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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

Comments


Oct. 31, 2002, 3:50 a.m.

A note about the use of "now" within stored procedures...it is evaluated at compile time only and then maintains that value for the life of the function. To get around this problem, use CURRENT_TIMESTAMP instead.


Nov. 19, 2002, 11:17 a.m.

Note a change between 7.1 and 7.2

Intervals have somewhat different syntax.

The following works with 7.1, but not 7.2:

select created_on + interval( period_value||'year' ) as expiry from table_with_duration;

The equivalent that /does/ work with 7.2 is the following:

select created_on + (period_value || 'year')::interval as expiry from table_with_duration ;

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