Convert HTML docs to more streamlined TXT

[ai-pg] / full-docs / txt / functions-datetime.txt

9.9. Date/Time Functions and Operators #

   9.9.1. EXTRACT, date_part
   9.9.2. date_trunc
   9.9.3. date_bin
   9.9.4. AT TIME ZONE and AT LOCAL
   9.9.5. Current Date/Time
   9.9.6. Delaying Execution

   Table 9.33 shows the available functions for date/time value
   processing, with details appearing in the following subsections.
   Table 9.32 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.

   In addition, the usual comparison operators shown in Table 9.1 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.

   Table 9.32. Date/Time Operators

   Operator

   Description

   Example(s)

   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

   Table 9.33. Date/Time Functions

   Function

   Description

   Example(s)

   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 ( ) → timestamp with time zone

   Current date and time (changes during statement execution); see
   Section 9.9.5

   clock_timestamp() → 2019-12-23 14:39:53.662522-05

   current_date → date

   Current date; see Section 9.9.5

   current_date → 2019-12-23

   current_time → time with time zone

   Current time of day; see Section 9.9.5

   current_time → 14:39:53.662522-05

   current_time ( integer ) → time with time zone

   Current time of day, with limited precision; see Section 9.9.5

   current_time(2) → 14:39:53.66-05

   current_timestamp → timestamp with time zone

   Current date and time (start of current transaction); see Section 9.9.5

   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 9.9.5

   current_timestamp(0) → 2019-12-23 14:39:53-05

   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 ( interval, timestamp, timestamp ) → timestamp

   Bin input into specified interval aligned with specified origin; see
   Section 9.9.3

   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 ( text, timestamp ) → double precision

   Get timestamp 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 interval subfield (equivalent to extract); see Section 9.9.1

   date_part('month', interval '2 years 3 months') → 3

   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 ( text, timestamp ) → timestamp

   Truncate to specified precision; see Section 9.9.2

   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 9.9.2

   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 9.9.2

   date_trunc('hour', interval '2 days 3 hours 40 minutes') → 2 days
   03:00:00

   extract ( field from timestamp ) → numeric

   Get timestamp subfield; see Section 9.9.1

   extract(hour from timestamp '2001-02-16 20:38:40') → 20

   extract ( field from interval ) → numeric

   Get interval subfield; see Section 9.9.1

   extract(month from interval '2 years 3 months') → 3

   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 (not +/-infinity)

   isfinite(interval '4 hours') → true

   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 ( 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 ( 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 → time

   Current time of day; see Section 9.9.5

   localtime → 14:39:53.662522

   localtime ( integer ) → time

   Current time of day, with limited precision; see Section 9.9.5

   localtime(0) → 14:39:53

   localtimestamp → timestamp

   Current date and time (start of current transaction); see Section 9.9.5

   localtimestamp → 2019-12-23 14:39:53.662522

   localtimestamp ( integer ) → timestamp

   Current date and time (start of current transaction), with limited
   precision; see Section 9.9.5

   localtimestamp(2) → 2019-12-23 14:39:53.66

   make_date ( year int, month int, 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 ( [ years int [, months int [, weeks int [, days int [,
   hours int [, mins int [, 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 ( hour int, min int, sec double precision ) → time

   Create time from hour, minute and seconds fields

   make_time(8, 15, 23.5) → 08:15:23.5

   make_timestamp ( year int, month int, day int, hour int, min int, 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 ( year int, month int, day int, hour int, min int, sec
   double 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 ( ) → timestamp with time zone

   Current date and time (start of current transaction); see Section 9.9.5

   now() → 2019-12-23 14:39:53.662522-05

   statement_timestamp ( ) → timestamp with time zone

   Current date and time (start of current statement); see Section 9.9.5

   statement_timestamp() → 2019-12-23 14:39:53.662522-05

   timeofday ( ) → text

   Current date and time (like clock_timestamp, but as a text string); see
   Section 9.9.5

   timeofday() → Mon Dec 23 14:39:53.662522 2019 EST

   transaction_timestamp ( ) → timestamp with time zone

   Current date and time (start of current transaction); see Section 9.9.5

   transaction_timestamp() → 2019-12-23 14:39:53.662522-05

   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

   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. PostgreSQL'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

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,
   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 B.7 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; for interval
          values, the month field divided by 3 plus 1

SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 1
SELECT EXTRACT(QUARTER FROM INTERVAL '1 year 6 months');
Result: 3

   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, PostgreSQL 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.

