Distro: Fedora Core 2

I'm looking for a way to automatically logoff a user who has been idle for more than a couple hours. I thought that I'd use the 'who -u' command to get a value for idle time, but none of the machines provide any information about idle time using the 'who -u' command. Is there something in my log configuration that messed up??

So, my questions are:

What's up with 'who -u'?
Is there a better way to get a user's idle time?