in reply to Cookie expires inaccurately
Your browser throws away the cookie when the date set by the server has passed. If your server's time and your local time are off (by 1 minute, ~10 seconds) cookies will expire prematurely.
I suggest using some other form of session-time verification, like time-in-url or record the session time on the server only. I also highly suggest against cookies altogether, considering they are unreliable in every browser.
Hope this helps... if not, post a reply!
John J Reiser