[BACK]Return to Theory CVS log [TXT][DIR] Up to [cvs.NetBSD.org] / src / lib / libc / time

Annotation of src/lib/libc/time/Theory, Revision 1.20

1.18      christos    1: Theory and pragmatics of the tz code and data
                      2:
1.2       perry       3:
                      4: ----- Outline -----
                      5:
1.10      christos    6:        Scope of the tz database
1.18      christos    7:        Names of time zone rules
1.2       perry       8:        Time zone abbreviations
1.18      christos    9:        Accuracy of the tz database
                     10:        Time and date functions
1.4       kleink     11:        Calendrical issues
1.8       kleink     12:        Time and time zones on Mars
1.2       perry      13:
                     14:
1.18      christos   15: ----- Scope of the tz database -----
                     16:
                     17: The tz database attempts to record the history and predicted future of
                     18: all computer-based clocks that track civil time.  To represent this
                     19: data, the world is partitioned into regions whose clocks all agree
                     20: about time stamps that occur after the somewhat-arbitrary cutoff point
                     21: of the POSIX Epoch (1970-01-01 00:00:00 UTC).  For each such region,
                     22: the database records all known clock transitions, and labels the region
                     23: with a notable location.  Although 1970 is a somewhat-arbitrary
                     24: cutoff, there are significant challenges to moving the cutoff earlier
                     25: even by a decade or two, due to the wide variety of local practices
                     26: before computer timekeeping became prevalent.
                     27:
                     28: Clock transitions before 1970 are recorded for each such location,
                     29: because most systems support time stamps before 1970 and could
                     30: misbehave if data entries were omitted for pre-1970 transitions.
                     31: However, the database is not designed for and does not suffice for
                     32: applications requiring accurate handling of all past times everywhere,
                     33: as it would take far too much effort and guesswork to record all
                     34: details of pre-1970 civil timekeeping.
                     35:
                     36: As described below, reference source code for using the tz database is
                     37: also available.  The tz code is upwards compatible with POSIX, an
                     38: international standard for UNIX-like systems.  As of this writing, the
                     39: current edition of POSIX is:
1.2       perry      40:
1.14      christos   41:   The Open Group Base Specifications Issue 7
                     42:   IEEE Std 1003.1, 2013 Edition
                     43:   <http://pubs.opengroup.org/onlinepubs/9699919799/>
1.2       perry      44:
                     45:
1.1       jtc        46:
1.18      christos   47: ----- Names of time zone rules -----
1.2       perry      48:
1.18      christos   49: Each of the database's time zone rules has a unique name.
                     50: Inexperienced users are not expected to select these names unaided.
                     51: Distributors should provide documentation and/or a simple selection
                     52: interface that explains the names; for one example, see the 'tzselect'
                     53: program in the tz code.  The Unicode Common Locale Data Repository
                     54: <http://cldr.unicode.org/> contains data that may be useful for other
                     55: selection interfaces.
1.2       perry      56:
1.18      christos   57: The time zone rule naming conventions attempt to strike a balance
                     58: among the following goals:
1.6       kleink     59:
1.18      christos   60:  * Uniquely identify every region where clocks have agreed since 1970.
                     61:    This is essential for the intended use: static clocks keeping local
                     62:    civil time.
                     63:
                     64:  * Indicate to experts where that region is.
                     65:
                     66:  * Be robust in the presence of political changes.  For example, names
                     67:    of countries are ordinarily not used, to avoid incompatibilities
                     68:    when countries change their name (e.g. Zaire->Congo) or when
                     69:    locations change countries (e.g. Hong Kong from UK colony to
                     70:    China).
1.2       perry      71:
1.18      christos   72:  * Be portable to a wide variety of implementations.
1.9       mlelstv    73:
1.18      christos   74:  * Use a consistent naming conventions over the entire world.
1.9       mlelstv    75:
1.18      christos   76: Names normally have the form AREA/LOCATION, where AREA is the name
                     77: of a continent or ocean, and LOCATION is the name of a specific
                     78: location within that region.  North and South America share the same
                     79: area, 'America'.  Typical names are 'Africa/Cairo', 'America/New_York',
                     80: and 'Pacific/Honolulu'.
1.9       mlelstv    81:
1.18      christos   82: Here are the general rules used for choosing location names,
                     83: in decreasing order of importance:
1.9       mlelstv    84:
1.18      christos   85:        Use only valid POSIX file name components (i.e., the parts of
                     86:                names other than '/').  Do not use the file name
                     87:                components '.' and '..'.  Within a file name component,
                     88:                use only ASCII letters, '.', '-' and '_'.  Do not use
                     89:                digits, as that might create an ambiguity with POSIX
                     90:                TZ strings.  A file name component must not exceed 14
                     91:                characters or start with '-'.  E.g., prefer 'Brunei'
                     92:                to 'Bandar_Seri_Begawan'.  Exceptions: see the discussion
                     93:                of legacy names below.
                     94:        A name must not be empty, or contain '//', or start or end with '/'.
                     95:        Do not use names that differ only in case.  Although the reference
                     96:                implementation is case-sensitive, some other implementations
                     97:                are not, and they would mishandle names differing only in case.
                     98:        If one name A is an initial prefix of another name AB (ignoring case),
                     99:                then B must not start with '/', as a regular file cannot have
                    100:                the same name as a directory in POSIX.  For example,
                    101:                'America/New_York' precludes 'America/New_York/Bronx'.
                    102:        Uninhabited regions like the North Pole and Bouvet Island
                    103:                do not need locations, since local time is not defined there.
