Date/Time Support

PostgreSQL User's Guide
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# Appendix UG1. Date/Time Support

# Time Zones

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.

Table UG1-1. Postgres Recognized 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

## Australian Time Zones

Australian time zones and their naming variants account for fully one quarter of all time zones in the Postgres time zone lookup table. There are two naming conflicts with common time zones defined in the United States, `CST` and `EST`.

If the compiler option USE_AUSTRALIAN_RULES is set then `CST` and `EST` will be interpreted using Australian conventions.

Table UG1-2. Postgres Australian Time Zones

Time Zone | Offset from UTC | Description
--- | --- | ---
CST | +10:30 | Australian Central Standard Time
EST | +10:00 | Australian Eastern Standard Time

## Date/Time Input Interpretation

The date/time types are all decoded using a common set of routines.

Date/Time Input Interpretation

1. Break the input string into tokens and categorize each token as a string, time, time zone, or number.

   1. If the token contains a colon (":"), this is a time string.

   2. If the token contains a dash ("-"), slash ("/"), or dot ("."), this is a date string which may have a text month.

   3. 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).

   4. If the token starts with a plus ("+") or minus ("-"), then it is either a time zone or a special field.

2. If the token is a text string, match up with possible strings.

   1. 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`.

   2. If not found, do a similar binary-search table lookup to match the token with a time zone.

   3. If not found, throw an error.

3. The token is a number or number field.

   1. 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.

   2. If the token is three digits and a year has already been decoded, then interpret as day of year.

   3. If four or more digits, then interpret as a year.

   4. 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.

   5. 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.

   6. 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.

   7. If two digits or four or more digits, then interpret as a year.

   8. Otherwise, throw an error.

4. 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).

5. 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.

   > Tip: Gregorian years 1-99AD may be entered by using 4 digits with leading zeros (e.g. 0099 is 99AD). Previous versions of Postgres accepted years with three digits and with single digits, but as of v7.0 the rules have been tightened up to reduce the possibility of ambiguity.

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