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The date/time functions provide a powerful set of tools for manipulating various date/time types.
Table 4-5. Date/Time Functions
| Function | Returns | Description | Example |
|---|---|---|---|
| abstime(datetime) | abstime | convert to abstime | abstime('now'::datetime) |
| age(datetime,datetime) | timespan | preserve months and years | age('now','1957-06-13'::datetime) |
| datetime(abstime) | datetime | convert to datetime | datetime('now'::abstime) |
| datetime(date) | datetime | convert to datetime | datetime('today'::date) |
| datetime(date,time) | datetime | convert to datetime | datetime('1998-02-24'::datetime, '23:07'::time); |
| date_part(text,datetime) | float8 | portion of date | date_part('dow','now'::datetime) |
| date_part(text,timespan) | float8 | portion of time | date_part('hour','4 hrs 3 mins'::timespan) |
| date_trunc(text,datetime) | datetime | truncate date | date_trunc('month','now'::abstime) |
| isfinite(abstime) | bool | a finite time? | isfinite('now'::abstime) |
| isfinite(datetime) | bool | a finite time? | isfinite('now'::datetime) |
| isfinite(timespan) | bool | a finite time? | isfinite('4 hrs'::timespan) |
| reltime(timespan) | reltime | convert to reltime | reltime('4 hrs'::timespan) |
| timespan(reltime) | timespan | convert to timespan | timespan('4 hours'::reltime) |
For the date_part and date_trunc functions, arguments can be `year', `month', `day', `hour', `minute', and `second', as well as the more specialized quantities `decade', `century', `millenium', `millisecond', and `microsecond'. date_part allows `dow' to return day of week and `epoch' to return seconds since 1970 (for datetime) or 'epoch' to return total elapsed seconds (for timespan).