                    104:        There should typically be at least one name for each ISO 3166-1
                    105:                officially assigned two-letter code for an inhabited country
                    106:                or territory.
                    107:        If all the clocks in a region have agreed since 1970,
                    108:                don't bother to include more than one location
                    109:                even if subregions' clocks disagreed before 1970.
                    110:                Otherwise these tables would become annoyingly large.
                    111:        If a name is ambiguous, use a less ambiguous alternative;
                    112:                e.g. many cities are named San José and Georgetown, so
                    113:                prefer 'Costa_Rica' to 'San_Jose' and 'Guyana' to 'Georgetown'.
                    114:        Keep locations compact.  Use cities or small islands, not countries
                    115:                or regions, so that any future time zone changes do not split
                    116:                locations into different time zones.  E.g. prefer 'Paris'
                    117:                to 'France', since France has had multiple time zones.
                    118:        Use mainstream English spelling, e.g. prefer 'Rome' to 'Roma', and
                    119:                prefer 'Athens' to the Greek 'Αθήνα' or the Romanized 'Athína'.
                    120:                The POSIX file name restrictions encourage this rule.
                    121:        Use the most populous among locations in a zone,
                    122:                e.g. prefer 'Shanghai' to 'Beijing'.  Among locations with
                    123:                similar populations, pick the best-known location,
                    124:                e.g. prefer 'Rome' to 'Milan'.
                    125:        Use the singular form, e.g. prefer 'Canary' to 'Canaries'.
                    126:        Omit common suffixes like '_Islands' and '_City', unless that
                    127:                would lead to ambiguity.  E.g. prefer 'Cayman' to
                    128:                'Cayman_Islands' and 'Guatemala' to 'Guatemala_City',
                    129:                but prefer 'Mexico_City' to 'Mexico' because the country
                    130:                of Mexico has several time zones.
                    131:        Use '_' to represent a space.
                    132:        Omit '.' from abbreviations in names, e.g. prefer 'St_Helena'
                    133:                to 'St._Helena'.
                    134:        Do not change established names if they only marginally
                    135:                violate the above rules.  For example, don't change
                    136:                the existing name 'Rome' to 'Milan' merely because
                    137:                Milan's population has grown to be somewhat greater
                    138:                than Rome's.
                    139:        If a name is changed, put its old spelling in the 'backward' file.
                    140:                This means old spellings will continue to work.
1.1       jtc       141:
1.18      christos  142: The file 'zone1970.tab' lists geographical locations used to name time
                    143: zone rules.  It is intended to be an exhaustive list of names for
                    144: geographic regions as described above; this is a subset of the names
                    145: in the data.  Although a 'zone1970.tab' location's longitude
                    146: corresponds to its LMT offset with one hour for every 15 degrees east
                    147: longitude, this relationship is not exact.
1.1       jtc       148:
1.18      christos  149: Older versions of this package used a different naming scheme,
                    150: and these older names are still supported.
                    151: See the file 'backward' for most of these older names
                    152: (e.g., 'US/Eastern' instead of 'America/New_York').
                    153: The other old-fashioned names still supported are
                    154: 'WET', 'CET', 'MET', and 'EET' (see the file 'europe').
1.1       jtc       155:
1.18      christos  156: Older versions of this package defined legacy names that are
                    157: incompatible with the first rule of location names, but which are
                    158: still supported.  These legacy names are mostly defined in the file
                    159: 'etcetera'.  Also, the file 'backward' defines the legacy names
                    160: 'GMT0', 'GMT-0', 'GMT+0' and 'Canada/East-Saskatchewan', and the file
                    161: 'northamerica' defines the legacy names 'EST5EDT', 'CST6CDT',
                    162: 'MST7MDT', and 'PST8PDT'.
1.14      christos  163:
1.18      christos  164: Excluding 'backward' should not affect the other data.  If
                    165: 'backward' is excluded, excluding 'etcetera' should not affect the
                    166: remaining data.
1.1       jtc       167:
                    168:
1.18      christos  169: ----- Time zone abbreviations -----
1.1       jtc       170:
1.18      christos  171: When this package is installed, it generates time zone abbreviations
                    172: like 'EST' to be compatible with human tradition and POSIX.
                    173: Here are the general rules used for choosing time zone abbreviations,
                    174: in decreasing order of importance:
1.1       jtc       175:
1.19      christos  176:        Use three or more characters that are ASCII alphanumerics or '+' or '-'.
1.18      christos  177:                Previous editions of this database also used characters like
                    178:                ' ' and '?', but these characters have a special meaning to
                    179:                the shell and cause commands like
                    180:                        set `date`
                    181:                to have unexpected effects.
                    182:                Previous editions of this rule required upper-case letters,
                    183:                but the Congressman who introduced Chamorro Standard Time
1.19      christos  184:                preferred "ChST", so lower-case letters are now allowed.
                    185:                Also, POSIX from 2001 on relaxed the rule to allow '-', '+',
                    186:                and alphanumeric characters from the portable character set
                    187:                in the current locale.  In practice ASCII alphanumerics and
                    188:                '+' and '-' are safe in all locales.
1.1       jtc       189:
1.19      christos  190:                In other words, in the C locale the POSIX extended regular
                    191:                expression [-+[:alnum:]]{3,} should match the abbreviation.
                    192:                This guarantees that all abbreviations could have been
                    193:                specified by a POSIX TZ string.
1.1       jtc       194:
1.18      christos  195:        Use abbreviations that are in common use among English-speakers,
                    196:                e.g. 'EST' for Eastern Standard Time in North America.
