Date/Time Functions and Operators
Section Date/Time Functions shows the available functions for date/time value
processing, with details appearing in the following subsections. Section Date/Time Operators
illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For
formatting functions, refer to Section Data Type Formatting Functions. You
should be familiar with the background information on date/time data types from
Section Date/Time Types.
In addition, the usual comparison operators shown in Section Comparison Operators are available for the date/time types. Dates and timestamps (with or without time zone) are all comparable, while times (with or without time zone) and intervals can only be compared to other values of the same data type. When comparing a timestamp without time zone to a timestamp with time zone, the former value is assumed to be given in the time zone specified by the TimeZone configuration parameter, and is rotated to UTC for comparison to the latter value (which is already in UTC internally). Similarly, a date value is assumed to represent midnight in the TimeZone zone when comparing it to a timestamp.
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.
Date/Time Operators
date + integer → date
Add a number of days to a date.
date '2001-09-28' + 7 → 2001-10-05
date + interval → timestamp
Add an interval to a date.
date '2001-09-28' + interval '1 hour' → 2001-09-28 01:00:00
date + time → timestamp
Add a time-of-day to a date.
date '2001-09-28' + time '03:00' → 2001-09-28 03:00:00
interval + interval → interval
Add intervals.
interval '1 day' + interval '1 hour' → 1 day 01:00:00
timestamp + interval → timestamp
Add an interval to a timestamp.
timestamp '2001-09-28 01:00' + interval '23 hours' → 2001-09-29 00:00:00
time + interval → time
Add an interval to a time.
time '01:00' + interval '3 hours' → 04:00:00
- interval → interval
Negate an interval.
- interval '23 hours' → -23:00:00
date - date → integer
Subtract dates, producing the number of days elapsed.
date '2001-10-01' - date '2001-09-28' → 3
date - integer → date
Subtract a number of days from a date.
date '2001-10-01' - 7 → 2001-09-24
date - interval → timestamp
Subtract an interval from a date.
date '2001-09-28' - interval '1 hour' → 2001-09-27 23:00:00
time - time → interval
Subtract times.
time '05:00' - time '03:00' → 02:00:00
time - interval → time
Subtract an interval from a time.
time '05:00' - interval '2 hours' → 03:00:00
timestamp - interval → timestamp
Subtract an interval from a timestamp.
timestamp '2001-09-28 23:00' - interval '23 hours' → 2001-09-28 00:00:00
interval - interval → interval
Subtract intervals.
interval '1 day' - interval '1 hour' → 1 day -01:00:00
timestamp - timestamp → interval
Subtract timestamps (converting 24-hour intervals into days, similarly to justify_hours()).
timestamp '2001-09-29 03:00' - timestamp '2001-07-27 12:00' → 63 days 15:00:00
interval * double precision → interval
Multiply an interval by a scalar.
interval '1 second' * 900 → 00:15:00
interval '1 day' * 21 → 21 days
interval '1 hour' * 3.5 → 03:30:00
interval / double precision → interval
Divide an interval by a scalar.
interval '1 hour' / 1.5 → 00:40:00
Date/Time Functions
age
age ( timestamp, timestamp ) → interval
Subtract arguments, producing a “symbolic” result that uses years and months, rather than just days.
age(timestamp '2001-04-10', timestamp '1957-06-13') → 43 years 9 mons 27 days
age ( timestamp ) → interval
Subtract argument from current_date (at midnight).
age(timestamp '1957-06-13') → 62 years 6 mons 10 days
clock_timestamp
clock_timestamp ( ) → timestamp with time zone
Current date and time (changes during statement execution); see Section Current Date/Time.
clock_timestamp() → 2019-12-23 14:39:53.662522-05
current_date
current_date → date
Current date; see Section Current Date/Time.
current_date → 2019-12-23
current_time
current_time → time with time zone
Current time of day; see Section Current Date/Time.
