Postgres must have 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. 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 Std Time NZT +12:00 New Zealand Time AESST +11:00 Australia Eastern Summer Std Time ACSST +10:30 Central Australia Summer Std Time CADT +10:30 Central Australia Daylight Savings Time SADT +10:30 South Australian Daylight Time AEST +10:00 Australia Eastern Std Time EAST +10:00 East Australian Std Time GST +10:00 Guam Std Time, USSR Zone 9 LIGT +10:00 Melbourne, Australia ACST +09:30 Central Australia Std Time CAST +09:30 Central Australia Std Time SAT +9:30 South Australian Std Time AWSST +9:00 Australia Western Summer Std Time JST +9:00 Japan Std Time,USSR Zone 8 KST +9:00 Korea Standard Time WDT +9:00 West Australian Daylight Time MT +8:30 Moluccas Time AWST +8:00 Australia Western Std Time CCT +8:00 China Coastal Time WADT +8:00 West Australian Daylight Time WST +8:00 West Australian Std Time JT +7:30 Java Time WAST +7:00 West Australian Std Time IT +3:30 Iran Time BT +3:00 Baghdad Time EETDST +3:00 Eastern Europe Daylight Savings Time CETDST +2:00 Central European Daylight Savings Time EET +2:00 Eastern Europe, USSR Zone 1 FWT +2:00 French Winter Time IST +2:00 Israel Std Time MEST +2:00 Middle Europe Summer Time METDST +2:00 Middle Europe Daylight Time SST +2:00 Swedish Summer Time BST +1:00 British Summer Time CET +1:00 Central European Time DNT +1:00 Dansk Normal Tid DST +1:00 Dansk Standard Time (?) FST +1:00 French Summer Time MET +1:00 Middle Europe Time MEWT +1:00 Middle Europe Winter Time MEZ +1:00 Middle Europe Zone NOR +1:00 Norway Standard Time SET +1:00 Seychelles Time SWT +1:00 Swedish Winter Time WETDST +1:00 Western Europe Daylight Savings Time GMT 0:00 Greenwish Mean Time WET 0:00 Western Europe WAT -1:00 West Africa Time NDT -2:30 Newfoundland Daylight Time ADT -03:00 Atlantic Daylight Time NFT -3:30 Newfoundland Standard Time NST -3:30 Newfoundland Standard Time AST -4:00 Atlantic Std Time (Canada) EDT -4:00 Eastern Daylight Time ZP4 -4:00 GMT +4 hours CDT -5:00 Central Daylight Time EST -5:00 Eastern Standard Time ZP5 -5:00 GMT +5 hours CST -6:00 Central Std Time MDT -6:00 Mountain Daylight Time ZP6 -6:00 GMT +6 hours MST -7:00 Mountain Standard Time PDT -7:00 Pacific Daylight Time PST -8:00 Pacific Std Time YDT -8:00 Yukon Daylight Time HDT -9:00 Hawaii/Alaska Daylight Time YST -9:00 Yukon Standard Time AHST -10:00 Alaska-Hawaii Std Time CAT -10:00 Central Alaska Time NT -11:00 Nome Time IDLW -12:00 International Date Line, West If the compiler option USE_AUSTRALIAN_RULES is set then EST refers to Australia Eastern Std Time, which has an offset of +10:00 hours from UTC. Australian time zones and their naming variants account for fully one quarter of all time zones in the Postgres time zone lookup table. 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 token contains a colon (":"), this is a time string. If the token contains a dash ("-"), slash ("/"), or dot ("."), 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. 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 longer than two 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. Otherwise, interpret as a year. If BC has been specified, negate the year and offset by 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). Three digits are also accepted as a year under most circumstances, though depending on position the numeric string may be interpreted as doy instead. Contributed by José Soares. 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 1 day in approximately 128 days. 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.