How do you do no new posts/new posts function

[liam1412]

Hi

 

Hope this makes sense

 

On forums you have the little image at the side.  Usually one is faded which means no new posts and the other is the same image but darker which means new posts.

 

How do you go about logging when people have looked in a forum. Surely you can't have a table with an activity time for every forum as this would be imense.

 

ANy ideas how this is done.

 

Thanks