Custom Forum, How to set recent posts, or updated threads since last login?

Hey, currently I’m building a forum, and it’s going pretty good so far, however, how do i implement the updated since you were gone things? Like here on kirupa (or any forum, really), when somebody posts and you sign on, the picture by the post is not greyed out (usually the color yellow (to indicate ‘new’) is involved, like on newgrounds). How do i do that in my forum? At least what sort of thing am I looking to do to achieve this?

BTW, everything is custom, so i can modify jsut about anything.