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8.5. Date/Time Types

PostgreSQL supports the full set of SQL date and time types, shown in Table 8-9.

Table 8-9. Date/Time Types

Name Storage Size Description Low Value High Value Resolution
timestamp [ (p) ] [ without time zone ] 8 bytes both date and time 4713 BC 5874897 AD 1 microsecond / 14 digits
timestamp [ (p) ] with time zone 8 bytes both date and time, with time zone 4713 BC 5874897 AD 1 microsecond / 14 digits
interval [ (p) ] 12 bytes time intervals -178000000 years 178000000 years 1 microsecond
date 4 bytes dates only 4713 BC 32767 AD 1 day
time [ (p) ] [ without time zone ] 8 bytes times of day only 00:00:00.00 23:59:59.99 1 microsecond
time [ (p) ] with time zone 12 bytes times of day only, with time zone 00:00:00.00+12 23:59:59.99-12 1 microsecond

Note: Prior to PostgreSQL 7.3, writing just timestamp was equivalent to timestamp with time zone. This was changed for SQL compliance.

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.

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. timestamp values are stored as seconds before or after midnight 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 timestamp values are stored as eight-byte integers (a compile-time option), microsecond precision is available over the full range of values. However eight-byte integer timestamps have a more limited range of dates than shown above: from 4713 BC up to 294276 AD.

For the time types, the allowed range of p is from 0 to 6 when eight-byte integer storage is used, or from 0 to 10 when floating-point storage is used.

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.

8.5.1. Date/Time Input

Date and time input is accepted in almost any reasonable format, including ISO 8601, SQL-compatible, traditional POSTGRES, and others. For some formats, ordering of month, day, and year in date input is ambiguous and there is support for specifying the expected ordering of these fields. Set the datestyle parameter to MDY to select month-day-year interpretation, DMY to select day-month-year interpretation, or YMD to select year-month-day interpretation.

PostgreSQL is more flexible in handling date/time input than the SQL standard requires. See Appendix B 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 4.1.2.4 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. The allowed values are mentioned above. If no precision is specified in a constant specification, it defaults to the precision of the literal value.

8.5.1.1. Dates

Table 8-10 shows some possible inputs for the date type.

Table 8-10. Date Input

Example Description
January 8, 1999 unambiguous in any datestyle input mode
1999-01-08 ISO 8601; January 8 in any mode (recommended format)
1/8/1999 January 8 in MDY mode; August 1 in DMY mode
1/18/1999 January 18 in MDY mode; rejected in other modes
01/02/03 January 2, 2003 in MDY mode; February 1, 2003 in DMY mode; February 3, 2001 in YMD mode
1999-Jan-08 January 8 in any mode
Jan-08-1999 January 8 in any mode
08-Jan-1999 January 8 in any mode
99-Jan-08 January 8 in YMD mode, else error
08-Jan-99 January 8, except error in YMD mode
Jan-08-99 January 8, except error in YMD mode
19990108 ISO 8601; January 8, 1999 in any mode
990108 ISO 8601; January 8, 1999 in any mode
1999.008 year and day of year
J2451187 Julian day
January 8, 99 BC year 99 before the Common Era

8.5.1.2. Times

The time-of-day types are time [ (p) ] without time zone and time [ (p) ] with time zone. Writing just time is equivalent to time without time zone.

Valid input for these types consists of a time of day followed by an optional time zone. (See Table 8-11 and Table 8-12.) If a time zone is specified in the input for time without time zone, it is silently ignored.

Table 8-11. Time Input

Example Description
04:05:06.789 ISO 8601
04:05:06 ISO 8601
04:05 ISO 8601
040506 ISO 8601
04:05 AM same as 04:05; AM does not affect value
04:05 PM same as 16:05; input hour must be <= 12
04:05:06.789-8 ISO 8601
04:05:06-08:00 ISO 8601
04:05-08:00 ISO 8601
040506-08 ISO 8601
04:05:06 PST time zone specified by name

Table 8-12. Time Zone Input

Example Description
PST Pacific Standard Time
-8:00 ISO-8601 offset for PST
-800 ISO-8601 offset for PST
-8 ISO-8601 offset for PST
zulu Military abbreviation for UTC
z Short form of zulu

8.5.1.3. Time Stamps

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. Thus

1999-01-08 04:05:06

and

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

is supported.

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 (Universal Coordinated Time, traditionally known as Greenwich Mean Time, 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 9.8.3).

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.

8.5.1.4. Intervals

interval values can be written with the following syntax:

[@] 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.

8.5.1.5. Special Values

The following SQL-compatible functions can be used as date or time values for the corresponding data type: CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, LOCALTIMESTAMP. The latter four accept an optional precision specification. (See also Section 9.8.4.)

PostgreSQL also supports several special date/time input values for convenience, as shown in Table 8-13. 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. All of these values are treated as normal constants and need to be written in single quotes.

Table 8-13. Special Date/Time Inputs

Input String Valid Types Description
epoch date, timestamp 1970-01-01 00:00:00+00 (Unix system time zero)
infinity timestamp later than all other time stamps
-infinity timestamp earlier than all other time stamps
now date, time, timestamp current transaction's start time
today date, timestamp midnight today
tomorrow date, timestamp midnight tomorrow
yesterday date, timestamp midnight yesterday
allballs time 00:00:00.00 UTC

8.5.2. Date/Time Output

The output format of the date/time types can be set to one of the four styles ISO 8601, SQL (Ingres), traditional POSTGRES, and German, using the command 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 8-14 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 8-14. Date/Time Output Styles

Style Specification Description Example
ISO ISO 8601/SQL standard 1997-12-17 07:37:16-08
SQL traditional style 12/17/1997 07:37:16.00 PST
POSTGRES original style Wed Dec 17 07:37:16 1997 PST
German regional style 17.12.1997 07:37:16.00 PST

In the SQL and POSTGRES styles, day appears before month if DMY field ordering has been specified, otherwise month appears before day. (See Section 8.5.1 for how this setting also affects interpretation of input values.) Table 8-15 shows an example.

Table 8-15. Date Order Conventions

datestyle Setting Input Ordering Example Output
SQL, DMY day/month/year 17/12/1997 15:37:16.00 CET
SQL, MDY month/day/year 12/17/1997 07:37:16.00 PST
Postgres, DMY day/month/year Wed 17 Dec 07:37:16 1997 PST

interval output looks like the input format, except that units like century or wek are converted to years and days and that ago is converted to an appropriate sign. In ISO mode the output looks like

[ quantity unit [ ... ] ] [ days ] [ hours:minutes:sekunden ]

The date/time styles can be selected by the user using the SET datestyle command, the datestyle parameter in the postgresql.conf configuration file, or the PGDATESTYLE environment variable on the server or client. The formatting function to_char (see Section 9.7) is also available as a more flexible way to format the date/time output.

8.5.3. Time Zones

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. But since time zone support is derived from the underlying operating system time-zone capabilities, it can handle daylight-saving time and other special behavior.

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 numeric offset from 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.

All dates and times are stored internally in UTC. Times are converted to local time on the database server before being sent to the client, 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 the file 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 B for a list of available time zones.

8.5.4. Internals

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.

Comments


July 23, 2004, 7:14 a.m.

To get a timestamp that is automatically set to row create time,
set the column\'s default value to \"now()\". Using a special value
such as \"now\" will not work, as it is interpreted at definition
time.

E.g.
create table example (
a integer,
ts timestamp with time zone default now()
);


Aug. 17, 2004, 3:13 p.m.

To retrieve the value of a TIMESTAMP in seconds since the epoch (integer) use:

select extract(epoch from your_column)::integer from your_table;


April 20, 2005, 5:05 p.m.

Just to clarify what is contained in this section, to change the output type of the date fields you would issue the following query,

SET DateStyle TO 'SQL';

This for example would change the DateStyle to SQL (m/d/Y). Notice also, this is a per connection variable; the next time you connect to the database the format will be reset to whatever the column type is set to (USUALLY ISO).

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