Goto the discussion forums link. Goto a specific forum. At the bottom of the forum page there's a series of dropdown menus that let you filter in or out however many days, specific times, and the order in which they were posted. I typically don't use that feature, but it's there, and can be done for every forum this site supports (and there's a lot of 'em).

Normally if a thread catches my interest the first time I read it, I subscribe to it and receive email notifications about new additions to that thread.