EXTRACT

Return a specific date part, such as the month, day, or hour, from a date, timestamp, or time expression, or an INTERVAL literal.

EXTRACT( datepart FROM { date | timestamp | time | interval } )

See Supported Dateparts.

For example, extract the day from a timestamp column:
premdb=# SELECT matchday, EXTRACT(day FROM matchday) 
FROM match WHERE htid=26 AND seasonid=10 ORDER BY 1 LIMIT 5;
      matchday       | date_part 
---------------------+-----------
 2001-08-19 00:00:00 |        19
 2001-09-08 00:00:00 |         8
 2001-09-22 00:00:00 |        22
 2001-10-20 00:00:00 |        20
 2001-10-27 00:00:00 |        27
(5 rows)
When you specify a literal expression, include the TIMESTAMP, TIMESTAMPTZ, TIME, or INTERVAL keyword. For example:
premdb=# SELECT EXTRACT(dow FROM TIMESTAMP '2016-04-17 12:00:00');
 date_part 
-----------
         0
(1 row)

premdb=# SELECT EXTRACT(hr FROM TIME '11:21:15');
 date_part 
-----------
        11
(1 row)