7cb6842015-08-15Henrik Grubbström (Grubba) Theory and pragmatics of the tz code and data
1e27ed2014-09-04Henrik Grubbström (Grubba)  ----- Outline ----- Scope of the tz database
7cb6842015-08-15Henrik Grubbström (Grubba)  Names of time zone rules
1e27ed2014-09-04Henrik Grubbström (Grubba)  Time zone abbreviations
7cb6842015-08-15Henrik Grubbström (Grubba)  Accuracy of the tz database Time and date functions
1e27ed2014-09-04Henrik Grubbström (Grubba)  Calendrical issues Time and time zones on Mars
7cb6842015-08-15Henrik Grubbström (Grubba) ----- 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:
1e27ed2014-09-04Henrik Grubbström (Grubba)  The Open Group Base Specifications Issue 7 IEEE Std 1003.1, 2013 Edition <http://pubs.opengroup.org/onlinepubs/9699919799/>
7cb6842015-08-15Henrik Grubbström (Grubba) ----- Names of time zone rules -----
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) The time zone rule naming conventions attempt to strike a balance among the following goals:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  * Uniquely identify every region where clocks have agreed since 1970. This is essential for the intended use: static clocks keeping local civil time.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  * Indicate to experts where that region is.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  * 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).
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  * Be portable to a wide variety of implementations.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  * Use a consistent naming conventions over the entire world.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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'.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) Here are the general rules used for choosing location names, in decreasing order of importance:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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').
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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'.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) Excluding 'backward' should not affect the other data. If 'backward' is excluded, excluding 'etcetera' should not affect the remaining data.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) ----- Time zone abbreviations -----
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
eb15d12016-04-10Henrik Grubbström (Grubba)  Use three or more characters that are ASCII alphanumerics or '+' or '-'.
7cb6842015-08-15Henrik Grubbström (Grubba)  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
eb15d12016-04-10Henrik Grubbström (Grubba)  preferred "ChST", so lower-case letters are now allowed. Also, POSIX from 2001 on relaxed the rule to allow '-', '+',
7cb6842015-08-15Henrik Grubbström (Grubba)  and alphanumeric characters from the portable character set
eb15d12016-04-10Henrik Grubbström (Grubba)  in the current locale. In practice ASCII alphanumerics and '+' and '-' are safe in all locales. In other words, in the C locale the POSIX extended regular expression [-+[:alnum:]]{3,} should match the abbreviation. This guarantees that all abbreviations could have been specified by a POSIX TZ string.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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'.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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'.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  Use 'LMT' for local mean time of locations before the introduction of standard time; see "Scope of the tz database".
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  If there is no common English abbreviation, use numeric offsets like -05 and +0830 that are generated by zic's %z notation.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  [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.]
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  Use UT (with time zone abbreviation 'zzz') for locations while uninhabited. The 'zzz' mnemonic is that these locations are, in some sense, asleep.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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.
1e27ed2014-09-04Henrik Grubbström (Grubba)  ----- 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 will be incorrect after future governments change the rules. For example, if today someone schedules a meeting for 13:00 next October 1, Casablanca time, and tomorrow Morocco changes its daylight saving rules, software can mess up after the rule change if it blithely relies on conversions made before the change. * The pre-1970 entries in this database cover only a tiny sliver of how clocks actually behaved; the vast majority of the necessary information was lost or never recorded. Thousands more zones would be needed if the tz database's scope were extended to cover even just the known or guessed history of standard time; for example, the current single entry for France would need to split into dozens
ac5f542016-02-07Henrik Grubbström (Grubba)  of entries, perhaps hundreds. And in most of the world even this approach would be misleading due to widespread disagreement or indifference about what times should be observed. In her 2015 book "The Global Transformation of Time, 1870-1950", Vanessa Ogle writes "Outside of Europe and North America there was no system of time zones at all, often not even a stable landscape of mean times, prior to the middle decades of the twentieth century". See: Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17 https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle
1e27ed2014-09-04Henrik Grubbström (Grubba)  * Most of the pre-1970 data entries come from unreliable sources, often astrology books that lack citations and whose compilers evidently invented entries when the true facts were unknown, without reporting which entries were known and which were invented. These books often contradict each other or give implausible entries, and on the rare occasions when they are checked they are typically found to be incorrect. * For the UK the tz database relies on years of first-class work done by Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>. Other countries are not done nearly as well. * Sometimes, different people in the same city would maintain clocks that differed significantly. Railway time was used by railroad companies (which did not always agree with each other), church-clock time was used for birth certificates, etc. Often this was merely common practice, but sometimes it was set by law. For example, from 1891 to 1911 the UT offset in France was legally 0:09:21 outside train stations and 0:04:21 inside. * Although a named location in the tz database stands for the containing region, its pre-1970 data entries are often accurate for only a small subset of that region. For example, Europe/London stands for the United Kingdom, but its pre-1847 times are valid only for locations that have London's exact meridian, and its 1847 transition to GMT is known to be valid only for the L&NW and the Caledonian railways. * The tz database does not record the earliest time for which a zone's data entries are thereafter valid for every location in the region. For example, Europe/London is valid for all locations in its region after GMT was made the standard time, but the date of standardization (1880-08-02) is not in the tz database, other than in commentary. For many zones the earliest time of validity is unknown. * The tz database does not record a region's boundaries, and in many cases the boundaries are not known. For example, the zone America/Kentucky/Louisville represents a region around the city of Louisville, the boundaries of which are unclear. * Changes that are modeled as instantaneous transitions in the tz database were often spread out over hours, days, or even decades. * Even if the time is specified by law, locations sometimes deliberately flout the law. * Early timekeeping practices, even assuming perfect clocks, were often not specified to the accuracy that the tz database requires. * Sometimes historical timekeeping was specified more precisely than what the tz database can handle. For example, from 1909 to 1937 Netherlands clocks were legally UT+00:19:32.13, but the tz database cannot represent the fractional second. * Even when all the timestamp transitions recorded by the tz database are correct, the tz rules that generate them may not faithfully reflect the historical rules. For example, from 1922 until World War II the UK moved clocks forward the day following the third Saturday in April unless that was Easter, in which case it moved clocks forward the previous Sunday. Because the tz database has no way to specify Easter, these exceptional years are entered as separate tz Rule lines, even though the legal rules did not change.
a6d1842014-10-16Henrik Grubbström (Grubba)  * The tz database models pre-standard time using the proleptic Gregorian
1e27ed2014-09-04Henrik Grubbström (Grubba)  calendar and local mean time (LMT), but many people used other calendars and other timescales. For example, the Roman Empire used the Julian calendar, and had 12 varying-length daytime hours with a non-hour-based system at night. * Early clocks were less reliable, and data entries do not represent this unreliability. * As for leap seconds, civil time was not based on atomic time before 1972, and we don't know the history of earth's rotation accurately enough to map SI seconds to historical solar time to more than about one-hour accuracy. See: Morrison LV, Stephenson FR. Historical values of the Earth's clock error Delta T and the calculation of eclipses. J Hist Astron. 2004;35:327-36 <http://adsabs.harvard.edu/full/2004JHA....35..327M>; Historical values of the Earth's clock error. J Hist Astron. 2005;36:339 <http://adsabs.harvard.edu/full/2005JHA....36..339M>. * The relationship between POSIX time (that is, UTC but ignoring leap seconds) and UTC is not agreed upon after 1972. Although the POSIX clock officially stops during an inserted leap second, at least one proposed standard has it jumping back a second instead; and in practice POSIX clocks more typically either progress glacially during a leap second, or are slightly slowed while near a leap second. * The tz database does not represent how uncertain its information is. Ideally it would contain information about when data entries are incomplete or dicey. Partial temporal knowledge is a field of active research, though, and it's not clear how to apply it here. 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.
7cb6842015-08-15Henrik Grubbström (Grubba) ----- Time and date functions -----
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) The tz code contains time and date functions that are upwards compatible with those of POSIX. 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: stdoffset[dst[offset][,date[/time],date[/time]]]
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  where: 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  Here is an example POSIX TZ string, for US Pacific time using rules appropriate from 1987 through 2006:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  TZ='America/Los_Angeles'
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.)
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * POSIX requires that systems ignore leap seconds.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) These are the extensions that have been made to the POSIX functions:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba)  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).
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.")
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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).
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.)
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * Negative time_t values are supported, on systems where time_t is signed.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * These functions can account for leap seconds, thanks to Bradley White.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) Points of interest to folks with other systems:
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) * 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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.
1e27ed2014-09-04Henrik Grubbström (Grubba) 
7cb6842015-08-15Henrik Grubbström (Grubba) 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.
1e27ed2014-09-04Henrik Grubbström (Grubba)  ----- 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 -----
29be4f2015-03-21Henrik Grubbström (Grubba) 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
1e27ed2014-09-04Henrik Grubbström (Grubba) 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.
29be4f2015-03-21Henrik Grubbström (Grubba) 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/>
1e27ed2014-09-04Henrik Grubbström (Grubba)  -----
7cb6842015-08-15Henrik Grubbström (Grubba)  This file is in the public domain, so clarified as of 2009-05-17 by Arthur David Olson. -----
1e27ed2014-09-04Henrik Grubbström (Grubba) Local Variables: coding: utf-8 End: