Date/Time Functions and Operators

PostgreSQL 7.2 Documentation
Prev | Chapter 4. Functions and Operators | Next

# 4.8. Date/Time Functions and Operators

Table 4-17 shows the available functions for date/time value processing. Table 4-16 illustrates the behaviors of the basic arithmetic operators (`+`, `*`, etc.). For formatting functions, refer to Section 4.7. You should be familiar with the background information on date/time data types (see Section 3.5).

The date/time operators described below behave similarly for types involving time zones as well as those without.

Table 4-16. Date/Time Operators

Name | Example | Result
--- | --- | ---
`+` | `timestamp` '2001-09-28 01:00' + `interval` '23 hours' | `timestamp` '2001-09-29 00:00'
`+` | `date` '2001-09-28' + `interval` '1 hour' | `timestamp` '2001-09-28 01:00'
`+` | `time` '01:00' + `interval` '3 hours' | `time` '04:00'
`-` | `timestamp` '2001-09-28 23:00' - `interval` '23 hours' | `timestamp` '2001-09-28'
`-` | `date` '2001-09-28' - `interval` '1 hour' | `timestamp` '2001-09-27 23:00'
`-` | `time` '05:00' - `interval` '2 hours' | `time` '03:00'
`-` | `interval` '2 hours' - `time` '05:00' | `time` '03:00:00'
`*` | `interval` '1 hour' * `int` '3' | `interval` '03:00'
`/` | `interval` '1 hour' / `int` '3' | `interval` '00:20'

The date/time functions are summarized below, with additional details in subsequent sections.

Table 4-17. Date/Time Functions

Name | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
`age`(`timestamp`) | `interval` | Subtract from today | `age(timestamp '1957-06-13')` | `43 years 8 mons 3 days`
`age`(`timestamp`, `timestamp`) | `interval` | Subtract arguments | `age('2001-04-10', timestamp '1957-06-13')` | `43 years 9 mons 27 days`
`current_date` | `date` | Today's date; see below |  |
`current_time` | `time` | Time of day; see below |  |
`current_timestamp` | `timestamp` | Date and time; see below |  |
`date_part`(`text`, `timestamp`) | `double precision` | Get subfield (equivalent to `extract`); see also below | `date_part('hour', timestamp '2001-02-16 20:38:40')` | `20`
`date_part`(`text`, `interval`) | `double precision` | Get subfield (equivalent to `extract`); see also below | `date_part('month', interval '2 years 3 months')` | `3`
`date_trunc`(`text`, `timestamp`) | `timestamp` | Truncate to specified precision; see also below | `date_trunc('hour', timestamp '2001-02-16 20:38:40')` | `2001-02-16 20:00:00+00`
`extract`(`field` from `timestamp`) | `double precision` | Get subfield; see also below | `extract(hour from timestamp '2001-02-16 20:38:40')` | `20`
`extract`(`field` from `interval`) | `double precision` | Get subfield; see also below | `extract(month from interval '2 years 3 months')` | `3`
`isfinite`(`timestamp`) | `boolean` | Test for finite time stamp (neither invalid nor infinity) | `isfinite(timestamp '2001-02-16 21:28:30')` | `true`
`isfinite`(`interval`) | `boolean` | Test for finite interval | `isfinite(interval '4 hours')` | `true`
`now`() | `timestamp` | Current date and time (equivalent to `current_timestamp`); see below |  |
`timeofday()` | `text` | Current date and time; see below | `timeofday()` | `Wed Feb 21 17:01:13.000126 2001 EST`
`timestamp`(`date`) | `timestamp` | `date` to `timestamp` | `timestamp(date '2000-12-25')` | `2000-12-25 00:00:00`
`timestamp`(`date`, `time`) | `timestamp` | `date` and `time` to `timestamp` | `timestamp(date '1998-02-24',time '23:07')` | `1998-02-24 23:07:00`

## 4.8.1. `EXTRACT`, `date_part`

    EXTRACT (field FROM source)

The `extract` function retrieves sub-fields from date/time values, such as year or hour. `source` is a value expression that evaluates to type `timestamp` or `interval`. (Expressions of type `date` or `time` 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 values:

`century`

  The year field divided by 100

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

  Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.

`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

`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 (Result may be negative.); for `interval` values, the total number of seconds in the interval

      SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 982352320

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

`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 year field divided by 1000

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

  Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.

`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_hour`

  The hour component of the time zone offset.

`timezone_minute`

  The minute component of the time zone offset.

`week`

  From a `timestamp` value, calculate 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 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.

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

`year`

  The year field

      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 4.7.

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

    date_part('field', source)

Note that here the `field` value needs to be a string. The valid field values 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

## 4.8.2. `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` (values of type `date` and `time` are cast automatically). `field` selects to which precision to truncate the time stamp value. The return value is of type `timestamp` with all fields that are less than the selected one set to zero (or one, for day and month).

Valid values for `field` are:

microseconds
milliseconds
second
minute
hour
day
month
year
decade
century
millennium

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

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

## 4.8.3. 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 )

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

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

> Note: The SQL99 standard requires these functions to be written without any parentheses, unless a precision parameter is given. As of PostgreSQL 7.2, an empty pair of parentheses can be written, but this is deprecated and may be removed in a future release.

    SELECT CURRENT_TIME;
    14:39:53.662522-05

    SELECT CURRENT_DATE;
    2001-12-23

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

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

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

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

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

It is quite important to realize that `CURRENT_TIMESTAMP` and related functions all return the time as of the start of the current transaction; their values do not increment while a transaction is running. But `timeofday()` returns the actual current time.

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';

> Note: You do not want to use the third form when specifying a DEFAULT value 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

Prev | Home | Next
Data Type Formatting Functions | Up | Geometric Functions and Operators