From 46651ce6fe013220ed397add242004d764fc0153 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 4 May 2024 14:15:05 +0200 Subject: Adding upstream version 14.5. Signed-off-by: Daniel Baumann --- doc/src/sgml/datetime.sgml | 930 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 930 insertions(+) create mode 100644 doc/src/sgml/datetime.sgml (limited to 'doc/src/sgml/datetime.sgml') diff --git a/doc/src/sgml/datetime.sgml b/doc/src/sgml/datetime.sgml new file mode 100644 index 0000000..adaf72d --- /dev/null +++ b/doc/src/sgml/datetime.sgml @@ -0,0 +1,930 @@ + + + + Date/Time Support + + + PostgreSQL uses an internal heuristic + parser for all date/time input support. Dates and times are input as + strings, and are broken up into distinct fields with a preliminary + determination of what kind of information can be in the + field. Each field is interpreted and either assigned a numeric + value, ignored, or rejected. + The parser contains internal lookup tables for all textual fields, + including months, days of the week, and time zones. + + + + This appendix includes information on the content of these + lookup tables and describes the steps used by the parser to decode + dates and times. + + + + Date/Time Input Interpretation + + + Date/time input strings are decoded using the following procedure. + + + + + + Break the input string into tokens and categorize each token as + a string, time, time zone, or number. + + + + + + If the numeric token contains a colon (:), this is + a time string. Include all subsequent digits and colons. + + + + + + If the numeric token contains a dash (-), slash + (/), or two or more dots (.), this is + a date string which might have a text month. If a date token has + already been seen, it is instead interpreted as a time zone + name (e.g., America/New_York). + + + + + + If the token is numeric only, then it is either a single field + or an ISO 8601 concatenated date (e.g., + 19990113 for January 13, 1999) or time + (e.g., 141516 for 14:15:16). + + + + + + If the token starts with a plus (+) or minus + (-), then it is either a numeric time zone or a special + field. + + + + + + + + If the token is an alphabetic string, match up with possible strings: + + + + + + See if the token matches any known time zone abbreviation. + These abbreviations are supplied by the configuration file + described in . + + + + + + If not found, search an internal table to match + the token as either a special string (e.g., today), + day (e.g., Thursday), + month (e.g., January), + or noise word (e.g., at, on). + + + + + + If still not found, throw an error. + + + + + + + + When the token is a number or number field: + + + + + + If there are eight or six digits, + and if no other date fields have been previously read, then interpret + as a concatenated date (e.g., + 19990118 or 990118). + The interpretation is YYYYMMDD or YYMMDD. + + + + + + If the token is three digits + and a year has already been read, then interpret as day of year. + + + + + + If four or six digits and a year has already been read, then + interpret as a time (HHMM or HHMMSS). + + + + + + If three or more digits and no date fields have yet been found, + interpret as a year (this forces yy-mm-dd ordering of the remaining + date fields). + + + + + + Otherwise the date field ordering is assumed to follow the + DateStyle setting: mm-dd-yy, dd-mm-yy, or yy-mm-dd. + Throw an error if a month or day field is found to be out of range. + + + + + + + + If BC has been specified, negate the year and add one for + internal storage. (There is no year zero in the Gregorian + calendar, so numerically 1 BC becomes year zero.) + + + + + + If BC was not specified, and if the year field was two digits in length, + then adjust the year to four digits. If the field is less than 70, then + add 2000, otherwise add 1900. + + + + Gregorian years AD 1–99 can be entered by using 4 digits with leading + zeros (e.g., 0099 is AD 99). + + + + + + + + + + Handling of Invalid or Ambiguous Timestamps + + + Ordinarily, if a date/time string is syntactically valid but contains + out-of-range field values, an error will be thrown. For example, input + specifying the 31st of February will be rejected. + + + + During a daylight-savings-time transition, it is possible for a + seemingly valid timestamp string to represent a nonexistent or ambiguous + timestamp. Such cases are not rejected; the ambiguity is resolved by + determining which UTC offset to apply. For example, supposing that the + parameter is set + to America/New_York, consider + +=> SELECT '2018-03-11 02:30'::timestamptz; + timestamptz +------------------------ + 2018-03-11 03:30:00-04 +(1 row) + + Because that day was a spring-forward transition date in that time zone, + there was no civil time instant 2:30AM; clocks jumped forward from 2AM + EST to 3AM EDT. PostgreSQL interprets the + given time as if it were standard time (UTC-5), which then renders as + 3:30AM EDT (UTC-4). + + + + Conversely, consider the behavior during a fall-back transition: + +=> SELECT '2018-11-04 01:30'::timestamptz; + timestamptz +------------------------ + 2018-11-04 01:30:00-05 +(1 row) + + On that date, there were two possible interpretations of 1:30AM; there + was 1:30AM EDT, and then an hour later after clocks jumped back from + 2AM EDT to 1AM EST, there was 1:30AM EST. + Again, PostgreSQL interprets the given time + as if it were standard time (UTC-5). We can force the other + interpretation by specifying daylight-savings time: + +=> SELECT '2018-11-04 01:30 EDT'::timestamptz; + timestamptz +------------------------ + 2018-11-04 01:30:00-04 +(1 row) + + + + + The precise rule that is applied in such cases is that an invalid + timestamp that appears to fall within a jump-forward daylight savings + transition is assigned the UTC offset that prevailed in the time zone + just before the transition, while an ambiguous timestamp that could fall + on either side of a jump-back transition is assigned the UTC offset that + prevailed just after the transition. In most time zones this is + equivalent to saying that the standard-time interpretation is + preferred when in doubt. + + + + In all cases, the UTC offset associated with a timestamp can be + specified explicitly, using either a numeric UTC offset or a time zone + abbreviation that corresponds to a fixed UTC offset. The rule just + given applies only when it is necessary to infer a UTC offset for a time + zone in which the offset varies. + + + + + + Date/Time Key Words + + + shows the tokens that are + recognized as names of months. + + + + Month Names + + + + Month + Abbreviations + + + + + January + Jan + + + February + Feb + + + March + Mar + + + April + Apr + + + May + + + + June + Jun + + + July + Jul + + + August + Aug + + + September + Sep, Sept + + + October + Oct + + + November + Nov + + + December + Dec + + + +
+ + + shows the tokens that are + recognized as names of days of the week. + + + + Day of the Week Names + + + + Day + Abbreviations + + + + + Sunday + Sun + + + Monday + Mon + + + Tuesday + Tue, Tues + + + Wednesday + Wed, Weds + + + Thursday + Thu, Thur, Thurs + + + Friday + Fri + + + Saturday + Sat + + + +
+ + + shows the tokens that serve + various modifier purposes. + + + + Date/Time Field Modifiers + + + + Identifier + Description + + + + + AM + Time is before 12:00 + + + AT + Ignored + + + JULIAN, JD, J + Next field is Julian Date + + + ON + Ignored + + + PM + Time is on or after 12:00 + + + T + Next field is time + + + +
+ + + + Date/Time Configuration Files + + + time zone + input abbreviations + + + + Since timezone abbreviations are not well standardized, + PostgreSQL provides a means to customize + the set of abbreviations accepted by the server. The + run-time parameter + determines the active set of abbreviations. While this parameter + can be altered by any database user, the possible values for it + are under the control of the database administrator — they + are in fact names of configuration files stored in + .../share/timezonesets/ of the installation directory. + By adding or altering files in that directory, the administrator + can set local policy for timezone abbreviations. + + + + timezone_abbreviations can be set to any file name + found in .../share/timezonesets/, if the file's name + is entirely alphabetic. (The prohibition against non-alphabetic + characters in timezone_abbreviations prevents reading + files outside the intended directory, as well as reading editor + backup files and other extraneous files.) + + + + A timezone abbreviation file can contain blank lines and comments + beginning with #. Non-comment lines must have one of + these formats: + + +zone_abbreviation offset +zone_abbreviation offset D +zone_abbreviation time_zone_name +@INCLUDE file_name +@OVERRIDE + + + + + A zone_abbreviation is just the abbreviation + being defined. An offset is an integer giving + the equivalent offset in seconds from UTC, positive being east from + Greenwich and negative being west. For example, -18000 would be five + hours west of Greenwich, or North American east coast standard time. + D indicates that the zone name represents local + daylight-savings time rather than standard time. + + + + Alternatively, a