PostgreSQL uses an internal heuristic parser for all date/time 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. 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. Day Abbreviation Sunday Sun Monday Mon Tuesday Tue, Tues Wednesday Wed, Weds Thursday Thu, Thur, Thurs Friday Fri Saturday Sat Field Modifiers Identifier Description ABSTIME Keyword ignored AM Time is before 12:00 AT Keyword ignored JULIAN, JD, J Next field is Julian Day ON Keyword ignored PM Time is on or after after 12:00 T Next field is time The keyword ABSTIME is ignored for historical reasons; in very old releases of PostgreSQL invalid ABSTIME fields were emitted as Invalid Abstime. This is no longer the case however and this keyword will likely be dropped in a future release. time zones PostgreSQL contains internal tabular information for time zone decoding, since there is no *nix standard system interface to provide access to general, cross-timezone information. The underlying OS is used to provide time zone information for output, however. The following table of time zones recognized by PostgreSQL is organized by time zone offset from UTC, rather than alphabetically; this is intended to faciliate matching local usage with recognized abreviations for cases where these might differ. Time Zones Time Zone Offset from UTC Description NZDT +13:00 New Zealand Daylight 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 Savings Time SADT +10:30 South Australian Daylight Time AEST +10:00 Australia Eastern Standard Time EAST +10:00 East Australian Standard Time GST +10:00 Guam Standard Time, USSR 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,USSR Zone 8 KST +09:00 Korea Standard Time MHT +09:00 Kwajalein Time WDT +09:00 West Australian Daylight 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 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 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 Afganistan Time EAST +04:00 Antananarivo Savings Time MUT +04:00 Mauritius Island Time RET +04:00 Reunion Island Time SCT +04:00 Mahe Island Time IT +03:30 Iran Time EAT +03:00 Antananarivo, Comoro Time BT +03:00 Baghdad Time EETDST +03:00 Eastern Europe Daylight Savings Time HMT +03:00 Hellas Mediterranean Time (?) BDST +02:00 British Double Standard Time CEST +02:00 Central European Savings Time CETDST +02:00 Central European Daylight Savings Time EET +02:00 Eastern Europe, USSR Zone 1 FWT +02:00 French Winter Time IST +02:00 Israel Standard Time MEST +02:00 Middle Europe Summer Time METDST +02:00 Middle Europe Daylight 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 Europe Time MEWT +01:00 Middle Europe Winter Time MEZ +01:00 Middle Europe Zone NOR +01:00 Norway Standard Time SET +01:00 Seychelles Time SWT +01:00 Swedish Winter Time WETDST +01:00 Western Europe Daylight Savings Time GMT +00:00 Greenwich Mean Time UT +00:00 Universal Time UTC +00:00 Universal Time, Coordinated Z +00:00 Same as UTC ZULU +00:00 Same as UTC WET +00:00 Western Europe WAT -01:00 West Africa Time NDT -02:30 Newfoundland Daylight Time ADT -03:00 Atlantic Daylight Time AWT -03:00 (unknown) 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 ACT -05:00 Atlantic/Porto Acre Standard Time EDT -04:00 Eastern Daylight Time CDT -05:00 Central Daylight Time EST -05:00 Eastern Standard Time CST -06:00 Central Standard Time MDT -06:00 Mountain Daylight Time MST -07:00 Mountain Standard Time PDT -07:00 Pacific Daylight Time AKDT -08:00 Alaska Daylight Time PST -08:00 Pacific Standard Time YDT -08:00 Yukon Daylight Time AKST -09:00 Alaska Standard Time HDT -09:00 Hawaii/Alaska Daylight Time YST -09:00 Yukon Standard 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 Australian time zones and their naming variants account for fully one quarter of all time zones in the PostgreSQL time zone lookup table. There are two naming conflicts with time zones commonly used in the United States, CST and EST. If the runtime option AUSTRALIAN_TIMEZONES is set then CST, EST, and SAT will be interpreted as Australian timezone names. Without this option, CST and EST are taken as American timezone names, while SAT is interpreted as a noise word indicating Saturday. Australian Time Zones 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 date/time types are all decoded using a common set of routines. 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 not found, throw an error. The token is a number or number field. If there are more than 4 digits, and if no other date fields have been previously read, then interpret as a concatenated date (e.g. 19990118). 8 and 6 digits are interpreted as year, month, and day, while 7 and 5 digits are interpreted as year, day of year, respectively. If the token is three digits and a year has already been decoded, then interpret as day of year. If four or six digits and a year has already been read, then interpret as a time. If four or more digits, then interpret as a year. If in European date mode, and if the day field has not yet been read, and if the value is less than or equal to 31, then interpret as a day. If the month field has not yet been read, and if the value is less than or equal to 12, then interpret as a month. If the day field has not yet been read, and if the value is less than or equal to 31, then interpret as a day. If two digits or four or more digits, then interpret as a year. Otherwise, throw an error. 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 1BC becomes year zero). If BC was not specified, and if the year field was two digits in length, then adjust the year to 4 digits. If the field was less than 70, then add 2000; otherwise, add 1900. Gregorian years 1-99AD may be entered by using 4 digits with leading zeros (e.g. 0099 is 99AD). 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. Contributed by José Soares (jose@sferacarta.com) The Julian Day was invented by the French scholar Joseph Justus Scaliger (1540-1609) and probably takes its name from the 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 Day (JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC to noon UTC on 2 January 4713 BC. Julian Day is different from Julian Date. The Julian calendar was introduced by Julius Caesar in 45 BC. It was in common use until the 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 only 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 this century. The reform was observed by Great Britain and Dominions (including what is now the USA) in 1752. Thus 2 Sep 1752 was followed by 14 Sep 1752. This is why Unix systems have cal 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 SQL92 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. Chinese calendar is used for determining festivals.