current_time → 14:39:53.662522-05
current_time ( integer ) → time with time zone
Current time of day, with limited precision; see Section Current Date/Time.
current_time(2) → 14:39:53.66-05
current_timestamp
current_timestamp → timestamp with time zone
Current date and time (start of current transaction); see Section Current Date/Time.
current_timestamp → 2019-12-23 14:39:53.662522-05
current_timestamp ( integer ) → timestamp with time zone
Current date and time (start of current transaction), with limited precision; see Section Current Date/Time.
current_timestamp(0) → 2019-12-23 14:39:53-05
date_add
date_add ( timestamp with time zone, interval [, text ] ) → timestamp with time zone
Add an interval to a timestamp with time zone, computing times of day and
daylight-savings adjustments according to the time zone named by the third
argument, or the current TimeZone setting if that is omitted. The form with
two arguments is equivalent to the timestamp with time zone + interval operator.
date_add('2021-10-31 00:00:00+02'::timestamptz, '1 day'::interval, 'Europe/Warsaw') → 2021-10-31 23:00:00+00
date_bin
date_bin ( interval, timestamp, timestamp ) → timestamp
Bin input into specified interval aligned with specified origin; see Section Function date_bin.
date_bin('15 minutes', timestamp '2001-02-16 20:38:40', timestamp '2001-02-16 20:05:00') → 2001-02-16 20:35:00
date_part
date_part ( text, timestamp ) → double precision
Get timestamp subfield (equivalent to extract); see Section EXTRACT, date_part.
date_part('hour', timestamp '2001-02-16 20:38:40') → 20
date_part ( text, interval ) → double precision
Get interval subfield (equivalent to extract); see Section EXTRACT, date_part.
date_part('month', interval '2 years 3 months') → 3
date_subtract
date_subtract ( timestamp with time zone, interval [, text ] ) → timestamp with time zone
Subtract an interval from a timestamp with time zone, computing times of day
and daylight-savings adjustments according to the time zone named by the third
argument, or the current TimeZone setting if that is omitted. The form with
two arguments is equivalent to the timestamp with time zone - interval operator.
date_subtract('2021-11-01 00:00:00+01'::timestamptz, '1 day'::interval, 'Europe/Warsaw') → 2021-10-30 22:00:00+00
date_trunc
date_trunc ( text, timestamp ) → timestamp
Truncate to specified precision; see Section Function date_trunc.
date_trunc('hour', timestamp '2001-02-16 20:38:40') → 2001-02-16 20:00:00
date_trunc ( text, timestamp with time zone, text ) → timestamp with time zone
Truncate to specified precision in the specified time zone; see Section Function date_trunc.
date_trunc('day', timestamptz '2001-02-16 20:38:40+00', 'Australia/Sydney') → 2001-02-16 13:00:00+00
date_trunc ( text, interval ) → interval
Truncate to specified precision; see Section Function date_trunc.
date_trunc('hour', interval '2 days 3 hours 40 minutes') → 2 days 03:00:00
extract
extract ( field from timestamp ) → numeric
Get timestamp subfield; see Section EXTRACT, date_part.
extract(hour from timestamp '2001-02-16 20:38:40') → 20
extract ( field from interval ) → numeric
Get interval subfield; see Section EXTRACT, date_part.
extract(month from interval '2 years 3 months') → 3
isfinite
isfinite ( date ) → boolean
Test for finite date (not +/-infinity).
isfinite(date '2001-02-16') → true
isfinite ( timestamp ) → boolean
Test for finite timestamp (not +/-infinity).
isfinite(timestamp 'infinity') → false
isfinite ( interval ) → boolean
Test for finite interval (currently always true).
isfinite(interval '4 hours') → true
justify_days
justify_days ( interval ) → interval
Adjust interval, converting 30-day time periods to months.
justify_days(interval '1 year 65 days') → 1 year 2 mons 5 days
justify_hours
justify_hours ( interval ) → interval
Adjust interval, converting 24-hour time periods to days.
justify_hours(interval '50 hours 10 minutes') → 2 days 02:10:00
justify_interval
justify_interval ( interval ) → interval
Adjust interval using justify_days and justify_hours, with additional sign adjustments.
justify_interval(interval '1 mon -1 hour') → 29 days 23:00:00
localtime
localtime → time
Current time of day; see Section Current Date/Time.
localtime → 14:39:53.662522
localtime ( integer ) → time
Current time of day, with limited precision; see Section Current Date/Time.
localtime(0) → 14:39:53
localtimestamp
localtimestamp → timestamp
Current date and time (start of current transaction); see Section Current Date/Time.
localtimestamp → 2019-12-23 14:39:53.662522
localtimestamp ( integer ) → timestamp
Current date and time (start of current transaction), with limited precision; see Section Current Date/Time.
localtimestamp(2) → 2019-12-23 14:39:53.66
make_date
make_date ( yearint,monthint,day int ) → date
Create date from year, month and day fields (negative years signify BC).
make_date(2013, 7, 15) → 2013-07-15
make_interval
make_interval ( [ yearsint [,monthsint [,weeksint [,daysint [,hoursint [,minsint [,secs double precision ]]]]]]] ) → interval
Create interval from years, months, weeks, days, hours, minutes and seconds fields, each of which can default to zero.
make_interval(days => 10) → 10 days
make_time
make_time ( hourint,minint,sec double precision ) → time
Create time from hour, minute and seconds fields.
make_time(8, 15, 23.5) → 08:15:23.5
make_timestamp
make_timestamp ( yearint,monthint,dayint,hourint,minint,sec double precision ) → timestamp
Create timestamp from year, month, day, hour, minute and seconds fields (negative years signify BC).
make_timestamp(2013, 7, 15, 8, 15, 23.5) → 2013-07-15 08:15:23.5
make_timestamptz
make_timestamptz ( yearint,monthint,dayint,hourint,minint,secdouble precision [,timezone text ] ) → timestamp with time zone
Create timestamp with time zone from year, month, day, hour, minute and seconds fields (negative years signify BC). If timezone is not specified, the current time zone is used; the examples assume the session time zone is Europe/London.
make_timestamptz(2013, 7, 15, 8, 15, 23.5) → 2013-07-15 08:15:23.5+01
make_timestamptz(2013, 7, 15, 8, 15, 23.5, 'America/New\_York') → 2013-07-15 13:15:23.5+01
now
now ( ) → timestamp with time zone
Current date and time (start of current transaction); see Section Current Date/Time.
now() → 2019-12-23 14:39:53.662522-05
statement_timestamp
statement_timestamp ( ) → timestamp with time zone
Current date and time (start of current statement); see Section Current Date/Time.
statement_timestamp() → 2019-12-23 14:39:53.662522-05
timeofday
timeofday ( ) → text
Current date and time (like clock_timestamp, but as a text string); see Section Current Date/Time.
timeofday() → Mon Dec 23 14:39:53.662522 2019 EST
transaction_timestamp
transaction_timestamp ( ) → timestamp with time zone
Current date and time (start of current transaction); see Section Current Date/Time.
transaction_timestamp() → 2019-12-23 14:39:53.662522-05
to_timestamp
to_timestamp ( double precision ) → timestamp with time zone
Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp with time zone.
to_timestamp(1284352323) → 2010-09-13 04:32:03+00
Notes
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. When a pair of values is provided, either the start or the end
can be written first; OVERLAPS automatically takes the earlier value of the
pair as the start. Each time period is considered to represent the half-open
interval start <= time < end, unless start and end are
equal in which case it represents that single time instant. This means for
instance that two time periods with only an endpoint in common do not overlap.