                    197:                We assume that applications translate them to other languages
                    198:                as part of the normal localization process; for example,
                    199:                a French application might translate 'EST' to 'HNE'.
1.1       jtc       200:
1.18      christos  201:        For zones whose times are taken from a city's longitude, use the
                    202:                traditional xMT notation, e.g. 'PMT' for Paris Mean Time.
                    203:                The only name like this in current use is 'GMT'.
1.14      christos  204:
1.18      christos  205:        Use 'LMT' for local mean time of locations before the introduction
                    206:                of standard time; see "Scope of the tz database".
1.1       jtc       207:
1.18      christos  208:        If there is no common English abbreviation, use numeric offsets like
                    209:                -05 and +0830 that are generated by zic's %z notation.
1.2       perry     210:
1.18      christos  211:     [The remaining guidelines predate the introduction of %z.
                    212:     They are problematic as they mean tz data entries invent
                    213:     notation rather than record it.  These guidelines are now
                    214:     deprecated and the plan is to gradually move to %z for
                    215:     inhabited locations and to "-00" for uninhabited locations.]
1.2       perry     216:
1.18      christos  217:        If there is no common English abbreviation, abbreviate the English
                    218:                translation of the usual phrase used by native speakers.
                    219:                If this is not available or is a phrase mentioning the country
                    220:                (e.g. "Cape Verde Time"), then:
1.2       perry     221:
1.18      christos  222:                When a country is identified with a single or principal zone,
                    223:                        append 'T' to the country's ISO code, e.g. 'CVT' for
                    224:                        Cape Verde Time.  For summer time append 'ST';
                    225:                        for double summer time append 'DST'; etc.
                    226:                Otherwise, take the first three letters of an English place
                    227:                        name identifying each zone and append 'T', 'ST', etc.
                    228:                        as before; e.g. 'VLAST' for VLAdivostok Summer Time.
1.1       jtc       229:
1.20    ! christos  230:        Use UT (with time zone abbreviation '-00') for locations while
        !           231:                uninhabited.  The leading '-' is a flag that the time
        !           232:                zone is in some sense undefined; this notation is
        !           233:                derived from Internet RFC 3339.
1.2       perry     234:
1.18      christos  235: Application writers should note that these abbreviations are ambiguous
                    236: in practice: e.g. 'CST' has a different meaning in China than
                    237: it does in the United States.  In new applications, it's often better
                    238: to use numeric UT offsets like '-0600' instead of time zone
                    239: abbreviations like 'CST'; this avoids the ambiguity.
1.10      christos  240:
1.14      christos  241:
                    242: ----- Accuracy of the tz database -----
                    243:
                    244: The tz database is not authoritative, and it surely has errors.
1.16      christos  245: Corrections are welcome and encouraged; see the file CONTRIBUTING.
                    246: Users requiring authoritative data should consult national standards
                    247: bodies and the references cited in the database's comments.
1.10      christos  248:
1.14      christos  249: Errors in the tz database arise from many sources:
                    250:
                    251:  * The tz database predicts future time stamps, and current predictions
                    252:    will be incorrect after future governments change the rules.
                    253:    For example, if today someone schedules a meeting for 13:00 next
                    254:    October 1, Casablanca time, and tomorrow Morocco changes its
                    255:    daylight saving rules, software can mess up after the rule change
                    256:    if it blithely relies on conversions made before the change.
                    257:
1.16      christos  258:  * The pre-1970 entries in this database cover only a tiny sliver of how
1.14      christos  259:    clocks actually behaved; the vast majority of the necessary
                    260:    information was lost or never recorded.  Thousands more zones would
                    261:    be needed if the tz database's scope were extended to cover even
                    262:    just the known or guessed history of standard time; for example,
                    263:    the current single entry for France would need to split into dozens
1.19      christos  264:    of entries, perhaps hundreds.  And in most of the world even this
                    265:    approach would be misleading due to widespread disagreement or
                    266:    indifference about what times should be observed.  In her 2015 book
                    267:    "The Global Transformation of Time, 1870-1950", Vanessa Ogle writes
                    268:    "Outside of Europe and North America there was no system of time
                    269:    zones at all, often not even a stable landscape of mean times,
                    270:    prior to the middle decades of the twentieth century".  See:
                    271:    Timothy Shenk, Booked: A Global History of Time. Dissent 2015-12-17
                    272:    https://www.dissentmagazine.org/blog/booked-a-global-history-of-time-vanessa-ogle
1.14      christos  273:
1.16      christos  274:  * Most of the pre-1970 data entries come from unreliable sources, often
1.14      christos  275:    astrology books that lack citations and whose compilers evidently
                    276:    invented entries when the true facts were unknown, without
                    277:    reporting which entries were known and which were invented.
                    278:    These books often contradict each other or give implausible entries,
1.16      christos  279:    and on the rare occasions when they are checked they are
1.14      christos  280:    typically found to be incorrect.
                    281:
                    282:  * For the UK the tz database relies on years of first-class work done by
                    283:    Joseph Myers and others; see <http://www.polyomino.org.uk/british-time/>.
                    284:    Other countries are not done nearly as well.
                    285:
                    286:  * Sometimes, different people in the same city would maintain clocks
                    287:    that differed significantly.  Railway time was used by railroad
                    288:    companies (which did not always agree with each other),
                    289:    church-clock time was used for birth certificates, etc.
                    290:    Often this was merely common practice, but sometimes it was set by law.
                    291:    For example, from 1891 to 1911 the UT offset in France was legally
                    292:    0:09:21 outside train stations and 0:04:21 inside.
