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 may 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. The date/time type inputs are all 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 may have a text month. 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 time zone or a special field. If the token is a text string, match up with possible strings. Do a binary-search table lookup for 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). Set field values and bit mask for fields. For example, set year, month, day for today, and additionally hour, minute, second for now. If not found, do a similar binary-search table lookup to match the token with a time zone. 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 may be entered by using 4 digits with leading zeros (e.g., 0099 is AD 99). Previous versions of PostgreSQL accepted years with three digits and with single digits, but as of version 7.0 the rules have been tightened up to reduce the possibility of ambiguity. shows the tokens that are permissible as abbreviations for the names of the month. Month Abbreviations April Apr August Aug December Dec February Feb January Jan July Jul June Jun March Mar November Nov October Oct September Sep, Sept The month May has no explicit abbreviation, for obvious reasons. shows the tokens that are permissible as abbreviations for the names of the days of the week. Day Abbreviation 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. Identifier Description ABSTIME Key word ignored AM Time is before 12:00 AT Key word ignored JULIAN, JD, J Next field is Julian Day ON Key word ignored PM Time is on or after 12:00 T Next field is time The key word ABSTIME is ignored for historical reasons: In very old releases of PostgreSQL, invalid values of type abstime were emitted as Invalid Abstime. This is no longer the case however and this key word will likely be dropped in a future release. time zone abbreviations shows the time zone abbreviations recognized by PostgreSQL in date/time input values. PostgreSQL uses internal tables for time zone input decoding, since there is no standard operating system interface to provide access to general, cross-time zone information. The underlying operating system is used to provide time zone information for output, however. Keep in mind also that the time zone names recognized by SET TIMEZONE are operating-system dependent and may have little to do with . For example, some systems recognize values like 'Europe/Rome' in SET TIMEZONE. The table is organized by time zone offset from UTC, rather than alphabetically. This is intended to facilitate matching local usage with recognized abbreviations for cases where these might differ. Time Zone Offset from UTC Description NZDT +13:00 New Zealand Daylight-Saving Time IDLE +12:00 International Date Line, East NZST +12:00 New Zealand Standard Time NZT +12:00 New Zealand Time AESST +11:00 Australia Eastern Summer Standard Time ACSST +10:30 Central Australia Summer Standard Time CADT +10:30 Central Australia Daylight-Saving Time SADT +10:30 South Australian Daylight-Saving Time AEST +10:00 Australia Eastern Standard Time EAST +10:00 East Australian Standard Time GST +10:00 Guam Standard Time, Russia zone 9 LIGT +10:00 Melbourne, Australia SAST +09:30 South Australia Standard Time CAST +09:30 Central Australia Standard Time AWSST +09:00 Australia Western Summer Standard Time JST +09:00 Japan Standard Time, Russia zone 8 KST +09:00 Korea Standard Time MHT +09:00 Kwajalein Time WDT +09:00 West Australian Daylight-Saving Time MT +08:30 Moluccas Time AWST +08:00 Australia Western Standard Time CCT +08:00 China Coastal Time WADT +08:00 West Australian Daylight-Saving Time WST +08:00 West Australian Standard Time JT +07:30 Java Time ALMST +07:00 Almaty Summer Time WAST +07:00 West Australian Standard Time CXT +07:00 Christmas (Island) Time MMT +06:30 Myanmar Time ALMT +06:00 Almaty Time MAWT +06:00 Mawson (Antarctica) Time IOT +05:00 Indian Chagos Time MVT +05:00 Maldives Island Time TFT +05:00 Kerguelen Time