PostgreSQL 8.0.0rc4 Documentation
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# 9.9. Date/Time Functions and Operators

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (`+`, `*`, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take `time` or `timestamp` inputs actually come in two variants: one that takes `time with time zone` or `timestamp with time zone`, and one that takes `time without time zone` or `timestamp without time zone`. For brevity, these variants are not shown separately. Also, the `+` and `*` operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-25. Date/Time Operators

Operator | Example | Result
--- | --- | ---
`+` | `date '2001-09-28' + integer '7'` | `date '2001-10-05'`
`+` | `date '2001-09-28' + interval '1 hour'` | `timestamp '2001-09-28 01:00'`
`+` | `date '2001-09-28' + time '03:00'` | `timestamp '2001-09-28 03:00'`
`+` | `interval '1 day' + interval '1 hour'` | `interval '1 day 01:00'`
`+` | `timestamp '2001-09-28 01:00' + interval '23 hours'` | `timestamp '2001-09-29 00:00'`
`+` | `time '01:00' + interval '3 hours'` | `time '04:00'`
`-` | `- interval '23 hours'` | `interval '-23:00'`
`-` | `date '2001-10-01' - date '2001-09-28'` | `integer '3'`
`-` | `date '2001-10-01' - integer '7'` | `date '2001-09-24'`
`-` | `date '2001-09-28' - interval '1 hour'` | `timestamp '2001-09-27 23:00'`
`-` | `time '05:00' - time '03:00'` | `interval '02:00'`
`-` | `time '05:00' - interval '2 hours'` | `time '03:00'`
`-` | `timestamp '2001-09-28 23:00' - interval '23 hours'` | `timestamp '2001-09-28 00:00'`
`-` | `interval '1 day' - interval '1 hour'` | `interval '23:00'`
`-` | `timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'` | `interval '1 day 15:00'`
`*` | `interval '1 hour' * double precision '3.5'` | `interval '03:30'`
`/` | `interval '1 hour' / double precision '1.5'` | `interval '00:40'`

Table 9-26. Date/Time Functions

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
``age`(`timestamp`, `timestamp`)` | `interval` | Subtract arguments, producing a "symbolic" result that uses years and months | `age(timestamp '2001-04-10', timestamp '1957-06-13')` | `43 years 9 mons 27 days`
``age`(`timestamp`)` | `interval` | Subtract from `current_date` | `age(timestamp '1957-06-13')` | `43 years 8 mons 3 days`
``current_date`` | `date` | Today's date; see Section 9.9.4 |  |
``current_time`` | `time with time zone` | Time of day; see Section 9.9.4 |  |
``current_timestamp`` | `timestamp with time zone` | Date and time; see Section 9.9.4 |  |
``date_part`(`text`, `timestamp`)` | `double precision` | Get subfield (equivalent to `extract`); see Section 9.9.1 | `date_part('hour', timestamp '2001-02-16 20:38:40')` | `20`
``date_part`(`text`, `interval`)` | `double precision` | Get subfield (equivalent to `extract`); see Section 9.9.1 | `date_part('month', interval '2 years 3 months')` | `3`
``date_trunc`(`text`, `timestamp`)` | `timestamp` | Truncate to specified precision; see also Section 9.9.2 | `date_trunc('hour', timestamp '2001-02-16 20:38:40')` | `2001-02-16 20:00:00`
``extract`(`field` from `timestamp`)` | `double precision` | Get subfield; see Section 9.9.1 | `extract(hour from timestamp '2001-02-16 20:38:40')` | `20`
``extract`(`field` from `interval`)` | `double precision` | Get subfield; see Section 9.9.1 | `extract(month from interval '2 years 3 months')` | `3`
``isfinite`(`timestamp`)` | `boolean` | Test for finite time stamp (not equal to infinity) | `isfinite(timestamp '2001-02-16 21:28:30')` | `true`
``isfinite`(`interval`)` | `boolean` | Test for finite interval | `isfinite(interval '4 hours')` | `true`
``localtime`` | `time` | Time of day; see Section 9.9.4 |  |
``localtimestamp`` | `timestamp` | Date and time; see Section 9.9.4 |  |
``now`()` | `timestamp with time zone` | Current date and time (equivalent to `current_timestamp`); see Section 9.9.4 |  |
``timeofday()`` | `text` | Current date and time; see Section 9.9.4 |  |

In addition to these functions, the SQL `OVERLAPS` operator is supported:

    ( start1, end1 ) OVERLAPS ( start2, end2 )
    ( start1, length1 ) OVERLAPS ( start2, length2 )

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

    SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS
           (DATE '2001-10-30', DATE '2002-10-30');
    Result: true
    SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS
           (DATE '2001-10-30', DATE '2002-10-30');
    Result: false

## 9.9.1. `EXTRACT`, `date_part`

    EXTRACT (field FROM source)

The `extract` function retrieves subfields such as year or hour from date/time values. `source` must be a value expression of type `timestamp`, `time`, or `interval`. (Expressions of type `date` will be cast to `timestamp` and can therefore be used as well.) `field` is an identifier or string that selects what field to extract from the source value. The `extract` function returns values of type `double precision`. The following are valid field names:

`century`

  The century

      SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
      Result: 20
      SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 21

  The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

  PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

`day`

  The day (of the month) field (1 - 31)

      SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 16

`decade`

  The year field divided by 10

      SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 200

`dow`

  The day of the week (0 - 6; Sunday is 0) (for `timestamp` values only)

      SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 5

  Note that `extract`'s day of the week numbering is different from that of the `to_char` function.