                    293:
                    294:  * Although a named location in the tz database stands for the
                    295:    containing region, its pre-1970 data entries are often accurate for
                    296:    only a small subset of that region.  For example, Europe/London
                    297:    stands for the United Kingdom, but its pre-1847 times are valid
                    298:    only for locations that have London's exact meridian, and its 1847
                    299:    transition to GMT is known to be valid only for the L&NW and the
                    300:    Caledonian railways.
                    301:
1.16      christos  302:  * The tz database does not record the earliest time for which a zone's
                    303:    data entries are thereafter valid for every location in the region.
1.14      christos  304:    For example, Europe/London is valid for all locations in its
                    305:    region after GMT was made the standard time, but the date of
                    306:    standardization (1880-08-02) is not in the tz database, other than
                    307:    in commentary.  For many zones the earliest time of validity is
                    308:    unknown.
                    309:
                    310:  * The tz database does not record a region's boundaries, and in many
                    311:    cases the boundaries are not known.  For example, the zone
                    312:    America/Kentucky/Louisville represents a region around the city of
                    313:    Louisville, the boundaries of which are unclear.
                    314:
                    315:  * Changes that are modeled as instantaneous transitions in the tz
                    316:    database were often spread out over hours, days, or even decades.
                    317:
                    318:  * Even if the time is specified by law, locations sometimes
                    319:    deliberately flout the law.
                    320:
                    321:  * Early timekeeping practices, even assuming perfect clocks, were
                    322:    often not specified to the accuracy that the tz database requires.
                    323:
                    324:  * Sometimes historical timekeeping was specified more precisely
                    325:    than what the tz database can handle.  For example, from 1909 to
                    326:    1937 Netherlands clocks were legally UT+00:19:32.13, but the tz
                    327:    database cannot represent the fractional second.
                    328:
                    329:  * Even when all the timestamp transitions recorded by the tz database
                    330:    are correct, the tz rules that generate them may not faithfully
                    331:    reflect the historical rules.  For example, from 1922 until World
                    332:    War II the UK moved clocks forward the day following the third
                    333:    Saturday in April unless that was Easter, in which case it moved
                    334:    clocks forward the previous Sunday.  Because the tz database has no
                    335:    way to specify Easter, these exceptional years are entered as
                    336:    separate tz Rule lines, even though the legal rules did not change.
                    337:
1.16      christos  338:  * The tz database models pre-standard time using the proleptic Gregorian
1.14      christos  339:    calendar and local mean time (LMT), but many people used other
                    340:    calendars and other timescales.  For example, the Roman Empire used
                    341:    the Julian calendar, and had 12 varying-length daytime hours with a
                    342:    non-hour-based system at night.
                    343:
1.16      christos  344:  * Early clocks were less reliable, and data entries do not represent
                    345:    this unreliability.
1.14      christos  346:
                    347:  * As for leap seconds, civil time was not based on atomic time before
                    348:    1972, and we don't know the history of earth's rotation accurately
                    349:    enough to map SI seconds to historical solar time to more than
                    350:    about one-hour accuracy.  See: Morrison LV, Stephenson FR.
                    351:    Historical values of the Earth's clock error Delta T and the
                    352:    calculation of eclipses. J Hist Astron. 2004;35:327-36
                    353:    <http://adsabs.harvard.edu/full/2004JHA....35..327M>;
                    354:    Historical values of the Earth's clock error. J Hist Astron. 2005;36:339
                    355:    <http://adsabs.harvard.edu/full/2005JHA....36..339M>.
                    356:
                    357:  * The relationship between POSIX time (that is, UTC but ignoring leap
                    358:    seconds) and UTC is not agreed upon after 1972.  Although the POSIX
                    359:    clock officially stops during an inserted leap second, at least one
                    360:    proposed standard has it jumping back a second instead; and in
                    361:    practice POSIX clocks more typically either progress glacially during
                    362:    a leap second, or are slightly slowed while near a leap second.
                    363:
                    364:  * The tz database does not represent how uncertain its information is.
1.16      christos  365:    Ideally it would contain information about when data entries are
1.14      christos  366:    incomplete or dicey.  Partial temporal knowledge is a field of
                    367:    active research, though, and it's not clear how to apply it here.
                    368:
                    369: In short, many, perhaps most, of the tz database's pre-1970 and future
                    370: time stamps are either wrong or misleading.  Any attempt to pass the
                    371: tz database off as the definition of time should be unacceptable to
                    372: anybody who cares about the facts.  In particular, the tz database's
                    373: LMT offsets should not be considered meaningful, and should not prompt
                    374: creation of zones merely because two locations differ in LMT or
                    375: transitioned to standard time at different dates.
                    376:
1.10      christos  377:
1.18      christos  378: ----- Time and date functions -----
                    379:
                    380: The tz code contains time and date functions that are upwards
                    381: compatible with those of POSIX.
                    382:
                    383: POSIX has the following properties and limitations.
                    384:
                    385: *      In POSIX, time display in a process is controlled by the
                    386:        environment variable TZ.  Unfortunately, the POSIX TZ string takes
                    387:        a form that is hard to describe and is error-prone in practice.
                    388:        Also, POSIX TZ strings can't deal with other (for example, Israeli)
                    389:        daylight saving time rules, or situations where more than two
                    390:        time zone abbreviations are used in an area.
                    391:
                    392:        The POSIX TZ string takes the following form:
                    393:
                    394:                stdoffset[dst[offset][,date[/time],date[/time]]]
                    395:
                    396:        where:
                    397:
                    398:        std and dst
                    399:                are 3 or more characters specifying the standard
                    400:                and daylight saving time (DST) zone names.