time_zone_name can be given, referencing + a zone name defined in the IANA timezone database. The zone's definition + is consulted to see whether the abbreviation is or has been in use in + that zone, and if so, the appropriate meaning is used — that is, + the meaning that was currently in use at the timestamp whose value is + being determined, or the meaning in use immediately before that if it + wasn't current at that time, or the oldest meaning if it was used only + after that time. This behavior is essential for dealing with + abbreviations whose meaning has historically varied. It is also allowed + to define an abbreviation in terms of a zone name in which that + abbreviation does not appear; then using the abbreviation is just + equivalent to writing out the zone name. + + + + + Using a simple integer offset is preferred + when defining an abbreviation whose offset from UTC has never changed, + as such abbreviations are much cheaper to process than those that + require consulting a time zone definition. + + + + + The @INCLUDE syntax allows inclusion of another file in the + .../share/timezonesets/ directory. Inclusion can be nested, + to a limited depth. + + + + The @OVERRIDE syntax indicates that subsequent entries in the + file can override previous entries (typically, entries obtained from + included files). Without this, conflicting definitions of the same + timezone abbreviation are considered an error. + + + + In an unmodified installation, the file Default contains + all the non-conflicting time zone abbreviations for most of the world. + Additional files Australia and India are + provided for those regions: these files first include the + Default file and then add or modify abbreviations as needed. + + + + For reference purposes, a standard installation also contains files + Africa.txt, America.txt, etc, containing + information about every time zone abbreviation known to be in use + according to the IANA timezone database. The zone name + definitions found in these files can be copied and pasted into a custom + configuration file as needed. Note that these files cannot be directly + referenced as timezone_abbreviations settings, because of + the dot embedded in their names. + + + + + If an error occurs while reading the time zone abbreviation set, no new + value is applied and the old set is kept. If the error occurs while + starting the database, startup fails. + + + + + + Time zone abbreviations defined in the configuration file override + non-timezone meanings built into PostgreSQL. + For example, the Australia configuration file defines + SAT (for South Australian Standard Time). When this + file is active, SAT will not be recognized as an abbreviation + for Saturday. + + + + + + If you modify files in .../share/timezonesets/, + it is up to you to make backups — a normal database dump + will not include this directory. + + + + + + + <acronym>POSIX</acronym> Time Zone Specifications + + + time zone + POSIX-style specification + + + + PostgreSQL can accept time zone specifications + that are written according to the POSIX standard's rules + for the TZ environment + variable. POSIX time zone specifications are + inadequate to deal with the complexity of real-world time zone history, + but there are sometimes reasons to use them. + + + + A POSIX time zone specification has the form + +STD offset DST dstoffset , rule + + (For readability, we show spaces between the fields, but spaces should + not be used in practice.) The fields are: + + + + STD is the zone abbreviation to be used + for standard time. + + + + + offset is the zone's standard-time offset + from UTC. + + + + + DST is the zone abbreviation to be used + for daylight-savings time. If this field and the following ones are + omitted, the zone uses a fixed UTC offset with no daylight-savings + rule. + + + + + dstoffset is the daylight-savings offset + from UTC. This field is typically omitted, since it defaults to one + hour less than the standard-time offset, + which is usually the right thing. + + + + + rule defines the rule for when daylight + savings is in effect, as described below. + + + + + + + In this syntax, a zone abbreviation can be a string of letters, such + as EST, or an arbitrary string surrounded by angle + brackets, such as <UTC-05>. + Note that the zone abbreviations given here are only used for output, + and even then only in some timestamp output formats. The zone + abbreviations recognized in timestamp input are determined as explained + in . + + + + The offset fields specify the hours, and optionally minutes and seconds, + difference from UTC. They