Substracting two timestamp values

Hi,

Can anyone help me with getting a resonable value when subtracting two timestamps??
I'm using the AUD$ table and I wan to get the amount of time the session lasted so I'm taking the logoff tiem and subtracting it from the time stampstamp, both are datetime values. Unfortunately I can't seem to get seconds and minutes out of the result.

For Example:
SELECT
to_char(SYS.AUD$.LOGOFF$TIME,'hh:mm:ss'), to_char(SYS.AUD$.TIMESTAMP,'hh:mm:ss'),
(SYS.AUD$.LOGOFF$TIME - SYS.AUD$.TIMESTAMP) SessionTime FROM SYS.AUD$
WHERE SYS.AUD$.TIMESTAMP>'15-AUG-97' I would appreciate any thoughts!!
Thanks,
Bob Received on Fri Aug 15 1997 - 00:00:00 CDT