Highlight Unread Posts

I understand that this forum uses some sort of cookie system to detect which threads have been read by users. I am trying to accomplish the same thing. My thought is to have a cookie with two keys: one with the last activity of the user, and the other with the posts that have been read.

Now, the only problem, is that I more than likely will need to update these reads threads based on a date/timeframe criteria, correct? Has anyone wrote one of these? In my threads table, I have a column named 'forumLastPostID' for each thread.