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
SELECT (DATE '2001-10-29', DATE '2001-10-30') OVERLAPS
(DATE '2001-10-30', DATE '2001-10-31');
Result: false
SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS
(DATE '2001-10-30', DATE '2001-10-31');
Result: true
When adding an interval value to (or subtracting an interval value from) a
timestamp or timestamp with time zone value, the months, days, and microseconds
fields of the interval value are handled in turn. First, a nonzero months field
advances or decrements the date of the timestamp by the indicated number of months,
keeping the day of month the same unless it would be past the end of the new month,
in which case the last day of that month is used. (For example, March 31 plus 1
month becomes April 30, but March 31 plus 2 months becomes May 31.) Then the days
field advances or decrements the date of the timestamp by the indicated number
of days. In both these steps the local time of day is kept the same. Finally, if
there is a nonzero microseconds field, it is added or subtracted literally. When
doing arithmetic on a timestamp with time zone value in a time zone that
recognizes DST, this means that adding or subtracting (say) interval '1 day'
does not necessarily have the same result as adding or subtracting interval '24 hours'. For example, with the session time zone set to America/Denver:
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '1 day';
Result: 2005-04-03 12:00:00-06
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '24 hours';
Result: 2005-04-03 13:00:00-06
This happens because an hour was skipped due to a change in daylight saving time at 2005-04-03 02:00:00 in time zone America/Denver.
Note there can be ambiguity in the months field returned by age because
different months have different numbers of days. QHB's approach
uses the month from the earlier of the two dates when calculating partial months.
For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day,
while using May would yield 1 mon 2 days because May has 31 days, while April
has only 30.
Subtraction of dates and timestamps can also be complex. One conceptually simple
way to perform subtraction is to convert each value to a number of seconds using
EXTRACT(EPOCH FROM ...), then subtract the results; this produces the number of
seconds between the two values. This will adjust for the number of days in
each month, timezone changes, and daylight saving time adjustments. Subtraction
of date or timestamp values with the “-” operator returns the number of days
(24-hours) and hours/minutes/seconds between the values, making the same
adjustments. The age function returns years, months, days, and hours/minutes/
seconds, performing field-by-field subtraction and then adjusting for negative
field values. The following queries illustrate the differences in these approaches.
The sample results were produced with timezone = 'US/Eastern'; there is a
daylight saving time change between the two dates used:
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
Result: 10537200.000000
SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'))
/ 60 / 60 / 24;
Result: 121.9583333333333333
SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00';
Result: 121 days 23:00:00
SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00');
Result: 4 mons
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, date, time, or interval. (Timestamps and times can be with or without time zone.) field is an identifier or string that selects what field to extract from the source value. Not all fields are valid for every input data type; for example, fields smaller than a day cannot be extracted from a date, while fields of a day or more cannot be extracted from a time. The extract function returns values of type numeric.
The following are valid field names:
century
The century; for interval values, the year field divided by 100
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
SELECT EXTRACT(CENTURY FROM DATE '0001-01-01 AD');
Result: 1
SELECT EXTRACT(CENTURY FROM DATE '0001-12-31 BC');
Result: -1
SELECT EXTRACT(CENTURY FROM INTERVAL '2001 years');
Result: 20
day
The day of the month (1–31); for interval values, the number of days
SELECT EXTRACT(DAY FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 16
SELECT EXTRACT(DAY FROM INTERVAL ’40 days 1 minute’);
Result: 40
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 as Sunday (0) to Saturday (6)
SELECT EXTRACT(DOW FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 5
Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.
doy
The day of the year (1–365/366)
SELECT EXTRACT(DOY FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 47
epoch
For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (negative for timestamps before that); for date and timestamp values, the nominal number of seconds since 1970-01-01 00:00:00, without regard to timezone or daylight-savings rules; for interval values, the total number of seconds in the interval
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08');
Result: 982384720.120000
SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12');
Result: 982355920.120000
SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
Result: 442800.000000
You can convert an epoch value back to a timestamp with time zone with to_timestamp:
SELECT to_timestamp(982384720.12);
Result: 2001-02-17 04:38:40.12+00
Beware that applying to_timestamp to an epoch extracted from a date or timestamp value could produce a misleading result: the result will effectively assume that the original value had been given in UTC, which might not be the case.