                    401:                Starting with POSIX.1-2001, std and dst may also be
                    402:                in a quoted form like "<UTC+10>"; this allows
                    403:                "+" and "-" in the names.
                    404:        offset
                    405:                is of the form '[+-]hh:[mm[:ss]]' and specifies the
                    406:                offset west of UT.  'hh' may be a single digit; 0<=hh<=24.
                    407:                The default DST offset is one hour ahead of standard time.
                    408:        date[/time],date[/time]
                    409:                specifies the beginning and end of DST.  If this is absent,
                    410:                the system supplies its own rules for DST, and these can
                    411:                differ from year to year; typically US DST rules are used.
                    412:        time
                    413:                takes the form 'hh:[mm[:ss]]' and defaults to 02:00.
                    414:                This is the same format as the offset, except that a
                    415:                leading '+' or '-' is not allowed.
                    416:        date
                    417:                takes one of the following forms:
                    418:                Jn (1<=n<=365)
                    419:                        origin-1 day number not counting February 29
                    420:                n (0<=n<=365)
                    421:                        origin-0 day number counting February 29 if present
                    422:                Mm.n.d (0[Sunday]<=d<=6[Saturday], 1<=n<=5, 1<=m<=12)
                    423:                        for the dth day of week n of month m of the year,
                    424:                        where week 1 is the first week in which day d appears,
                    425:                        and '5' stands for the last week in which day d appears
                    426:                        (which may be either the 4th or 5th week).
                    427:                        Typically, this is the only useful form;
                    428:                        the n and Jn forms are rarely used.
                    429:
                    430:        Here is an example POSIX TZ string, for US Pacific time using rules
                    431:        appropriate from 1987 through 2006:
1.2       perry     432:
1.18      christos  433:                TZ='PST8PDT,M4.1.0/02:00,M10.5.0/02:00'
1.6       kleink    434:
1.18      christos  435:        This POSIX TZ string is hard to remember, and mishandles time stamps
                    436:        before 1987 and after 2006.  With this package you can use this
                    437:        instead:
1.6       kleink    438:
1.18      christos  439:                TZ='America/Los_Angeles'
1.6       kleink    440:
1.18      christos  441: *      POSIX does not define the exact meaning of TZ values like "EST5EDT".
                    442:        Typically the current US DST rules are used to interpret such values,
                    443:        but this means that the US DST rules are compiled into each program
                    444:        that does time conversion.  This means that when US time conversion
                    445:        rules change (as in the United States in 1987), all programs that
                    446:        do time conversion must be recompiled to ensure proper results.
1.6       kleink    447:
1.18      christos  448: *      In POSIX, there's no tamper-proof way for a process to learn the
                    449:        system's best idea of local wall clock.  (This is important for
                    450:        applications that an administrator wants used only at certain times -
                    451:        without regard to whether the user has fiddled the "TZ" environment
                    452:        variable.  While an administrator can "do everything in UTC" to get
                    453:        around the problem, doing so is inconvenient and precludes handling
                    454:        daylight saving time shifts - as might be required to limit phone
                    455:        calls to off-peak hours.)
1.2       perry     456:
1.18      christos  457: *      POSIX requires that systems ignore leap seconds.
1.2       perry     458:
1.18      christos  459: *      The tz code attempts to support all the time_t implementations
                    460:        allowed by POSIX.  The time_t type represents a nonnegative count of
                    461:        seconds since 1970-01-01 00:00:00 UTC, ignoring leap seconds.
                    462:        In practice, time_t is usually a signed 64- or 32-bit integer; 32-bit
                    463:        signed time_t values stop working after 2038-01-19 03:14:07 UTC, so
                    464:        new implementations these days typically use a signed 64-bit integer.
                    465:        Unsigned 32-bit integers are used on one or two platforms,
                    466:        and 36-bit and 40-bit integers are also used occasionally.
                    467:        Although earlier POSIX versions allowed time_t to be a
                    468:        floating-point type, this was not supported by any practical
                    469:        systems, and POSIX.1-2013 and the tz code both require time_t
                    470:        to be an integer type.
1.2       perry     471:
1.18      christos  472: These are the extensions that have been made to the POSIX functions:
1.2       perry     473:
1.18      christos  474: *      The "TZ" environment variable is used in generating the name of a file
                    475:        from which time zone information is read (or is interpreted a la
                    476:        POSIX); "TZ" is no longer constrained to be a three-letter time zone
                    477:        name followed by a number of hours and an optional three-letter
                    478:        daylight time zone name.  The daylight saving time rules to be used
                    479:        for a particular time zone are encoded in the time zone file;
                    480:        the format of the file allows U.S., Australian, and other rules to be
                    481:        encoded, and allows for situations where more than two time zone
                    482:        abbreviations are used.
1.2       perry     483:
1.18      christos  484:        It was recognized that allowing the "TZ" environment variable to
                    485:        take on values such as "America/New_York" might cause "old" programs
                    486:        (that expect "TZ" to have a certain form) to operate incorrectly;
                    487:        consideration was given to using some other environment variable
                    488:        (for example, "TIMEZONE") to hold the string used to generate the
                    489:        time zone information file name.  In the end, however, it was decided
                    490:        to continue using "TZ": it is widely used for time zone purposes;
                    491:        separately maintaining both "TZ" and "TIMEZONE" seemed a nuisance;
                    492:        and systems where "new" forms of "TZ" might cause problems can simply
                    493:        use TZ values such as "EST5EDT" which can be used both by
                    494:        "new" programs (a la POSIX) and "old" programs (as zone names and
                    495:        offsets).
