PostgreSQL supports the full set of SQL date and time types, shown in Table 5-9.
Table 5-9. Date/Time Types
|timestamp [ (p) ] [ without time zone ]||both date and time||8 bytes||4713 BC||AD 1465001||1 microsecond / 14 digits|
|timestamp [ (p) ] with time zone||both date and time||8 bytes||4713 BC||AD 1465001||1 microsecond / 14 digits|
|interval [ (p) ]||time intervals||12 bytes||-178000000 years||178000000 years||1 microsecond|
|date||dates only||4 bytes||4713 BC||32767 AD||1 day|
|time [ (p) ] [ without time zone ]||times of day only||8 bytes||00:00:00.00||23:59:59.99||1 microsecond|
|time [ (p) ] with time zone||times of day only||12 bytes||00:00:00.00+12||23:59:59.99-12||1 microsecond|
time, timestamp, and interval accept an optional precision value p which specifies the number of fractional digits retained in the seconds field. By default, there is no explicit bound on precision. The allowed range of p is from 0 to 6 for the timestamp and interval types, 0 to 13 for the time types.
Note: When timestamp values are stored as double precision floating-point numbers (currently the default), the effective limit of precision may be less than 6, since timestamp values are stored as seconds since 2000-01-01. Microsecond precision is achieved for dates within a few years of 2000-01-01, but the precision degrades for dates further away. When timestamps are stored as eight-byte integers (a compile-time option), microsecond precision is available over the full range of values.
Time zones, and time-zone conventions, are influenced by political decisions, not just earth geometry. Time zones around the world became somewhat standardized during the 1900's, but continue to be prone to arbitrary changes. PostgreSQL uses your operating system's underlying features to provide output time-zone support, and these systems usually contain information for only the time period 1902 through 2038 (corresponding to the full range of conventional Unix system time). timestamp with time zone and time with time zone will use time zone information only within that year range, and assume that times outside that range are in UTC.
The type time with time zone is defined by the SQL standard, but the definition exhibits properties which lead to questionable usefulness. In most cases, a combination of date, time, timestamp without time zone and timestamp with time zone should provide a complete range of date/time functionality required by any application.
The types abstime and reltime are lower precision types which are used internally. You are discouraged from using these types in new applications and are encouraged to move any old ones over when appropriate. Any or all of these internal types might disappear in a future release.
Date and time input is accepted in almost any reasonable format, including ISO 8601, SQL-compatible, traditional PostgreSQL, and others. For some formats, ordering of month and day in date input can be ambiguous and there is support for specifying the expected ordering of these fields. The command SET DateStyle TO 'US' or SET DateStyle TO 'NonEuropean' specifies the variant "month before day", the command SET DateStyle TO 'European' sets the variant "day before month".
PostgreSQL is more flexible in handling date/time than the SQL standard requires. See Appendix A for the exact parsing rules of date/time input and for the recognized text fields including months, days of the week, and time zones.
Remember that any date or time literal input needs to be enclosed in single quotes, like text strings. Refer to Section 188.8.131.52 for more information. SQL requires the following syntax
type [ (p) ] 'value'
where p in the optional precision specification is an integer corresponding to the number of fractional digits in the seconds field. Precision can be specified for time, timestamp, and interval types.
Table 5-10 shows some possible inputs for the date type.
Table 5-10. Date Input
|January 8, 1999||unambiguous|
|1999-01-08||ISO-8601 format, preferred|
|1/8/1999||U.S.; read as August 1 in European mode|
|8/1/1999||European; read as August 1 in U.S. mode|
|1/18/1999||U.S.; read as January 18 in any mode|
|19990108||ISO-8601 year, month, day|
|990108||ISO-8601 year, month, day|
|1999.008||year and day of year|
|99008||year and day of year|
|January 8, 99 BC||year 99 before the Common Era|
The time type can be specified as time or as time without time zone. The optional precision p should be between 0 and 13, and defaults to the precision of the input time literal.
Table 5-11 shows the valid time inputs.
Table 5-11. Time Input
|04:05 AM||same as 04:05; AM does not affect value|
|04:05 PM||same as 16:05; input hour must be <= 12|
|allballs||same as 00:00:00|
The type time with time zone accepts all input also legal for the time type, appended with a legal time zone, as shown in Table 5-12.
Table 5-12. Time With Time Zone Input
Refer to Table 5-13 for more examples of time zones.
The time stamp types are timestamp [ (p) ] without time zone and timestamp [ (p) ] with time zone. Writing just timestamp is equivalent to timestamp without time zone.
Note: Prior to PostgreSQL 7.3, writing just timestamp was equivalent to timestamp with time zone. This was changed for SQL spec compliance.
Valid input for the time stamp types consists of a concatenation of a date and a time, followed by an optional AD or BC, followed by an optional time zone. (See Table 5-13.) Thus
1999-01-08 04:05:06 -8:00
are valid values, which follow the ISO 8601 standard. In addition, the wide-spread format
January 8 04:05:06 1999 PST
The optional precision p should be between 0 and 6, and defaults to the precision of the input timestamp literal.
For timestamp without time zone, any explicit time zone specified in the input is silently ignored. That is, the resulting date/time value is derived from the explicit date/time fields in the input value, and is not adjusted for time zone.
For timestamp with time zone, the internally stored value is always in UTC (GMT). An input value that has an explicit time zone specified is converted to UTC using the appropriate offset for that time zone. If no time zone is stated in the input string, then it is assumed to be in the time zone indicated by the system's TimeZone parameter, and is converted to UTC using the offset for the TimeZone zone.
