussion
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of legacy names below.
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A name must not be empty, or contain '//', or start or end with '/'.
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Do not use names that differ only in case. Although the reference
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implementation is case-sensitive, some other implementations
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are not, and they would mishandle names differing only in case.
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If one name A is an initial prefix of another name AB (ignoring case),
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then B must not start with '/', as a regular file cannot have
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the same name as a directory in POSIX. For example,
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'America/New_York' precludes 'America/New_York/Bronx'.
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Uninhabited regions like the North Pole and Bouvet Island
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do not need locations, since local time is not defined there.
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There should typically be at least one name for each ISO 3166-1
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officially assigned two-letter code for an inhabited country
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or territory.
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If all the clocks in a 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 José 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 mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and
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prefer 'Athens' to the Greek 'Αθήνα' or the Romanized 'Athína'.
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The POSIX file name restrictions encourage this rule.
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Use the most populous among locations in a 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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Do not change established names if they only marginally
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violate the above rules. For example, don't change
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the existing name 'Rome' to 'Milan' merely because
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Milan's population has grown to be somewhat greater
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than Rome's.
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If a name is changed, put its old spelling in the 'backward' file.
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This means old spellings will continue to work.
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The file 'zone1970.tab' lists geographical locations used to name time
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zone rules. It is intended to be an exhaustive list of names for
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geographic regions as described above; this is a subset of the names
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in the data. Although a 'zone1970.tab' location's longitude
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corresponds to its LMT offset with one hour for every 15 degrees east
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longitude, this relationship is not exact.
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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 'backward' 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', and 'EET' (see the file 'europe').
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Older versions of this package defined legacy names that are
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incompatible with the first rule of location names, but which are
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still supported. These legacy names are mostly defined in the file
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'etcetera'. Also, the file 'backward' defines the legacy names
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'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file
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'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT',
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'MST7MDT', and 'PST8PDT'.
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Excluding 'backward' should not affect the other data. If
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'backward' is excluded, excluding 'etcetera' should not affect the
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remaining data.
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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.
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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 three or more characters that are ASCII alphanumerics or '+' or '-'.
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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.
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Previous editions of this rule required upper-case letters,
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but the Congressman who introduced Chamorro Standard Time
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preferred "ChST", so lower-case letters are now allowed.
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Also, POSIX from 2001 on relaxed the rule to allow '-', '+',
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and alphanumeric characters from the portable character set
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in the current locale. In practice ASCII alphanumerics and
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'+' and '-' are safe in all locales.
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In other words, in the C locale the POSIX extended regular
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expression [-+[:alnum:]]{3,} should match the abbreviation.
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This guarantees that all abbreviations could have been
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specified by a POSIX TZ string.
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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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Use 'LMT' for local mean time of locations before the introduction
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of standard time; see "Scope of the tz database".
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If there is no common English abbreviation, use numeric offsets like
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-05 and +0830 that are generated by zic's %z notation.
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[The remaining guidelines predate the introduction of %z.
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They are problematic as they mean tz data entries invent
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notation rather than record it. These guidelines are now
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deprecated and the plan is to gradually move to %z for
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inhabited locations and to "-00" for uninhabited locations.]
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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 is identified with a single or principal zone,
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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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Otherwise, take the first three letters of an English place
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name identifying each zone and append 'T', 'ST', etc.
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as before; e.g. 'VLAST' for VLAdivostok Summer Time.
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Use UT (with time zone abbreviation '-00') for locations while
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uninhabited. The leading '-' is a flag that the time
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zone is in some sense undefined; this notation is
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derived from Internet RFC 3339.
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Application writers should note that these abbreviations are ambiguous
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in practice: e.g. 'CST' has a different meaning in China 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 UT offsets like '-0600' instead of time zone
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abbreviations like 'CST'; this avoids the ambiguity.
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----- Accuracy of the tz database -----
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The tz database is not authoritative, and it surely has errors.
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Corrections are welcome and encouraged; see the file CONTRIBUTING.
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Users requiring authoritative data should consult national standards
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bodies and the references cited in the database's comments.
