PostgreSQL 8.0.1 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: trueSELECT (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: 20SELECT 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: 982384720SELECT 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: 2SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');Result: 3SELECT 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: 40SELECT 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: 16SELECT 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 20:00:00SELECT 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 19:38:40-08SELECT 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_DATECURRENT_TIMECURRENT_TIMESTAMPCURRENT_TIME ( precision )CURRENT_TIMESTAMP ( precision )LOCALTIMELOCALTIMESTAMPLOCALTIME ( 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: 14:39:53.662522-05SELECT CURRENT_DATE;Result: 2001-12-23SELECT CURRENT_TIMESTAMP;Result: 2001-12-23 14:39:53.662522-05SELECT CURRENT_TIMESTAMP(2);Result: 2001-12-23 14:39:53.66-05SELECT 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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9.9. Date/Time Functions and Operators

9.9.1. EXTRACT, date_part

9.9.2. date_trunc

9.9.3. AT TIME ZONE

9.9.4. Current Date/Time

Notes

9.9.1. `EXTRACT`, `date_part`

9.9.2. `date_trunc`

9.9.3. `AT TIME ZONE`