6.8.1. EXTRACT, date_part

EXTRACT (field FROM source)

The extract function retrieves subfields 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:

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 6.7.

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 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

6.8.2. 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:

Examples:

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

6.8.3. AT TIME ZONE

The AT TIME ZONE construct allows conversions of timestamps to different timezones.

In these expressions, the desired time zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00').

Examples (supposing that TimeZone 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 timestamp and interprets it as MST time (GMT-7) to produce a UTC timestamp, which is then rotated to PST (GMT-8) for display. The second example takes a timestamp specified in EST (GMT-5) and converts it to local time in MST (GMT-7).

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

6.8.4. 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. Without a precision parameter, the result is given to the full available precision.

Some examples:

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

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

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 important to realize that CURRENT_TIMESTAMP and related functions return the start time of the current transaction; their values do not change during the transaction. timeofday() returns the wall clock time and does advance during transactions.

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';