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Errors in the tz database arise from many sources:
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* The tz database predicts future time stamps, and current predictions
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will be incorrect after future governments change the rules.
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For example, if today someone schedules a meeting for 13:00 next
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October 1, Casablanca time, and tomorrow Morocco changes its
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daylight saving rules, software can mess up after the rule change
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if it blithely relies on conversions made before the change.
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* The pre-1970 entries in this database cover only a tiny sliver of how
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clocks actually behaved; the vast majority of the necessary
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information was lost or never recorded. Thousands more zones would
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be needed if the tz database's scope were extended to cover even
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just the known or guessed history of standard time; for example,
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the current single entry for France would need to split into dozens
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of entries, perhaps hundreds. And in most of the world even this
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approach would be misleading due to widespread disagreement or
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indifference about what times should be observed. In her 2015 book
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"The Global Transformation of Time, 1870-1950", Vanessa Ogle writes
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"Outside of Europe and North America there was no system of time
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zones at all, often not even a stable landscape of mean times,
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prior to the middle decades of the twentieth century". See:
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Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17
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https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle
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* Most of the pre-1970 data entries come from unreliable sources, often
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astrology books that lack citations and whose compilers evidently
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invented entries when the true facts were unknown, without
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reporting which entries were known and which were invented.
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These books often contradict each other or give implausible entries,
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and on the rare occasions when they are checked they are
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typically found to be incorrect.
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* For the UK the tz database relies on years of first-class work done by
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Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>.
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Other countries are not done nearly as well.
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* Sometimes, different people in the same city would maintain clocks
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that differed significantly. Railway time was used by railroad
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companies (which did not always agree with each other),
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church-clock time was used for birth certificates, etc.
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Often this was merely common practice, but sometimes it was set by law.
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For example, from 1891 to 1911 the UT offset in France was legally
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0:09:21 outside train stations and 0:04:21 inside.
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* Although a named location in the tz database stands for the
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containing region, its pre-1970 data entries are often accurate for
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only a small subset of that region. For example, Europe/London
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stands for the United Kingdom, but its pre-1847 times are valid
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only for locations that have London's exact meridian, and its 1847
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transition to GMT is known to be valid only for the L&NW and the
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Caledonian railways.
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* The tz database does not record the earliest time for which a zone's
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data entries are thereafter valid for every location in the region.
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For example, Europe/London is valid for all locations in its
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region after GMT was made the standard time, but the date of
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standardization (1880-08-02) is not in the tz database, other than
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in commentary. For many zones the earliest time of validity is
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unknown.
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* The tz database does not record a region's boundaries, and in many
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cases the boundaries are not known. For example, the zone
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America/Kentucky/Louisville represents a region around the city of
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Louisville, the boundaries of which are unclear.
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* Changes that are modeled as instantaneous transitions in the tz
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database were often spread out over hours, days, or even decades.
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* Even if the time is specified by law, locations sometimes
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deliberately flout the law.
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* Early timekeeping practices, even assuming perfect clocks, were
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often not specified to the accuracy that the tz database requires.
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* Sometimes historical timekeeping was specified more precisely
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than what the tz database can handle. For example, from 1909 to
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1937 Netherlands clocks were legally UT +00:19:32.13, but the tz
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database cannot represent the fractional second.
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* Even when all the timestamp transitions recorded by the tz database
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are correct, the tz rules that generate them may not faithfully
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reflect the historical rules. For example, from 1922 until World
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War II the UK moved clocks forward the day following the third
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Saturday in April unless that was Easter, in which case it moved
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clocks forward the previous Sunday. Because the tz database has no
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way to specify Easter, these exceptional years are entered as
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separate tz Rule lines, even though the legal rules did not change.
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* The tz database models pre-standard time using the proleptic Gregorian
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calendar and local mean time (LMT), but many people used other
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calendars and other timescales. For example, the Roman Empire used
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the Julian calendar, and had 12 varying-length daytime hours with a
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non-hour-based system at night.
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* Early clocks were less reliable, and data entries do not represent
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clock error.