hour
The hour field (0–23 in timestamps, unrestricted in intervals)
SELECT EXTRACT(HOUR FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 20
isodow
The day of the week as Monday (1) to Sunday (7)
SELECT EXTRACT(ISODOW FROM TIMESTAMP ’2001-02-18 20:38:40’);
Result: 7
This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.
isoyear
The ISO 8601 week-numbering year that the date falls in
SELECT EXTRACT(ISOYEAR FROM DATE ’2006-01-01’);
Result: 2005
SELECT EXTRACT(ISOYEAR FROM DATE ’2006-01-02’);
Result: 2006
Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.
julian
The Julian Date corresponding to the date or timestamp. Timestamps that are not local midnight result in a fractional value. See Section Julian Dates for more information.
SELECT EXTRACT(JULIAN FROM DATE '2006-01-01');
Result: 2453737
SELECT EXTRACT(JULIAN FROM TIMESTAMP '2006-01-01 12:00');
Result: 2453737.50000000000000000000
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; for interval values, the year field divided by 1000
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 3
SELECT EXTRACT(MILLENNIUM FROM INTERVAL '2001 years');
Result: 2
Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.
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: 28500.000
minute
The minutes field (0-59)
SELECT EXTRACT(MINUTE FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 38
month
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 date is in
SELECT EXTRACT(QUARTER FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 1
second
The seconds field, including any fractional seconds
SELECT EXTRACT(SECOND FROM TIMESTAMP ’2001-02-16 20:38:40’);
Result: 40.000000
SELECT EXTRACT(SECOND FROM TIME ’17:12:28.5’);
Result: 28.500000
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. (Technically, QHB does not use UTC because leap seconds are not handled.)
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 ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.
In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next 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, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.
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
When processing an interval value, the extract function produces field values that match the interpretation used by the interval output function. This can produce surprising results if one starts with a non-normalized interval representation, for example:
SELECT INTERVAL '80 minutes';
Result: 01:20:00
SELECT EXTRACT(MINUTES FROM INTERVAL '80 minutes');
Result: 20
Note
When the input value is +/-Infinity, extract returns +/-Infinity for monotonically-increasing fields (epoch, julian, year, isoyear, decade, century, and millennium). For other fields, NULL is returned.
The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 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. For historical reasons, the date_part function returns values of type double precision. This can result in a loss of precision in certain uses. Using extract is recommended instead.
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
Result: 16
SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
Result: 4
Function date_trunc
The function date_trunc is conceptually similar to the trunc function for numbers.
date_trunc(field, source [, time_zone ])
source is a value expression of type timestamp, timestamp with time zone, 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 likewise of type timestamp, timestamp with time zone, or interval, and it has 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
When the input value is of type timestamp with time zone, the truncation is performed with respect to a particular time zone; for example, truncation to day produces a value that is midnight in that zone. By default, truncation is done with respect to the current TimeZone setting, but the optional time_zone argument can be provided to specify a different time zone. The time zone name can be specified in any of the ways described in Section Time Zones.
A time zone cannot be specified when processing timestamp without time zone or interval inputs. These are always taken at face value.
Examples (assuming the local time zone is America/New_York):
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
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
Result: 2001-02-16 00:00:00-05
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00', 'Australia/Sydney');
Result: 2001-02-16 08:00:00-05
SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
Result: 3 days 02:00:00
Function date_bin
The function date_bin “bins” the input timestamp into the specified interval (the stride) aligned with a specified origin.
date_bin(stride, source, origin)
source is a value expression of type timestamp or timestamp with time zone. (Values of type date are cast automatically to timestamp.) stride is a value expression of type interval. The return value is likewise of type timestamp or timestamp with time zone, and it marks the beginning of the bin into which the source is placed.
