timer on sessions?

Not sure how to go about setting up a function to parse the date, hour, minutes, seconds, take the seconds and register them in a session var, then do a check on the session var (seconds) vs. the seconds var + 5*60 (or 5 minutes) to time out the session and force the user to log back in. My problem is finding the correct way to check the seconds in php. Any help or pointers is appreciated. Jas