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* The tz database assumes Universal Time (UT) as an origin, even
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though UT is not standardized for older time stamps. In the tz
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database commentary, UT denotes a family of time standards that
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includes Coordinated Universal Time (UTC) along with other variants
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such as UT1 and GMT, with days starting at midnight. Although UT
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equals UTC for modern time stamps, UTC was not defined until 1960,
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so commentary uses the more-general abbreviation UT for time stamps
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that might predate 1960. Since UT, UT1, etc. disagree slightly,
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and since pre-1972 UTC seconds varied in length, interpretation of
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older time stamps can be problematic when subsecond accuracy is
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needed.
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* Civil time was not based on atomic time before 1972, and we don't
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know the history of earth's rotation accurately enough to map SI
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seconds to historical solar time to more than about one-hour
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accuracy. See: Morrison LV, Stephenson FR.
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Historical values of the Earth's clock error Delta T and the
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calculation of eclipses. J Hist Astron. 2004;35:327-36
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<http://adsabs.harvard.edu/full/2004JHA....35..327M>;
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Historical values of the Earth's clock error. J Hist Astron. 2005;36:339
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<http://adsabs.harvard.edu/full/2005JHA....36..339M>.
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* The relationship between POSIX time (that is, UTC but ignoring leap
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seconds) and UTC is not agreed upon after 1972. Although the POSIX
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clock officially stops during an inserted leap second, at least one
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proposed standard has it jumping back a second instead; and in
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practice POSIX clocks more typically either progress glacially during
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a leap second, or are slightly slowed while near a leap second.
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* The tz database does not represent how uncertain its information is.
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Ideally it would contain information about when data entries are
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incomplete or dicey. Partial temporal knowledge is a field of
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active research, though, and it's not clear how to apply it here.
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In short, many, perhaps most, of the tz database's pre-1970 and future
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time stamps are either wrong or misleading. Any attempt to pass the
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tz database off as the definition of time should be unacceptable to
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anybody who cares about the facts. In particular, the tz database's
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LMT offsets should not be considered meaningful, and should not prompt
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creation of zones merely because two locations differ in LMT or
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transitioned to standard time at different dates.
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----- Time and date functions -----
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The tz code contains time and date functions that are upwards
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compatible with those of POSIX.
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POSIX has the following properties and limitations.
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* In POSIX, time display in a process is controlled by the
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environment variable TZ. Unfortunately, the POSIX 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 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 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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Starting with POSIX.1-2001, std and dst may also be
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in a quoted form like "<UTC+10>"; this allows
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"+" and "-" in the 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 UT. 'hh' may be a single digit; 0<=hh<=24.
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The default DST offset is one hour 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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This is the same format as the offset, except that a
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leading '+' or '-' is not allowed.
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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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Typically, this is the only useful form;
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the n and Jn forms are rarely used.
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Here is an example POSIX TZ string, for US Pacific time using rules
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appropriate from 1987 through 2006:
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TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
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This POSIX TZ string is hard to remember, and mishandles time stamps
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before 1987 and after 2006. With this package you can use this
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instead:
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TZ='America/Los_Angeles'
|
|
|
|
* 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.
|
|
|
|
* The TZ environment variable is process-global, which makes it hard
|
|
to write efficient, thread-safe applications that need access
|
|
to multiple time zones.
|
|
|
|
* 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.)
|
|
|
|
* POSIX provides no convenient and efficient way to determine the UT
|
|
offset and time zone abbreviation of arbitrary time stamps,
|
|
particularly for time zone settings that do not fit into the
|
|
POSIX model.
|
|
|
|
* 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.
|
|
|
|
These are the extensions that have been made to the POSIX 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).
|
|
|
|
* The code supports platforms with a UT offset member in struct tm,
|
|
e.g., tm_gmtoff.
|
|
|
|
* The code supports platforms with a time zone abbreviation member in
|
|
struct tm, e.g., tm_zone.
|
|
|
|
* 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.")
|
|
|
|
* Functions tzalloc, tzfree, localtime_rz, and mktime_z for
|
|
more-efficient thread-safe applications that need to use
|
|
multiple time zones. The tzalloc and tzfree functions
|
|
allocate and free objects of type timezone_t, and localtime_rz
|
|
and mktime_z are like localtime_r and mktime with an extra
|
|
timezone_t argument. The functions were inspired by NetBSD.