1.2       perry     496:
1.18      christos  497: *      To handle places where more than two time zone abbreviations are used,
                    498:        the functions "localtime" and "gmtime" set tzname[tmp->tm_isdst]
                    499:        (where "tmp" is the value the function returns) to the time zone
                    500:        abbreviation to be used.  This differs from POSIX, where the elements
                    501:        of tzname are only changed as a result of calls to tzset.
1.15      christos  502:
1.18      christos  503: *      Since the "TZ" environment variable can now be used to control time
                    504:        conversion, the "daylight" and "timezone" variables are no longer
                    505:        needed.  (These variables are defined and set by "tzset"; however, their
                    506:        values will not be used by "localtime.")
1.15      christos  507:
1.18      christos  508: *      The "localtime" function has been set up to deliver correct results
                    509:        for near-minimum or near-maximum time_t values.  (A comment in the
                    510:        source code tells how to get compatibly wrong results).
1.2       perry     511:
1.18      christos  512: *      A function "tzsetwall" has been added to arrange for the system's
                    513:        best approximation to local wall clock time to be delivered by
                    514:        subsequent calls to "localtime."  Source code for portable
                    515:        applications that "must" run on local wall clock time should call
                    516:        "tzsetwall();" if such code is moved to "old" systems that don't
                    517:        provide tzsetwall, you won't be able to generate an executable program.
                    518:        (These time zone functions also arrange for local wall clock time to be
                    519:        used if tzset is called - directly or indirectly - and there's no "TZ"
                    520:        environment variable; portable applications should not, however, rely
                    521:        on this behavior since it's not the way SVR2 systems behave.)
1.2       perry     522:
1.18      christos  523: *      Negative time_t values are supported, on systems where time_t is signed.
1.2       perry     524:
1.18      christos  525: *      These functions can account for leap seconds, thanks to Bradley White.
1.6       kleink    526:
1.18      christos  527: Points of interest to folks with other systems:
1.6       kleink    528:
1.18      christos  529: *      This package is already part of many POSIX-compliant hosts,
                    530:        including BSD, HP, Linux, Network Appliance, SCO, SGI, and Sun.
                    531:        On such hosts, the primary use of this package
                    532:        is to update obsolete time zone rule tables.
                    533:        To do this, you may need to compile the time zone compiler
                    534:        'zic' supplied with this package instead of using the system 'zic',
                    535:        since the format of zic's input changed slightly in late 1994,
                    536:        and many vendors still do not support the new input format.
1.6       kleink    537:
1.18      christos  538: *      The UNIX Version 7 "timezone" function is not present in this package;
                    539:        it's impossible to reliably map timezone's arguments (a "minutes west
                    540:        of GMT" value and a "daylight saving time in effect" flag) to a
                    541:        time zone abbreviation, and we refuse to guess.
                    542:        Programs that in the past used the timezone function may now examine
                    543:        tzname[localtime(&clock)->tm_isdst] to learn the correct time
                    544:        zone abbreviation to use.  Alternatively, use
                    545:        localtime(&clock)->tm_zone if this has been enabled.
1.6       kleink    546:
1.18      christos  547: *      The 4.2BSD gettimeofday function is not used in this package.
                    548:        This formerly let users obtain the current UTC offset and DST flag,
                    549:        but this functionality was removed in later versions of BSD.
1.2       perry     550:
1.18      christos  551: *      In SVR2, time conversion fails for near-minimum or near-maximum
                    552:        time_t values when doing conversions for places that don't use UT.
                    553:        This package takes care to do these conversions correctly.
1.2       perry     554:
1.18      christos  555: The functions that are conditionally compiled if STD_INSPIRED is defined
                    556: should, at this point, be looked on primarily as food for thought.  They are
                    557: not in any sense "standard compatible" - some are not, in fact, specified in
                    558: *any* standard.  They do, however, represent responses of various authors to
                    559: standardization proposals.
1.14      christos  560:
1.18      christos  561: Other time conversion proposals, in particular the one developed by folks at
                    562: Hewlett Packard, offer a wider selection of functions that provide capabilities
                    563: beyond those provided here.  The absence of such functions from this package
                    564: is not meant to discourage the development, standardization, or use of such
                    565: functions.  Rather, their absence reflects the decision to make this package
                    566: contain valid extensions to POSIX, to ensure its broad acceptability.  If
                    567: more powerful time conversion functions can be standardized, so much the
                    568: better.
1.4       kleink    569:
                    570:
                    571: ----- Calendrical issues -----
                    572:
                    573: Calendrical issues are a bit out of scope for a time zone database,
                    574: but they indicate the sort of problems that we would run into if we
                    575: extended the time zone database further into the past.  An excellent
1.10      christos  576: resource in this area is Nachum Dershowitz and Edward M. Reingold,
1.15      christos  577: Calendrical Calculations: Third Edition, Cambridge University Press (2008)
                    578: <http://emr.cs.iit.edu/home/reingold/calendar-book/third-edition/>.
                    579: Other information and sources are given below.  They sometimes disagree.
1.4       kleink    580:
                    581:
                    582: France
                    583:
                    584: Gregorian calendar adopted 1582-12-20.
                    585: French Revolutionary calendar used 1793-11-24 through 1805-12-31,
                    586: and (in Paris only) 1871-05-06 through 1871-05-23.
                    587:
                    588:
                    589: Russia
                    590:
1.9       mlelstv   591: From Chris Carrier (1996-12-02):
1.14      christos  592: On 1929-10-01 the Soviet Union instituted an "Eternal Calendar"
1.4       kleink    593: with 30-day months plus 5 holidays, with a 5-day week.