`doy`

  The day of the year (1 - 365/366) (for `timestamp` values only)

      SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 47

`epoch`

  For `date` and `timestamp` values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for `interval` values, the total number of seconds in the interval

      SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-08');
      Result: 982384720

      SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
      Result: 442800

  Here is how you can convert an epoch value back to a time stamp:

      SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720 * interval '1 second';

`hour`

  The hour field (0 - 23)

      SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 20

`microseconds`

  The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

      SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
      Result: 28500000

`millennium`

  The millennium

      SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 3

  Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001.

  PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

`milliseconds`

  The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

      SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
      Result: 28500

`minute`

  The minutes field (0 - 59)

      SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 38

`month`

  For `timestamp` values, the number of the month within the year (1 - 12) ; for `interval` values the number of months, modulo 12 (0 - 11)

      SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 2

      SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
      Result: 3

      SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
      Result: 1

`quarter`

  The quarter of the year (1 - 4) that the day is in (for `timestamp` values only)

      SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 1

`second`

  The seconds field, including fractional parts (0 - 59[1])

      SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 40

      SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
      Result: 28.5

`timezone`

  The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

`timezone_hour`

  The hour component of the time zone offset

`timezone_minute`

  The minute component of the time zone offset

`week`

  The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for `timestamp` values only)

      SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 7

`year`

  The year field. Keep in mind there is no `0 AD`, so subtracting `BC` years from `AD` years should be done with care.

      SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 2001

The `extract` function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The `date_part` function is modeled on the traditional Ingres equivalent to the SQL-standard function `extract`:

    date_part('field', source)

Note that here the `field` parameter needs to be a string value, not a name. The valid field names for `date_part` are the same as for `extract`.

    SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
    Result: 16

    SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
    Result: 4

## 9.9.2. `date_trunc`

The function `date_trunc` is conceptually similar to the `trunc` function for numbers.

    date_trunc('field', source)

`source` is a value expression of type `timestamp` or `interval`. (Values of type `date` and `time` are cast automatically, to `timestamp` or `interval` respectively.) `field` selects to which precision to truncate the input value. The return value is of type `timestamp` or `interval` with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for `field` are:

`microseconds`
`milliseconds`
`second`
`minute`
`hour`
`day`
`week`
`month`
`year`
`decade`
`century`
`millennium`

Examples:

    SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
    Result: 2001-02-16 20:00:00

    SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
    Result: 2001-01-01 00:00:00

## 9.9.3. `AT TIME ZONE`

The `AT TIME ZONE` construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. `AT TIME ZONE` Variants

Expression | Return Type | Description
--- | --- | ---
``timestamp without time zone` AT TIME ZONE `zone`` | `timestamp with time zone` | Convert local time in given time zone to UTC
``timestamp with time zone` AT TIME ZONE `zone`` | `timestamp without time zone` | Convert UTC to local time in given time zone
``time with time zone` AT TIME ZONE `zone`` | `time with time zone` | Convert local time across time zones

In these expressions, the desired time zone `zone` can be specified either as a text string (e.g., `'PST'`) or as an interval (e.g., `INTERVAL '-08:00'`). In the text case, the available zone names are those shown in Table B-4. (It would be useful to support the more general names shown in Table B-6, but this is not yet implemented.)

Examples (supposing that the local time zone is `PST8PDT`):

    SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
    Result: 2001-02-16 19:38:40-08

    SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
    Result: 2001-02-16 18:38:40

The first example takes a zone-less time stamp and interprets it as MST time (UTC-7) to produce a UTC time stamp, which is then rotated to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function ``timezone`(`zone`, `timestamp`)` is equivalent to the SQL-conforming construct ``timestamp` AT TIME ZONE `zone``.

## 9.9.4. Current Date/Time

The following functions are available to obtain the current date and/or time:

    CURRENT_DATE
    CURRENT_TIME
    CURRENT_TIMESTAMP
    CURRENT_TIME ( precision )
    CURRENT_TIMESTAMP ( precision )
    LOCALTIME
    LOCALTIMESTAMP
    LOCALTIME ( precision )
    LOCALTIMESTAMP ( precision )

`CURRENT_TIME` and `CURRENT_TIMESTAMP` deliver values with time zone; `LOCALTIME` and `LOCALTIMESTAMP` deliver values without time zone.

`CURRENT_TIME`, `CURRENT_TIMESTAMP`, `LOCALTIME`, and `LOCALTIMESTAMP` can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

> Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Some examples:

    SELECT CURRENT_TIME;
    Result: 14:39:53.662522-05

    SELECT CURRENT_DATE;
    Result: 2001-12-23

    SELECT CURRENT_TIMESTAMP;
    Result: 2001-12-23 14:39:53.662522-05

    SELECT CURRENT_TIMESTAMP(2);
    Result: 2001-12-23 14:39:53.66-05

    SELECT LOCALTIMESTAMP;
    Result: 2001-12-23 14:39:53.662522

The function `now()` is the traditional PostgreSQL equivalent to `CURRENT_TIMESTAMP`.

There is also the function `timeofday()`, which for historical reasons returns a `text` string rather than a `timestamp` value:

    SELECT timeofday();
    Result: Sat Feb 17 19:07:32.000126 2001 EST

It is important to know that `CURRENT_TIMESTAMP` and related functions return the start time of the current transaction; their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp. `timeofday()` returns the wall-clock time and does advance during transactions.

> Note: Other database systems may advance these values more frequently.

All the date/time data types also accept the special literal value `now` to specify the current date and time. Thus, the following three all return the same result:

    SELECT CURRENT_TIMESTAMP;
    SELECT now();
    SELECT TIMESTAMP 'now';

> Tip: You do not want to use the third form when specifying a `DEFAULT` clause while creating a table. The system will convert `now` to a `timestamp` as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

### Notes

[1] | 60 if leap seconds are implemented by the operating system

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