AFT +04:30 Afghanistan Time EAST +04:00 Antananarivo Summer Time MUT +04:00 Mauritius Island Time RET +04:00 Reunion Island Time SCT +04:00 Mahe Island Time IRT, IT +03:30 Iran Time EAT +03:00 Antananarivo, Comoro Time BT +03:00 Baghdad Time EETDST +03:00 Eastern Europe Daylight-Saving Time HMT +03:00 Hellas Mediterranean Time (?) BDST +02:00 British Double Standard Time CEST +02:00 Central European Summer Time CETDST +02:00 Central European Daylight-Saving Time EET +02:00 Eastern European Time, Russia zone 1 FWT +02:00 French Winter Time IST +02:00 Israel Standard Time MEST +02:00 Middle European Summer Time METDST +02:00 Middle Europe Daylight-Saving Time SST +02:00 Swedish Summer Time BST +01:00 British Summer Time CET +01:00 Central European Time DNT +01:00 Dansk Normal Tid FST +01:00 French Summer Time MET +01:00 Middle European Time MEWT +01:00 Middle European Winter Time MEZ +01:00 Mitteleuropäische Zeit NOR +01:00 Norway Standard Time SET +01:00 Seychelles Time SWT +01:00 Swedish Winter Time WETDST +01:00 Western European Daylight-Saving Time GMT 00:00 Greenwich Mean Time UT 00:00 Universal Time UTC 00:00 Universal Coordinated Time Z 00:00 Same as UTC ZULU 00:00 Same as UTC WET 00:00 Western European Time WAT -01:00 West Africa Time FNST -01:00 Fernando de Noronha Summer Time FNT -02:00 Fernando de Noronha Time BRST -02:00 Brasilia Summer Time NDT -02:30 Newfoundland Daylight-Saving Time ADT -03:00 Atlantic Daylight-Saving Time AWT -03:00 (unknown) BRT -03:00 Brasilia Time NFT -03:30 Newfoundland Standard Time NST -03:30 Newfoundland Standard Time AST -04:00 Atlantic Standard Time (Canada) ACST -04:00 Atlantic/Porto Acre Summer Time EDT -04:00 Eastern Daylight-Saving Time ACT -05:00 Atlantic/Porto Acre Standard Time CDT -05:00 Central Daylight-Saving Time EST -05:00 Eastern Standard Time CST -06:00 Central Standard Time MDT -06:00 Mountain Daylight-Saving Time MST -07:00 Mountain Standard Time PDT -07:00 Pacific Daylight-Saving Time AKDT -08:00 Alaska Daylight-Saving Time PST -08:00 Pacific Standard Time YDT -08:00 Yukon Daylight-Saving Time AKST -09:00 Alaska Standard Time HDT -09:00 Hawaii/Alaska Daylight-Saving Time YST -09:00 Yukon Standard Time MART -09:30 Marquesas Time AHST -10:00 Alaska/Hawaii Standard Time HST -10:00 Hawaii Standard Time CAT -10:00 Central Alaska Time NT -11:00 Nome Time IDLW -12:00 International Date Line, West There are three naming conflicts between Australian time zone names and time zone names commonly used in North and South America: ACST, CST, and EST. If the run-time option australian_timezones is set to true then ACST, CST, EST, and SAT are interpreted as Australian time zone names, as shown in . If it is false (which is the default), then ACST, CST, and EST are taken as American time zone names, and SAT is interpreted as a noise word indicating Saturday. Time Zone Offset from UTC Description ACST +09:30 Central Australia Standard Time CST +10:30 Australian Central Standard Time EST +10:00 Australian Eastern Standard Time SAT +09:30 South Australian Standard Time The Julian Date 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). Astronomers have used the Julian period to assign a unique number to every day since 1 January 4713 BC. This is the so-called Julian Date (JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC to noon UTC on 2 January 4713 BC. The Julian Date is different from the Julian Calendar. The Julian calendar was introduced by Julius Caesar in 45 BC. It was in common use 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 Dominions (including what is now the USA) in 1752. Thus 2 September 1752 was followed by 14 September 1752. This is why Unix systems 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 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. Dates between 1752-09-03 and 1752-09-13, although eliminated in some countries by Papal fiat, conform to natural rules and are hence valid dates. 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 the 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.