Tracking user time on website during session

[bike5]

I am trying to write a tracking system to record the amount of time a user is logged in and actively doing stuff on a website. Are there any articles on how to do this? I just can't think of a good way to go about this. I was going to record the start of there session and update the current time on each page they visit and subtract the session start time from the current time but if they leave the website for an hour and come back on the same session it would update an hour. I also tried to verify the referer to see if they came from the website in question but that doesn't work because the referer isn't always set. Any ideas on how to go about this in the most accurate way?

Thanks.

[bbreslauer]

You could check the current time with the last current time (the last page that was updated), and if it's above a certain threshold (say 5 minutes), then it would record them as having left and come back.

You could also make it log the user out after that threshold, and they'd get a new session, and you wouldn't have to worry about it.