                    594: On 1931-12-01 it changed to a 6-day week; in 1934 it reverted to the
                    595: Gregorian calendar while retaining the 6-day week; on 1940-06-27 it
                    596: reverted to the 7-day week.  With the 6-day week the usual days
                    597: off were the 6th, 12th, 18th, 24th and 30th of the month.
                    598: (Source: Evitiar Zerubavel, _The Seven Day Circle_)
                    599:
                    600:
                    601: Mark Brader reported a similar story in "The Book of Calendars", edited
                    602: by Frank Parise (1982, Facts on File, ISBN 0-8719-6467-8), page 377.  But:
                    603:
                    604: From: Petteri Sulonen (via Usenet)
                    605: Date: 14 Jan 1999 00:00:00 GMT
1.9       mlelstv   606: ...
1.4       kleink    607:
1.15      christos  608: If your source is correct, how come documents between 1929 and 1940 were
1.4       kleink    609: still dated using the conventional, Gregorian calendar?
                    610:
                    611: I can post a scan of a document dated December 1, 1934, signed by
                    612: Yenukidze, the secretary, on behalf of Kalinin, the President of the
                    613: Executive Committee of the Supreme Soviet, if you like.
                    614:
                    615:
                    616:
                    617: Sweden (and Finland)
                    618:
1.9       mlelstv   619: From: Mark Brader
1.15      christos  620: Subject: Re: Gregorian reform - a part of locale?
                    621: <news:1996Jul6.012937.29190@sq.com>
1.4       kleink    622: Date: 1996-07-06
                    623:
                    624: In 1700, Denmark made the transition from Julian to Gregorian.  Sweden
                    625: decided to *start* a transition in 1700 as well, but rather than have one of
                    626: those unsightly calendar gaps :-), they simply decreed that the next leap
1.15      christos  627: year after 1696 would be in 1744 - putting the whole country on a calendar
1.4       kleink    628: different from both Julian and Gregorian for a period of 40 years.
                    629:
                    630: However, in 1704 something went wrong and the plan was not carried through;
                    631: they did, after all, have a leap year that year.  And one in 1708.  In 1712
                    632: they gave it up and went back to Julian, putting 30 days in February that
                    633: year!...
                    634:
                    635: Then in 1753, Sweden made the transition to Gregorian in the usual manner,
                    636: getting there only 13 years behind the original schedule.
                    637:
                    638: (A previous posting of this story was challenged, and Swedish readers
1.15      christos  639: produced the following references to support it: "Tideräkning och historia"
                    640: by Natanael Beckman (1924) and "Tid, en bok om tideräkning och
                    641: kalenderväsen" by Lars-Olof Lodén (1968).
1.4       kleink    642:
                    643:
                    644: Grotefend's data
                    645:
1.9       mlelstv   646: From: "Michael Palmer" [with one obvious typo fixed]
1.4       kleink    647: Subject: Re: Gregorian Calendar (was Re: Another FHC related question
                    648: Newsgroups: soc.genealogy.german
                    649: Date: Tue, 9 Feb 1999 02:32:48 -800
1.9       mlelstv   650: ...
1.4       kleink    651:
1.6       kleink    652: The following is a(n incomplete) listing, arranged chronologically, of
                    653: European states, with the date they converted from the Julian to the
1.4       kleink    654: Gregorian calendar:
                    655:
                    656: 04/15 Oct 1582 - Italy (with exceptions), Spain, Portugal, Poland (Roman
                    657:                  Catholics and Danzig only)
                    658: 09/20 Dec 1582 - France, Lorraine
                    659:
                    660: 21 Dec 1582/
                    661:    01 Jan 1583 - Holland, Brabant, Flanders, Hennegau
1.15      christos  662: 10/21 Feb 1583 - bishopric of Liege (Lüttich)
1.4       kleink    663: 13/24 Feb 1583 - bishopric of Augsburg
                    664: 04/15 Oct 1583 - electorate of Trier
                    665: 05/16 Oct 1583 - Bavaria, bishoprics of Freising, Eichstedt, Regensburg,
                    666:                  Salzburg, Brixen
1.15      christos  667: 13/24 Oct 1583 - Austrian Oberelsaß and Breisgau
1.4       kleink    668: 20/31 Oct 1583 - bishopric of Basel
1.15      christos  669: 02/13 Nov 1583 - duchy of Jülich-Berg
                    670: 02/13 Nov 1583 - electorate and city of Köln
                    671: 04/15 Nov 1583 - bishopric of Würzburg
1.4       kleink    672: 11/22 Nov 1583 - electorate of Mainz
                    673: 16/27 Nov 1583 - bishopric of Strassburg and the margraviate of Baden
1.15      christos  674: 17/28 Nov 1583 - bishopric of Münster and duchy of Cleve
1.4       kleink    675: 14/25 Dec 1583 - Steiermark
                    676:
                    677: 06/17 Jan 1584 - Austria and Bohemia
1.15      christos  678: 11/22 Jan 1584 - Lucerne, Uri, Schwyz, Zug, Freiburg, Solothurn
1.4       kleink    679: 12/23 Jan 1584 - Silesia and the Lausitz
                    680: 22 Jan/
                    681:    02 Feb 1584 - Hungary (legally on 21 Oct 1587)
                    682:       Jun 1584 - Unterwalden
                    683: 01/12 Jul 1584 - duchy of Westfalen
                    684:
                    685: 16/27 Jun 1585 - bishopric of Paderborn
                    686:
                    687: 14/25 Dec 1590 - Transylvania
                    688:
                    689: 22 Aug/
                    690:    02 Sep 1612 - duchy of Prussia
                    691:
                    692: 13/24 Dec 1614 - Pfalz-Neuburg
                    693:
                    694:           1617 - duchy of Kurland (reverted to the Julian calendar in
                    695:                  1796)
                    696:
1.15      christos  697:           1624 - bishopric of Osnabrück
1.4       kleink    698:
                    699:           1630 - bishopric of Minden
                    700:
                    701: 15/26 Mar 1631 - bishopric of Hildesheim
                    702:
                    703:           1655 - Kanton Wallis
                    704:
                    705: 05/16 Feb 1682 - city of Strassburg
                    706:
                    707: 18 Feb/
                    708:    01 Mar 1700 - Protestant Germany (including Swedish possessions in
                    709:                  Germany), Denmark, Norway
                    710: 30 Jun/
                    711:    12 Jul 1700 - Gelderland, Zutphen
                    712: 10 Nov/
                    713:    12 Dec 1700 - Utrecht, Overijssel
                    714:
                    715: 31 Dec 1700/
1.15      christos  716:    12 Jan 1701 - Friesland, Groningen, Zürich, Bern, Basel, Geneva,
1.4       kleink    717:                  Turgau, and Schaffhausen
                    718:
                    719:           1724 - Glarus, Appenzell, and the city of St. Gallen
                    720:
                    721: 01 Jan 1750    - Pisa and Florence
                    722:
                    723: 02/14 Sep 1752 - Great Britain
                    724:
                    725: 17 Feb/
                    726:    01 Mar 1753 - Sweden
                    727:
1.15      christos  728: 1760-1812      - Graubünden
1.4       kleink    729:
1.6       kleink    730: The Russian empire (including Finland and the Baltic states) did not
1.4       kleink    731: convert to the Gregorian calendar until the Soviet revolution of 1917.
                    732:
1.16      christos  733: Source: H. Grotefend, _Taschenbuch der Zeitrechnung des deutschen
1.6       kleink    734: Mittelalters und der Neuzeit_, herausgegeben von Dr. O. Grotefend
1.16      christos  735: (Hannover: Hahnsche Buchhandlung, 1941), pp. 26-28.
1.8       kleink    736:
                    737:
                    738: ----- Time and time zones on Mars -----
                    739:
1.17      christos  740: Some people's work schedules use Mars time.  Jet Propulsion Laboratory
                    741: (JPL) coordinators have kept Mars time on and off at least since 1997
                    742: for the Mars Pathfinder mission.  Some of their family members have
                    743: also adapted to Mars time.  Dozens of special Mars watches were built
                    744: for JPL workers who kept Mars time during the Mars Exploration
1.8       kleink    745: Rovers mission (2004).  These timepieces look like normal Seikos and
                    746: Citizens but use Mars seconds rather than terrestrial seconds.
                    747:
                    748: A Mars solar day is called a "sol" and has a mean period equal to
                    749: about 24 hours 39 minutes 35.244 seconds in terrestrial time.  It is
                    750: divided into a conventional 24-hour clock, so each Mars second equals
                    751: about 1.02749125 terrestrial seconds.
                    752:
                    753: The prime meridian of Mars goes through the center of the crater
                    754: Airy-0, named in honor of the British astronomer who built the
                    755: Greenwich telescope that defines Earth's prime meridian.  Mean solar
                    756: time on the Mars prime meridian is called Mars Coordinated Time (MTC).
                    757:
                    758: Each landed mission on Mars has adopted a different reference for
                    759: solar time keeping, so there is no real standard for Mars time zones.
                    760: For example, the Mars Exploration Rover project (2004) defined two
                    761: time zones "Local Solar Time A" and "Local Solar Time B" for its two
                    762: missions, each zone designed so that its time equals local true solar
                    763: time at approximately the middle of the nominal mission.  Such a "time
                    764: zone" is not particularly suited for any application other than the
                    765: mission itself.
                    766:
                    767: Many calendars have been proposed for Mars, but none have achieved
                    768: wide acceptance.  Astronomers often use Mars Sol Date (MSD) which is a
                    769: sequential count of Mars solar days elapsed since about 1873-12-29
                    770: 12:00 GMT.
                    771:
                    772: The tz database does not currently support Mars time, but it is
                    773: documented here in the hopes that support will be added eventually.
                    774:
                    775: Sources:
                    776:
                    777: Michael Allison and Robert Schmunk,
                    778: "Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock"
1.13      christos  779: <http://www.giss.nasa.gov/tools/mars24/help/notes.html> (2012-08-08).
1.8       kleink    780:
                    781: Jia-Rui Chong, "Workdays Fit for a Martian", Los Angeles Times
1.13      christos  782: <http://articles.latimes.com/2004/jan/14/science/sci-marstime14>
1.8       kleink    783: (2004-01-14), pp A1, A20-A21.
1.15      christos  784:
1.17      christos  785: Tom Chmielewski, "Jet Lag Is Worse on Mars", The Atlantic (2015-02-26)
                    786: <http://www.theatlantic.com/technology/archive/2015/02/jet-lag-is-worse-on-mars/386033/>
1.15      christos  787:
                    788: -----
1.18      christos  789:
                    790: This file is in the public domain, so clarified as of 2009-05-17 by
                    791: Arthur David Olson.
                    792:
                    793: -----
1.15      christos  794: Local Variables:
                    795: coding: utf-8
                    796: End:

CVSweb <webmaster@jp.NetBSD.org>