## 9.9. Date/Time Functions and Operators

Table 9.30 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9.29 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 9.29. 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'` (days)
`-` | `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 9.30. Date/Time Functions

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
``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 from `current_date` (at midnight) | `age(timestamp '1957-06-13')` | `43 years 8 mons 3 days`
``clock_timestamp()`` | `timestamp with time zone` | Current date and time (changes during statement execution); see Section 9.9.4 |  |
``current_date`` | `date` | Current date; see Section 9.9.4 |  |
``current_time`` | `time with time zone` | Current time of day; see Section 9.9.4 |  |
``current_timestamp`` | `timestamp with time zone` | Current date and time (start of current transaction); 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`
``date_trunc(`text`, `interval`)`` | `interval` | Truncate to specified precision; see also Section 9.9.2 | `date_trunc('hour', interval '2 days 3 hours 40 minutes')` | `2 days 03:00:00`
``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(`date`)`` | `boolean` | Test for finite date (not +/-infinity) | `isfinite(date '2001-02-16')` | `true`
``isfinite(`timestamp`)`` | `boolean` | Test for finite time stamp (not +/-infinity) | `isfinite(timestamp '2001-02-16 21:28:30')` | `true`
``isfinite(`interval`)`` | `boolean` | Test for finite interval | `isfinite(interval '4 hours')` | `true`
``justify_days(`interval`)`` | `interval` | Adjust interval so 30-day time periods are represented as months | `justify_days(interval '35 days')` | `1 mon 5 days`
``justify_hours(`interval`)`` | `interval` | Adjust interval so 24-hour time periods are represented as days | `justify_hours(interval '27 hours')` | `1 day 03:00:00`
``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 9.9.4 |  |
``localtimestamp`` | `timestamp` | Current date and time (start of current transaction); see Section 9.9.4 |  |
``make_date(`year` `int`, `month` `int`, `day` `int`)`` | `date` | Create date from year, month and day fields | `make_date(2013, 7, 15)` | `2013-07-15`
``make_interval(`years` `int` DEFAULT 0, `months` `int` DEFAULT 0, `weeks` `int` DEFAULT 0, `days` `int` DEFAULT 0, `hours` `int` DEFAULT 0, `mins` `int` DEFAULT 0, `secs` `double precision` DEFAULT 0.0)`` | `interval` | Create interval from years, months, weeks, days, hours, minutes and seconds fields | `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 | `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; if `timezone` is not specified, the current time zone is used | `make_timestamptz(2013, 7, 15, 8, 15, 23.5)` | `2013-07-15 08:15:23.5+01`
``now()`` | `timestamp with time zone` | Current date and time (start of current transaction); see Section 9.9.4 |  |
``statement_timestamp()`` | `timestamp with time zone` | Current date and time (start of current statement); see Section 9.9.4 |  |
``timeofday()`` | `text` | Current date and time (like `clock_timestamp`, but as a `text` string); see Section 9.9.4 |  |
``transaction_timestamp()`` | `timestamp with time zone` | Current date and time (start of current transaction); see Section 9.9.4 |  |
``to_timestamp(`double precision`)`` | `timestamp with time zone` | Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp | `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 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 (when the session time zone is 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`.

Note there can be ambiguity in the `months` field returned by `age` because different months have different numbers of days. Postgres Pro'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
    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: 121.958333333333
    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`, `time`, or `interval`. (Expressions of type `date` are 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 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

`day`

  For `timestamp` values, the day (of the month) field (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 (can be negative); for `date` and `timestamp` values, the number of seconds since 1970-01-01 00:00:00 local time; 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: 982384720.12

      SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
      Result: 442800

  You can convert an epoch value back to a time stamp with `to_timestamp`:

      SELECT to_timestamp(982384720.12);
      Result: 2001-02-17 04:38:40.12+00

`hour`

  The hour field (0 - 23)

      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 (not applicable to intervals)

      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.

  This field is not available in PostgreSQL releases prior to 8.3.

`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 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

`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 date is in

      SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
      Result: 1

`second`

  The seconds field, including fractional parts (0 - 59[7])

      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. (Technically, Postgres Pro 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

### 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. Postgres Pro 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`.

    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` converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones. Table 9.31 shows its variants.

Table 9.31. `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, with no time zone designation
``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., `'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.

Examples (assuming the local time zone 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. Converting time values to other time zones uses the currently active time zone rules since no date is supplied.

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

### 9.9.4. Current Date/Time

Postgres Pro 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.

Postgres Pro 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 command of a transaction, but might differ during subsequent commands. `clock_timestamp()` returns the actual current time, and therefore its value changes even within a single SQL command. `timeofday()` is a historical Postgres Pro 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 Postgres Pro 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';  -- 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.

### 9.9.5. Delaying Execution

The following functions are available to delay execution of the server process:

    pg_sleep(seconds)
    pg_sleep_for(interval)
    pg_sleep_until(timestamp with time zone)

`pg_sleep` makes the current session's process sleep until `seconds` seconds have elapsed. `seconds` is a value of type `double precision`, so fractional-second delays can be specified. `pg_sleep_for` is a convenience function for larger sleep times 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.

[7] 60 if leap seconds are implemented by the operating system