|
|
|
|
* 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.)
|
|
|
|
* Negative time_t values are supported, on systems where time_t is signed.
|
|
|
|
* These functions can account for leap seconds, thanks to Bradley White.
|
|
|
|
Points of interest to folks with other systems:
|
|
|
|
* Code compatible with this package is already part of many platforms,
|
|
including GNU/Linux, Android, the BSDs, Chromium OS, Cygwin, AIX, iOS,
|
|
BlackBery 10, macOS, Microsoft Windows, OpenVMS, and Solaris.
|
|
On such hosts, the primary use of this package
|
|
is to update obsolete time zone rule tables.
|
|
To do this, you may need to compile the time zone compiler
|
|
'zic' supplied with this package instead of using the system 'zic',
|
|
since the format of zic's input is occasionally extended,
|
|
and a platform may still be shipping an older zic.
|
|
|
|
* 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.
|
|
A comment in the source code tells how to get compatibly wrong
|
|
results.
|
|
|
|
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.
|
|
|
|
|
|
----- Interface stability -----
|
|
|
|
The tz code and data supply the following interfaces:
|
|
|
|
* A set of zone names as per "Names of time zone rules" above.
|
|
|
|
* Library functions described in "Time and date functions" above.
|
|
|
|
* The programs tzselect, zdump, and zic, documented in their man pages.
|
|
|
|
* The format of zic input files, documented in the zic man page.
|
|
|
|
* The format of zic output files, documented in the tzfile man page.
|
|
|
|
* The format of zone table files, documented in zone1970.tab.
|
|
|
|
* The format of the country code file, documented in iso3166.tab.
|
|
|
|
* The version number of the code and data, as the first line of
|
|
the text file 'version' in each release.
|
|
|
|
Interface changes in a release attempt to preserve compatibility with
|
|
recent releases. For example, tz data files typically do not rely on
|
|
recently-added zic features, so that users can run older zic versions
|
|
to process newer data files. The tz-link.htm file describes how
|
|
releases are tagged and distributed.
|
|
|
|
Interfaces not listed above are less stable. For example, users
|
|
should not rely on particular UT offsets or abbreviations for time
|
|
stamps, as data entries are often based on guesswork and these guesses
|
|
may be corrected or improved.
|
|
|
|
|
|
----- 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.
|
|
|
|
|
|
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 (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 and 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: Mark Brader
|
|
Subject: Re: Gregorian reform - a part of locale?
|
|
<news:1996Jul6.012937.29190@sq.com>
|
|
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: "Tideräkning och historia"
|
|
by Natanael Beckman (1924) and "Tid, en bok om tideräkning och
|
|
kalenderväsen" by Lars-Olof Lodén (1968).
|
|
|
|
|
|
Grotefend's data
|
|
|
|
From: "Michael Palmer" [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
|
|
...
|
|
|
|
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 (Lüttich)
|
|
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 Jülich-Berg
|
|
02/13 Nov 1583 - electorate and city of Köln
|
|
04/15 Nov 1583 - bishopric of Würzburg
|
|
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 Münster and duchy of Cleve
|
|
14/25 Dec 1583 - Steiermark
|
|
|
|
06/17 Jan 1584 - Austria and Bohemia
|
|
11/22 Jan 1584 - Lucerne, 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 Osnabrück
|
|
|
|
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, Zürich, 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 - Graubünden
|
|
|
|
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's work schedules use Mars time. Jet Propulsion Laboratory
|
|
(JPL) coordinators have kept Mars time on and off at least since 1997
|
|
for the Mars Pathfinder mission. Some of their family members have
|
|
also adapted to Mars time. Dozens of special Mars watches were built
|
|
for JPL workers who kept Mars time during the Mars Exploration
|
|
Rovers mission (2004). These timepieces look like normal Seikos and
|
|
Citizens but use Mars seconds rather than terrestrial seconds.
|
|
|
|
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> (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:
|