PostgreSQL 8.0.0rc2 Documentation
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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 4.1.2.5 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`

Refer to Appendix B for a list of time zone names that are recognized for input.

### 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 time zone, followed by an optional `AD` or `BC`. (Alternatively, `AD`/`BC` can appear before the time zone, but this is not the preferred ordering.) 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.

The SQL standard differentiates `timestamp without time zone` and `timestamp with time zone` literals by the existence of a "+"; or "-". Hence, according to the standard,

    TIMESTAMP '2004-10-19 10:23:54'

is a `timestamp without time zone`, while

    TIMESTAMP '2004-10-19 10:23:54+02'

is a `timestamp with time zone`. PostgreSQL differs from the standard by requiring that `timestamp with time zone` literals be explicitly typed:

    TIMESTAMP WITH TIME ZONE '2004-10-19 10:23:54+02'

If a literal is not explicitly indicated as being of `timestamp with time zone`, PostgreSQL will silently ignore any time zone indication in the literal. That is, the resulting date/time value is derived from the 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 9.9.3).

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

PostgreSQL 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

The following SQL-compatible functions can also be used to obtain the current time value for the corresponding data type: `CURRENT_DATE`, `CURRENT_TIME`, `CURRENT_TIMESTAMP`, `LOCALTIME`, `LOCALTIMESTAMP`. The latter four accept an optional precision specification. (See also Section 9.9.4.) Note however that these are SQL functions and are not recognized as data input strings.

## 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 `week` are converted to years and days and `ago` is converted to an appropriate sign. In ISO mode the output looks like

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

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 9.8) 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, particularly with respect to daylight-savings rules. PostgreSQL currently supports daylight-savings rules over the time period 1902 through 2038 (corresponding to the full range of conventional Unix system time). Times outside that range are taken to be in "standard time" for the selected time zone, no matter what part of the year they fall in.

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 have little 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 therefore 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 compliance with the SQL standard). PostgreSQL assumes your local time zone for any type containing only date or time.

All timezone-aware dates and times are stored internally in UTC. They are converted to local time in the zone specified by the timezone configuration parameter before being displayed to the client.

The timezone configuration parameter can be set in the file `postgresql.conf`, or in any of the other standard ways described in Section 16.4. There are also several special ways to set it:

- If `timezone` is not specified in `postgresql.conf` nor as a postmaster command-line switch, the server attempts to use the value of the `TZ` environment variable as the default time zone. If `TZ` is not defined or is not any of the time zone names known to PostgreSQL, the server attempts to determine the operating system's default time zone by checking the behavior of the C library function `localtime()`. The default time zone is selected as the closest match among PostgreSQL's known time zones.

- The SQL command `SET TIME ZONE` sets the time zone for the session. This is an alternative spelling of `SET TIMEZONE TO` with a more SQL-spec-compatible syntax.

- 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.

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.

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