Oracle:
http://docs.oracle.com/cd/B19306_01/server.102/b14200/sql_elements003.htm#BABGIGCJ
PostgreSQL:
http://www.postgresql.org/docs/9.1/static/datatype-datetime.html
MySQL:
http://dev.mysql.com/doc/refman/5.0/en/date-and-time-literals.html
DB2 v9.5