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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 01:00:00'
+ date '2001-09-28' + time '03:00' timestamp '2001-09-28 03:00:00'
+ interval '1 day' + interval '1 hour' interval '1 day 01:00:00'
+ timestamp '2001-09-28 01:00' + interval '23 hours' timestamp '2001-09-29 00:00:00'
+ time '01:00' + interval '3 hours' time '04:00:00'
- - interval '23 hours' interval '-23:00:00'
- 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 23:00:00'
- time '05:00' - time '03:00' interval '02:00:00'
- time '05:00' - interval '2 hours' time '03:00:00'
- timestamp '2001-09-28 23:00' - interval '23 hours' timestamp '2001-09-28 00:00:00'
- interval '1 day' - interval '1 hour' interval '1 day -01:00:00'
- timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00' interval '1 day 15:00:00'
* 900 * interval '1 second' interval '00:15:00'
* 21 * interval '1 day' interval '21 days'
* double precision '3.5' * interval '1 hour' interval '03:30:00'
/ interval '1 hour' / double precision '1.5' interval '00:40:00'

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
justify_hours(interval) interval Adjust interval so 24-hour time periods are represented as days justify_hours(interval '24 hours') 1 day
justify_days(interval) interval Adjust interval so 30-day time periods are represented as months justify_days(interval '30 days') 1 month
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    

If you are using both justify_hours and justify_days, it is best to use justify_hours first so any additional days will be included in the justify_days calculation.

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

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

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)

Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005.

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
quarter
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 Treat given time stamp without time zone as located in the specified time zone
timestamp with time zone AT TIME ZONE zone timestamp without time zone Convert given time stamp with time zone to the new time zone
time with time zone AT TIME ZONE zone time with time zone Convert given time with time zone to the new time zone

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 either Table B-6 or Table B-4.

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 time stamp without time zone and interprets it as MST time (UTC-7), which is then converted 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.

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.

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

There is also the function timeofday() which returns the wall-clock time and advances during transactions. For historical reasons timeofday() returns a text string rather than a timestamp value:

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

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';  -- incorrect for use with DEFAULT

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

Comments


Jan. 13, 2006, 1:26 p.m.

The EXTRACT(EPOCH FROM INTERVAL ...) is a nice way to convert a timestamp to an int, i.e.

SELECT EXTRACT(EPOCH FROM INTERVAL '2 years 3 months') / (24*60*60)

will give the number of days in the interval (~821)

(Wouldn't it be great if (SELECT INTERVAL '2 years 3 months' / INTERAL '1 day') was supported?)


March 13, 2006, 8:43 p.m.

Here's a way to create an INTERVAL with an INT and a CHAR:

CAST(textcat(text(2), text(' seconds')) as interval)

You can use it thus:

SELECT current_timestamp + CAST(textcat(text(2), text(' seconds')) as interval)


June 22, 2006, 9:54 p.m.

If you use MS Access as GUI frontend by linked tables, Access will not allow update of a row if the row has timestamp field, like the one created by now(). The problem is the microsecond part of the timestamp as generally known. If you use timenow() instead, there'll be no microsecond part and Access will no more complain and allow updates. (The column type may does not need to change.)

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