Need a way to automate user logout

Damn stateless nature of HTTP, hey everyone, can someone point me or give me an idea on how I would automatically log out (destroy user sessions and make note of it in the database) on some type of schedule. I have designed a security system that depends upon people logging out to determine the amount of time they have accumalated on their accounts. Of course people do not always (never) do this, so I need a way to do it for them. We are running redhat linux, I am sure there is a way to say run a certain script at a certain time blah blah etc etc. But I am only a programmer, and am absolutely clueless on the finer points of nix. I can write a script in perl, php, java, u name it, but how do I have it run on a set schedule? Thanks Christian