Examples:
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01');
Result: 2020-02-11 15:30:00
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01 00:02:30');
Result: 2020-02-11 15:32:30
In the case of full units (1 minute, 1 hour, etc.), it gives the same result as the analogous date_trunc call, but the difference is that date_bin can truncate to an arbitrary interval.
The stride interval must be greater than zero and cannot contain units of month or larger.
AT TIME ZONE
The AT TIME ZONE operator converts time stamp without time zone to/from
time stamp with time zone, and time with time zone values to different time
zones. Its variants are shown below.
timestamp without time zone AT TIME ZONE zone → timestamp with time zone
Converts given time stamp without time zone to time stamp with time zone, assuming the given value is in the named time zone.
timestamp '2001-02-16 20:38:40' at time zone 'America/Denver' → 2001-02-17 03:38:40+00
timestamp with time zone AT TIME ZONE zone → timestamp without time zone
Converts given time stamp with time zone to time stamp without time zone, as the time would appear in that zone.
timestamp with time zone '2001-02-16 20:38:40-05' at time zone 'America/Denver' → 2001-02-16 18:38:40
time with time zone AT TIME ZONE zone → time with time zone
Converts given time with time zone to a new time zone. Since no date is supplied, this uses the currently active UTC offset for the named destination zone.
time with time zone '05:34:17-05' at time zone 'UTC' → 10:34:17+00
In these expressions, the desired time zone zone can be specified either as a text value (e.g., 'America/Los_Angeles') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section Time Zones. The interval case is only useful for zones that have fixed offsets from UTC, so it is not very common in practice.
Examples (assuming the current TimeZone setting is America/Los_Angeles):
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver';
Result: 2001-02-16 19:38:40-08
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver';
Result: 2001-02-16 18:38:40
SELECT TIMESTAMP '2001-02-16 20:38:40-05' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago';
Result: 2001-02-16 05:38:40
The first example adds a time zone to a value that lacks it, and displays the value using the current TimeZone setting. The second example shifts the time stamp with time zone value to the specified time zone, and returns the value without a time zone. This allows storage and display of values different from the current TimeZone setting. The third example converts Tokyo time to Chicago time.
The function timezone(zone, timestamp) is equivalent to the SQL-conforming
construct timestamp AT TIME ZONE zone.
Current Date/Time
QHB provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:
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 take 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.
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
Since these 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 might advance these values more frequently.
QHB also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:
transaction_timestamp()
statement_timestamp()
clock_timestamp()
timeofday()
now()
transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first statement of a transaction, but might differ during subsequent statements. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL statement. timeofday() is a historical QHB function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional QHB equivalent to transaction_timestamp().
All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:
SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP ’now’; -- but see tip below
Tip
Do not use the third form when specifying a value to be evaluated later, for example in a DEFAULT clause for a table column. 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. (See also Section Special Values.)
Delaying Execution
The following functions are available to delay execution of the server process:
pg_sleep ( double precision )
pg_sleep_for ( interval )
pg_sleep_until ( timestamp with time zone )
pg_sleep makes the current session's process sleep until the given number of seconds have elapsed. Fractional-second delays can be specified. pg_sleep_for is a convenience function to allow the sleep time to be specified as an interval. pg_sleep_until is a convenience function for when a specific wake-up time is desired. For example:
SELECT pg_sleep(1.5);
SELECT pg_sleep_for('5 minutes');
SELECT pg_sleep_until('tomorrow 03:00');
Note
The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load. In particular, pg_sleep_until is not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.
WARNING!
Make sure that your session does not hold more locks than necessary when calling pg_sleep or its variants. Otherwise other sessions might have to wait for your sleeping process, slowing down the entire system.