Table 9.26, “Date/Time Functions” shows the available
functions for date/time value processing, with details appearing in
the following subsections. Table 9.25, “Date/Time Operators” illustrates the behaviors of
the basic arithmetic operators (+,
*, etc.). For formatting functions, refer to
Section 9.8, “Data Type Formatting Functions”. You should be familiar with
the background information on date/time data types from Section 8.5, “Date/Time Types”.
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 |
|---|---|---|---|---|
|
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 |
|
interval |
Subtract from current_date
|
age(timestamp '1957-06-13') |
43 years 8 mons 3 days |
|
date |
Today's date; see Section 9.9.4, “Current Date/Time” | ||
|
time with time zone |
Time of day; see Section 9.9.4, “Current Date/Time” | ||
|
timestamp with time zone |
Date and time; see Section 9.9.4, “Current Date/Time” | ||
|
double precision |
Get subfield (equivalent to
extract); see Section 9.9.1, “EXTRACT, date_part”
|
date_part('hour', timestamp '2001-02-16 20:38:40') |
20 |
|
double precision |
Get subfield (equivalent to
extract); see Section 9.9.1, “EXTRACT, date_part”
|
date_part('month', interval '2 years 3 months') |
3 |
|
timestamp |
Truncate to specified precision; see also Section 9.9.2, “date_trunc”
|
date_trunc('hour', timestamp '2001-02-16 20:38:40') |
2001-02-16 20:00:00 |
|
double precision |
Get subfield; see Section 9.9.1, “EXTRACT, date_part”
|
extract(hour from timestamp '2001-02-16 20:38:40') |
20 |
|
double precision |
Get subfield; see Section 9.9.1, “EXTRACT, date_part”
|
extract(month from interval '2 years 3 months') |
3 |
|
boolean |
Test for finite time stamp (not equal to infinity) | isfinite(timestamp '2001-02-16 21:28:30') |
true |
|
boolean |
Test for finite interval | isfinite(interval '4 hours') |
true |
|
interval |
Adjust interval so 24-hour time periods are represented as days | justify_hours(interval '24 hours') |
1 day |
|
interval |
Adjust interval so 30-day time periods are represented as months | justify_days(interval '30 days') |
1 month |
|
time |
Time of day; see Section 9.9.4, “Current Date/Time” | ||
|
timestamp |
Date and time; see Section 9.9.4, “Current Date/Time” | ||
|
timestamp with time zone |
Current date and time (equivalent to
current_timestamp); see Section 9.9.4, “Current Date/Time”
|
||
|
text |
Current date and time; see Section 9.9.4, “Current Date/Time” |
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.
EXTRACT(fieldFROMsource)
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:
centuryThe 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.
dayThe day (of the month) field (1 - 31)
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 16
decadeThe 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';
hourThe hour field (0 - 23)
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 20
microsecondsThe 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
millenniumThe 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.
millisecondsThe 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
minuteThe 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
secondThe seconds field, including fractional parts (0 - 59[5])
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40'); Result:40SELECT EXTRACT(SECOND FROM TIME '17:12:28.5'); Result:28.5
timezoneThe 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_hourThe hour component of the time zone offset
timezone_minuteThe 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, “Data Type Formatting Functions”.
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
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 20:00:00
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
Result: 2001-01-01 00:00:00
The AT TIME ZONE construct allows conversions
of time stamps to different time zones. Table 9.27, “AT TIME ZONE Variants” shows its
variants.
Table 9.27. AT TIME ZONE Variants
| Expression | Return Type | Description |
|---|---|---|
|
timestamp with time zone |
Treat given time stamp without time zone as located in the specified time zone |
|
timestamp without time zone |
Convert given time stamp with time zone to the new time 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, “Time Zone Names for Setting timezone” or
Table B.4, “Time Zone Abbreviations for Input”.
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 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 is equivalent to the SQL-conforming construct
timezone(zone,
timestamp).
timestamp AT TIME ZONE
zone
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.
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.
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.
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
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.