have the format + hh:mm:ss + optionally with a leading sign (+ + or -). The positive sign is used for + zones west of Greenwich. (Note that this is the + opposite of the ISO-8601 sign convention used elsewhere in + PostgreSQL.) hh + can have one or two digits; mm + and ss (if used) must have two. + + + + The daylight-savings transition rule has the + format + +dstdate / dsttime , stddate / stdtime + + (As before, spaces should not be included in practice.) + The dstdate + and dsttime fields define when daylight-savings + time starts, while stddate + and stdtime define when standard time + starts. (In some cases, notably in zones south of the equator, the + former might be later in the year than the latter.) The date fields + have one of these formats: + + + n + + + A plain integer denotes a day of the year, counting from zero to + 364, or to 365 in leap years. + + + + + Jn + + + In this form, n counts from 1 to 365, + and February 29 is not counted even if it is present. (Thus, a + transition occurring on February 29 could not be specified this + way. However, days after February have the same numbers whether + it's a leap year or not, so that this form is usually more useful + than the plain-integer form for transitions on fixed dates.) + + + + + Mm.n.d + + + This form specifies a transition that always happens during the same + month and on the same day of the week. m + identifies the month, from 1 to 12. n + specifies the n'th occurrence of the + weekday identified by d. + n is a number between 1 and 4, or 5 + meaning the last occurrence of that weekday in the month (which + could be the fourth or the fifth). d is + a number between 0 and 6, with 0 indicating Sunday. + For example, M3.2.0 means the second + Sunday in March. + + + + + + + + + The M format is sufficient to describe many common + daylight-savings transition laws. But note that none of these variants + can deal with daylight-savings law changes, so in practice the + historical data stored for named time zones (in the IANA time zone + database) is necessary to interpret past time stamps correctly. + + + + + The time fields in a transition rule have the same format as the offset + fields described previously, except that they cannot contain signs. + They define the current local time at which the change to the other + time occurs. If omitted, they default to 02:00:00. + + + + If a daylight-savings abbreviation is given but the + transition rule field is omitted, + the fallback behavior is to use the + rule M3.2.0,M11.1.0, which corresponds to USA + practice as of 2020 (that is, spring forward on the second Sunday of + March, fall back on the first Sunday of November, both transitions + occurring at 2AM prevailing time). Note that this rule does not + give correct USA transition dates for years before 2007. + + + + As an example, CET-1CEST,M3.5.0,M10.5.0/3 describes + current (as of 2020) timekeeping practice in Paris. This specification + says that standard time has the abbreviation CET and + is one hour ahead (east) of UTC; daylight savings time has the + abbreviation CEST and is implicitly two hours ahead + of UTC; daylight savings time begins on the last Sunday in March at 2AM + CET and ends on the last Sunday in October at 3AM CEST. + + + + The four timezone names EST5EDT, + CST6CDT, MST7MDT, + and PST8PDT look like they are POSIX zone + specifications. However, they actually are treated as named time zones + because (for historical reasons) there are files by those names in the + IANA time zone database. The practical implication of this is that + these zone names will produce valid historical USA daylight-savings + transitions, even when a plain POSIX specification would not. + + + + One should be wary that it is easy to misspell a POSIX-style time zone + specification, since there is no check on the reasonableness of the + zone abbreviation(s). For example, SET TIMEZONE TO + FOOBAR0 will work, leaving the system effectively using a + rather peculiar abbreviation for UTC. + + + + + + History of Units + + + Gregorian calendar + + + + The SQL standard states that Within the definition of a + datetime literal, the datetime + values are constrained by the natural rules for dates and + times according to the Gregorian calendar. + PostgreSQL follows the SQL + standard's lead by counting dates exclusively in the Gregorian + calendar, even for years before that calendar was in use. + This rule is known as the proleptic Gregorian calendar. + + + + The Julian calendar was introduced by Julius Caesar in 45 BC. + It was in common use in the Western world + until the year 1582, when