I suppose you're using MySQL. Then this might be what you are looking for:
TIMEDIFF(expr,expr2)
TIMEDIFF() returns the time between the start time expr and the end time expr2. expr and expr2 are time or date-and-time expressions, but both must be of the same type.
mysql> SELECT TIMEDIFF('2000:01:01 00:00:00', '2000:01:01 00:00:00.000001');
-> '-00:00:00.000001'
mysql> SELECT TIMEDIFF('1997-12-31 23:59:59.000001','1997-12-30 01:01:01.000002');
-> '46:58:57.999999'
http://www.mysql.com/doc/en/Date_and_time_functions.html