キャスト・切り捨て

 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

秒の小数点以下の切り捨て

 SELECT tm1 FROM tb1
 2016-09-20 09:23:29.73666
 
 SELECT date_trunc('second', tm1) FROM tb1;
 2016-09-20 09:23:29

参考

• http://www.postgresql.jp/document/9.3/html/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC