You probably need a cron job (available on any flavor of UN*X; Host restrictions not included) to run a check on weither or not posts are 7 (or more) days old. It all depends on how you'll be implementing it. I'd recommend a SQL backend (DBI would be nice). If you're gonna use plain files, stat is that nifty file stats function (and yeah -X has -M). if you want to play it safe, you can just ignore old(er) posts and only show newer ones (as in don't delete older posts).

Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.