extrait doc oracle :
Time is stored in 24-hour format, HH24:MI:SS. By default, the time in a date field is
12:00:00 A.M. (midnight) if no time portion is entered. In a time-only entry, the date portion defaults to the first day of the current month. To enter the time portion of a
date, use the TO_DATE function with a format mask indicating the time portion, as
in:
INSERT INTO Birthdays_tab (bname, bday) VALUES
('ANNIE',TO_DATE('13-NOV-92 10:56 A.M.','DD-MON-YY HH:MI A.M.');
To compare dates that have time data, use the SQL function TRUNC if you want to
ignore the time component. Use the SQL function SYSDATE to return the system
date and time. The FIXED_DATE initialization parameter lets you set SYSDATE to a
constant; this can be useful for testing.