PostgreSQL 7.1.3 Documentation
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# 3.4. Date/Time Types

Postgres supports the full set of SQL date and time types.

Table 3-6. Date/Time Types

Type | Description | Storage | Earliest | Latest | Resolution
--- | --- | --- | --- | --- | ---
`timestamp` | both date and time | 8 bytes | 4713 BC | AD 1465001 | 1 microsecond / 14 digits
`timestamp [ with time zone ]` | date and time with time zone | 8 bytes | 1903 AD | 2037 AD | 1 microsecond / 14 digits
`interval` | for time intervals | 12 bytes | -178000000 years | 178000000 years | 1 microsecond
`date` | dates only | 4 bytes | 4713 BC | 32767 AD | 1 day
`time [ without time zone ]` | times of day only | 4 bytes | 00:00:00.00 | 23:59:59.99 | 1 microsecond
`time with time zone` | times of day only | 4 bytes | 00:00:00.00+12 | 23:59:59.99-12 | 1 microsecond

> Note: To ensure compatibility to earlier versions of Postgres we also continue to provide `datetime` (equivalent to `timestamp`) and `timespan` (equivalent to `interval`), however support for these is now restricted to having an implicit translation to `timestamp` and `interval`. The types `abstime` and `reltime` are lower precision types which are used internally. You are discouraged from using any of 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.

## 3.4.1. Date/Time Input

Date and time input is accepted in almost any reasonable format, including ISO-8601, SQL-compatible, traditional Postgres, and others. The ordering of month and day in date input can be ambiguous, therefore a setting exists to specify how it should be interpreted in ambiguous cases. 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". The `ISO` style is the default but this default can be changed at compile time or at run time.

See Appendix A for the exact parsing rules of date/time input and for the recognized time zones.

Remember that any date or time input needs to be enclosed into single quotes, like text strings. Refer to Section 1.1.2.5 for more information. SQL requires the following syntax

    type 'value'

but Postgres is more flexible.

### 3.4.1.1. date

The following are possible inputs for the `date` type.

Table 3-7. Date Input

Example | Description
--- | ---
January 8, 1999 | Unambiguous
1999-01-08 | ISO-8601 format, preferred
1/8/1999 | US; read as August 1 in European mode
8/1/1999 | European; read as August 1 in US mode
1/18/1999 | US; 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

Table 3-8. Month Abbreviations

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

> Note: The month `May` has no explicit abbreviation, for obvious reasons.

Table 3-9. Day of the Week Abbreviations

Day | Abbreviation
--- | ---
Sunday | Sun
Monday | Mon
Tuesday | Tue, Tues
Wednesday | Wed, Weds
Thursday | Thu, Thur, Thurs
Friday | Fri
Saturday | Sat

### 3.4.1.2. time [ without time zone ]

Per SQL99, this type can be referenced as `time` and as `time without time zone`.

The following are valid `time` inputs.

Table 3-10. 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
z | Same as 00:00:00
zulu | Same as 00:00:00
allballs | Same as 00:00:00

### 3.4.1.3. time with time zone

This type is defined by SQL92, but the definition exhibits fundamental deficiencies that render the type nearly useless. In most cases, a combination of `date`, `time`, and `timestamp` should provide a complete range of date/time functionality required by any application.

`time with time zone` accepts all input also legal for the `time` type, appended with a legal time zone, as follows:

Table 3-11. Time With Time Zone Input

Example | Description
--- | ---
04:05:06.789-8 | ISO-8601
04:05:06-08:00 | ISO-8601
04:05-08:00 | ISO-8601
040506-08 | ISO-8601

Refer to Table 3-12 for more examples of time zones.

### 3.4.1.4. timestamp

Valid input for the `timestamp` type consists of a concatenation of a date and a time, followed by an optional `AD` or `BC`, followed by an optional time zone. (See below.) Thus

    1999-01-08 04:05:06 -8:00

is a valid `timestamp` value that is ISO-compliant. In addition, the wide-spread format

    January 8 04:05:06 1999 PST

is supported.

Table 3-12. Time Zone Input

Time Zone | Description
--- | ---
PST | Pacific Standard Time
-8:00 | ISO-8601 offset for PST
-800 | ISO-8601 offset for PST
-8 | ISO-8601 offset for PST

### 3.4.1.5. interval

`interval`s can be specified with the following syntax:

      Quantity Unit [Quantity Unit...] [Direction]
    @ Quantity Unit [Direction]

where: `Quantity` is ..., `-1`, `0`, `1`, `2`, ...; `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.

### 3.4.1.6. Special values

The following SQL-compatible functions can be used as date or time input for the corresponding data type: `CURRENT_DATE`, `CURRENT_TIME`, `CURRENT_TIMESTAMP`.

Postgres also supports several special constants for convenience.

Table 3-13. Special Date/Time Constants

Constant | Description
--- | ---
current | Current transaction time, deferred
epoch | 1970-01-01 00:00:00+00 (Unix system time zero)
infinity | Later than other valid times
-infinity | Earlier than other valid times
invalid | Illegal entry
now | Current transaction time
today | Midnight today
tomorrow | Midnight tomorrow
yesterday | Midnight yesterday

`'now'` is resolved when the value is inserted, `'current'` is resolved every time the value is retrieved. So you probably want to use `'now'` in most applications. (Of course you really want to use `CURRENT_TIMESTAMP`, which is equivalent to `'now'`.)

## 3.4.2. Date/Time Output

Output formats can be set to one of the four styles ISO-8601, SQL (Ingres), traditional Postgres, and German, using the SET DateStyle. The default is the ISO format.

Table 3-14. Date/Time Output Styles

Style Specification | Description | Example
--- | --- | ---
'ISO' | ISO-8601 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

The output of the `date` and `time` styles is of course only the date or time part in accordance with the above examples.

The SQL style has European and non-European (US) variants, which determines whether month follows day or vice versa. (See also above at Date/Time Input, how this setting affects interpretation of input values.)

Table 3-15. Date Order Conventions

Style Specification | Description | Example
--- | --- | ---
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 ]

There are several ways to affect the appearance of date/time types:

- The `PGDATESTYLE` environment variable used by the backend directly on postmaster start-up.

- The `PGDATESTYLE` environment variable used by the frontend libpq on session start-up.

- SET DATESTYLE SQL command.

## 3.4.3. Time Zones

Postgres endeavors to be compatible with SQL92 definitions for typical usage. However, the SQL92 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 or does. 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 savings time boundaries.

- The default time zone is specified as a constant integer offset from GMT/UTC. It is not possible to adapt to daylight savings 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 SQL92 type TIME WITH TIME ZONE (though it is supported by Postgres for legacy applications and for compatibility with other RDBMS implementations). Postgres assumes local time 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 savings time and other expected behavior.

Postgres 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 affect the time zone behavior:

- The TZ environment variable is used by the backend directly on postmaster start-up as the default time zone.

- The PGTZ environment variable set at the client used by libpq to send time zone information to the backend upon connection.

- The SQL command SET TIME ZONE sets the time zone for the session.

- The SQL92 qualifier on

      timestamp AT TIME ZONE 'zone'

  where `zone` can be specified as a text time zone (e.g. `'PST'`) or as an interval (e.g. `INTERVAL '-08:00'`).

> Note: If an invalid time zone is specified, the time zone becomes GMT (on most systems anyway).

> Note: If the compiler option USE_AUSTRALIAN_RULES is set then `EST` refers to Australia Eastern Standard Time, which has an offset of +10:00 hours from UTC.

## 3.4.4. Internals

Postgres uses Julian dates for all date/time calculations. They have the nice property of correctly predicting/calculating any date more recent than 4713BC 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.

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