2.7. Date/Time Functions and Operators

Table 2.18 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 2.17 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 2.6. You should be familiar with the background information on date/time data types from Section 1.4.

Most of 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.

Note

The examples in this section that show interval values use a human-readable style similar to PostgreSQL to represent them as strings. However, when outputing interval data as a string, Hyper uses the ISO-8601 style. An example of interval in this format is P1DT2H3M4S, which corresponds to 1 day, 2 hours, 3 minutes, and 4 seconds.

While all examples here work as expected, please note that when converting intervals to strings for output, you might see a different format than what is shown in this page.

Table 2.17. Date/Time Operators

OperatorExampleResult
+ 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 2.18. Date/Time Functions

FunctionReturn TypeDescriptionExampleResult
current_date dateCurrent date; see Section 2.7.3   
current_time time with time zoneCurrent time of day; see Section 2.7.3   
current_timestamp timestamp with time zoneCurrent date and time (start of current statement); see Section 2.7.3   
date_part(text, timestamp) double precisionGet subfield (equivalent to extract); see Section 2.7.1 date_part('hour', timestamp '2001-02-16 20:38:40')20
date_part(text, interval)double precisionGet subfield (equivalent to extract); see Section 2.7.1 date_part('month', interval '2 years 3 months')3
date_trunc(text, timestamp) timestampTruncate to specified precision; see Section 2.7.2 date_trunc('hour', timestamp '2001-02-16 20:38:40')2001-02-16 20:00:00
date_trunc(text, interval)intervalTruncate to specified precision; see Section 2.7.2 date_trunc('hour', interval '2 days 3 hours 40 minutes')2 days 03:00:00
extract(field from timestamp) double precisionGet subfield; see Section 2.7.1 extract(hour from timestamp '2001-02-16 20:38:40')20
extract(field from interval)double precisionGet subfield; see Section 2.7.1 extract(month from interval '2 years 3 months')3
justify_days(interval) intervalAdjust interval so 30-day time periods are represented as monthsjustify_days(interval '35 days')1 mon 5 days
justify_hours(interval) intervalAdjust interval so 24-hour time periods are represented as daysjustify_hours(interval '27 hours')1 day 03:00:00
justify_interval(interval) intervalAdjust interval using justify_days and justify_hours, with additional sign adjustmentsjustify_interval(interval '1 mon -1 hour')29 days 23:00:00
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_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
now() timestamp with time zoneCurrent date and time (start of current statement); see Section 2.7.3   
to_timestamp(double precision) timestamp with time zoneConvert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestampto_timestamp(1284352323)2010-09-13 04:32:03+00

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
      

2.7.1. 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.

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[1])

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

SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
Result: 28.5
timezone

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, Hyper 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

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 2.6.

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

2.7.2. 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

If source is of type interval, then field may not be week. This is because a month may contain a fractional number of weeks, and thus truncation to weeks is not possible in the general case.

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 time zone.

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('hour', INTERVAL '3 days 02:47:33');
Result: 3 days 02:00:00

SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
Result: 2001-02-16 00:00:00-05

2.7.3. Current Date/Time

Hyper 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 statement:

CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP
CURRENT_TIME(precision)
CURRENT_TIMESTAMP(precision)

The return value of both CURRENT_TIME and CURRENT_TIMESTAMP contains the time zone.

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


All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now';
CREATE TABLE a (t TIMESTAMP DEFAULT 'now');

In the fourth example, the current timestamp at time of insertion of a tuple will be used, and not the timestamp of table creation. This works differently in PostgreSQL, where the table creation timestamp is used in every insertion that uses the default value.



[1] can be 60 on leap seconds