countries started changing to the Gregorian + calendar. In the Julian calendar, the tropical year is + approximated as 365 1/4 days = 365.25 days. This gives an error of + about 1 day in 128 years. + + + + The accumulating calendar error prompted + Pope Gregory XIII to reform the calendar in accordance with + instructions from the Council of Trent. + In the Gregorian calendar, the tropical year is approximated as + 365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300 + years for the tropical year to shift one day with respect to the + Gregorian calendar. + + + + The approximation 365+97/400 is achieved by having 97 leap years + every 400 years, using the following rules: + + + + Every year divisible by 4 is a leap year. + + + However, every year divisible by 100 is not a leap year. + + + However, every year divisible by 400 is a leap year after all. + + + + So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, + 2000, and 2400 are leap years. + + By contrast, in the older Julian calendar all years divisible by 4 are leap + years. + + + + The papal bull of February 1582 decreed that 10 days should be dropped + from October 1582 so that 15 October should follow immediately after + 4 October. + This was observed in Italy, Poland, Portugal, and Spain. Other Catholic + countries followed shortly after, but Protestant countries were + reluctant to change, and the Greek Orthodox countries didn't change + until the start of the 20th century. + + The reform was observed by Great Britain and its dominions (including what + is now the USA) in 1752. + Thus 2 September 1752 was followed by 14 September 1752. + + This is why Unix systems that have the cal program + produce the following: + + +$ cal 9 1752 + September 1752 + S M Tu W Th F S + 1 2 14 15 16 +17 18 19 20 21 22 23 +24 25 26 27 28 29 30 + + + But, of course, this calendar is only valid for Great Britain and + dominions, not other places. + Since it would be difficult and confusing to try to track the actual + calendars that were in use in various places at various times, + PostgreSQL does not try, but rather follows the Gregorian + calendar rules for all dates, even though this method is not historically + accurate. + + + + Different calendars have been developed in various parts of the + world, many predating the Gregorian system. + + For example, + the beginnings of the Chinese calendar can be traced back to the 14th + century BC. Legend has it that the Emperor Huangdi invented that + calendar in 2637 BC. + + The People's Republic of China uses the Gregorian calendar + for civil purposes. The Chinese calendar is used for determining + festivals. + + + + + + Julian Dates + + + Julian date + + + + The Julian Date system is a method for + numbering days. It is + unrelated to the Julian calendar, though it is confusingly + named similarly to that calendar. + The Julian Date system was invented by the French scholar + Joseph Justus Scaliger (1540–1609) + and probably takes its name from Scaliger's father, + the Italian scholar Julius Caesar Scaliger (1484–1558). + + + + In the Julian Date system, each day has a sequential number, starting + from JD 0 (which is sometimes called the Julian Date). + JD 0 corresponds to 1 January 4713 BC in the Julian calendar, or + 24 November 4714 BC in the Gregorian calendar. Julian Date counting + is most often used by astronomers for labeling their nightly observations, + and therefore a date runs from noon UTC to the next noon UTC, rather than + from midnight to midnight: JD 0 designates the 24 hours from noon UTC on + 24 November 4714 BC to noon UTC on 25 November 4714 BC. + + + + Although PostgreSQL supports Julian Date notation for + input and output of dates (and also uses Julian dates for some internal + datetime calculations), it does not observe the nicety of having dates + run from noon to noon. PostgreSQL treats a Julian Date + as running from local midnight to local midnight, the same as a normal + date. + + + + This definition does, however, provide a way to obtain the astronomical + definition when you need it: do the arithmetic in time + zone UTC+12. For example, + +=> SELECT extract(julian from '2021-06-23 7:00:00-04'::timestamptz at time zone 'UTC+12'); + extract +------------------------------ + 2459388.95833333333333333333 +(1 row) +=> SELECT extract(julian from '2021-06-23 8:00:00-04'::timestamptz at time zone 'UTC+12'); + extract +-------------------------------------- + 2459389.0000000000000000000000000000 +(1 row) +=> SELECT extract(julian from date '2021-06-23'); + extract +--------- + 2459389 +(1 row) + + + + + -- cgit v1.2.3