I just got an idea that I don't have the knowledge to implement so I'll throw it out there. Who knows, maybe someone will be bored and make it.
Sometimes I'll post something about an upcoming event at my forum. It would be cool if once the event has happened, it automatically gets an expired tag.
I guess while posting there would have to be a new field such as, "Post expires: date."
I wouldn't want it to be deleted. I know some sites have that feature where the post will be completely deleted at a certain date/time. I'd want the post to stay but somehow automatically ensure people know it's a post about an event that has already happened.
I'm sure this is another "easier said than done" type thing. 😄