When a timestamp with time zone value is output, it is always converted from UTC to the current TimeZone zone, and displayed as local time in that zone. To see the time in another time zone, either change TimeZone or use the AT TIME ZONE construct (see Section 6.8.3).
Conversions between timestamp without time zone and timestamp with time zone normally assume that the timestamp without time zone value should be taken or given as TimeZone local time. A different zone reference can be specified for the conversion using AT TIME ZONE.
interval values can be written with the following syntax:
Quantity Unit [Quantity Unit...] [Direction] @ Quantity Unit [Quantity Unit...] [Direction]
where: Quantity is a number (possibly signed), Unit is second, minute, hour, day, week, month, year, decade, century, millennium, or abbreviations or plurals of these units; Direction can be ago or empty. The at sign (@) is optional noise. The amounts of different units are implicitly added up with appropriate sign accounting.
Quantities of days, hours, minutes, and seconds can be specified without explicit unit markings. For example, '1 12:59:10' is read the same as '1 day 12 hours 59 min 10 sec'.
The optional precision p should be between 0 and 6, and defaults to the precision of the input literal.
The following SQL-compatible functions can be used as date or time values for the corresponding data type: CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP. The latter two accept an optional precision specification. (See also Section 6.8.4.)
PostgreSQL also supports several special date/time input values for convenience, as shown in Table 5-14. The values infinity and -infinity are specially represented inside the system and will be displayed the same way; but the others are simply notational shorthands that will be converted to ordinary date/time values when read.
Table 5-14. Special Date/Time Inputs
|epoch||1970-01-01 00:00:00+00 (Unix system time zero)|
|infinity||later than all other timestamps (not available for type date)|
|-infinity||earlier than all other timestamps (not available for type date)|
|now||current transaction time|
|zulu, allballs, z||00:00:00.00 GMT|
Output formats can be set to one of the four styles ISO 8601, SQL (Ingres), traditional PostgreSQL, and German, using the SET DateStyle. The default is the ISO format. (The SQL standard requires the use of the ISO 8601 format. The name of the "SQL" output format is a historical accident.) Table 5-15 shows examples of each output style. The output of the date and time types is of course only the date or time part in accordance with the given examples.
Table 5-15. Date/Time Output Styles
|ISO||ISO 8601/SQL standard||1997-12-17 07:37:16-08|
|SQL||traditional style||12/17/1997 07:37:16.00 PST|
|PostgreSQL||original style||Wed Dec 17 07:37:16 1997 PST|
|German||regional style||17.12.1997 07:37:16.00 PST|
The SQL style has European and non-European (U.S.) variants, which determines whether month follows day or vice versa. (See Section 5.5.1 for how this setting also affects interpretation of input values.) Table 5-16 shows an example.
Table 5-16. Date Order Conventions
|European||day/month/year||17/12/1997 15:37:16.00 MET|
|US||month/day/year||12/17/1997 07:37:16.00 PST|
interval output looks like the input format, except that units like week or century are converted to years and days. In ISO mode the output looks like
[ Quantity Units [ ... ] ] [ Days ] Hours:Minutes [ ago ]
The date/time styles can be selected by the user using the SET DATESTYLE command, the datestyle parameter in the postgresql.conf configuration file, and the PGDATESTYLE environment variable on the server or client. The formatting function to_char (see Section 6.7) is also available as a more flexible way to format the date/time output.
PostgreSQL endeavors to be compatible with the SQL standard definitions for typical usage. However, the SQL standard has an odd mix of date and time types and capabilities. Two obvious problems are:
Although the date type does not have an associated time zone, the time type can. Time zones in the real world can have no meaning unless associated with a date as well as a time since the offset may vary through the year with daylight-saving time boundaries.
The default time zone is specified as a constant integer offset from GMT/UTC. It is not possible to adapt to daylight-saving time when doing date/time arithmetic across DST boundaries.
To address these difficulties, we recommend using date/time types that contain both date and time when using time zones. We recommend not using the type time with time zone (though it is supported by PostgreSQL for legacy applications and for compatibility with other SQL implementations). PostgreSQL assumes your local time zone for any type containing only date or time. Further, time zone support is derived from the underlying operating system time-zone capabilities, and hence can handle daylight-saving time and other expected behavior.
PostgreSQL obtains time-zone support from the underlying operating system for dates between 1902 and 2038 (near the typical date limits for Unix-style systems). Outside of this range, all dates are assumed to be specified and used in Universal Coordinated Time (UTC).
All dates and times are stored internally in UTC, traditionally known as Greenwich Mean Time (GMT). Times are converted to local time on the database server before being sent to the client frontend, hence by default are in the server time zone.
There are several ways to select the time zone used by the server:
The TZ environment variable on the server host is used by the server as the default time zone, if no other is specified.
The timezone configuration parameter can be set in postgresql.conf.
The PGTZ environment variable, if set at the client, is used by libpq applications to send a SET TIME ZONE command to the server upon connection.
The SQL command SET TIME ZONE sets the time zone for the session.
Note: If an invalid time zone is specified, the time zone becomes UTC (on most systems anyway).
Refer to Appendix A for a list of available time zones.
PostgreSQL uses Julian dates for all date/time calculations. They have the nice property of correctly predicting/calculating any date more recent than 4713 BC to far into the future, using the assumption that the length of the year is 365.2425 days.
Date conventions before the 19th century make for interesting reading, but are not consistent enough to warrant coding into a date/time handler.