          For interval values, the week field is simply the number of
          integral days divided by 7.

SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 7
SELECT EXTRACT(WEEK FROM INTERVAL '13 days 24 hours');
Result: 1

   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 timestamp inputs; epoch, hour, day, year,
   decade, century, and millennium for interval inputs). For other fields,
   NULL is returned. PostgreSQL versions before 9.6 returned zero for all
   cases of infinite input.

   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.
   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

9.9.2. 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 8.5.3.

   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', 'Aus
tralia/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

9.9.3. 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 '2001-0
1-01');
Result: 2020-02-11 15:30:00
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-0
1-01 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.

9.9.4. AT TIME ZONE and AT LOCAL #

   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. Table 9.34 shows its variants.

   Table 9.34. AT TIME ZONE and AT LOCAL Variants

   Operator

   Description

   Example(s)

   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 without time zone AT LOCAL → timestamp with time zone

   Converts given time stamp without time zone to time stamp with the
   session's TimeZone value as time zone.

   timestamp '2001-02-16 20:38:40' at local → 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

   timestamp with time zone AT LOCAL → timestamp without time zone

   Converts given time stamp with time zone to time stamp without time
   zone, as the time would appear with the session's TimeZone value as
   time zone.

   timestamp with time zone '2001-02-16 20:38:40-05' at local → 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

   time with time zone AT LOCAL → 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 session's
   TimeZone value.

   Assuming the session's TimeZone is set to UTC:

   time with time zone '05:34:17-05' at local → 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 8.5.3. The interval
   case is only useful for zones that have fixed offsets from UTC, so it
   is not very common in practice.

   The syntax AT LOCAL may be used as shorthand for AT TIME ZONE local,
   where local is the session's TimeZone value.

   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/D
enver';
Result: 2001-02-16 18:38:40
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'A
merica/Chicago';
Result: 2001-02-16 05:38:40
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT LOCAL;
Result: 2001-02-16 17:38:40
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE '+05';
Result: 2001-02-16 20:38:40
SELECT TIME WITH TIME ZONE '20:38:40-05' AT LOCAL;
Result: 17:38:40

   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
   fourth example shifts the time stamp with time zone value to the time
   zone currently specified by the TimeZone setting and returns the value
   without a time zone. The fifth example demonstrates that the sign in a
   POSIX-style time zone specification has the opposite meaning of the
   sign in an ISO-8601 datetime literal, as described in Section 8.5.3 and
   Appendix B.

   The sixth example is a cautionary tale. Due to the fact that there is
   no date associated with the input value, the conversion is made using
   the current date of the session. Therefore, this static example may
   show a wrong result depending on the time of the year it is viewed
   because 'America/Los_Angeles' observes Daylight Savings Time.

   The function timezone(zone, timestamp) is equivalent to the
   SQL-conforming construct timestamp AT TIME ZONE zone.

   The function timezone(zone, time) is equivalent to the SQL-conforming
   construct time AT TIME ZONE zone.

   The function timezone(timestamp) is equivalent to the SQL-conforming
   construct timestamp AT LOCAL.

   The function timezone(time) is equivalent to the SQL-conforming
   construct time AT LOCAL.

9.9.5. Current Date/Time #

   PostgreSQL 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: 2019-12-23
SELECT CURRENT_TIMESTAMP;
Result: 2019-12-23 14:39:53.662522-05
SELECT CURRENT_TIMESTAMP(2);
Result: 2019-12-23 14:39:53.66-05
SELECT LOCALTIMESTAMP;
Result: 2019-12-23 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.

   PostgreSQL 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 PostgreSQL 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 PostgreSQL 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 8.5.1.4.)

9.9.6. 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.