Expiry date on threads with a twist

[Julchen]

Hi,

I'd really love to add a certain feature to my forum (3.6.7):
- Adding a new radio button to all "edit thread" pages which says "Automatically move this thread to $forum after $days"
Ok. This would be quite easy ... but
- actually processing this new input isn't - at least not for me cause I don't know where to place a hook for this
- and, of course, moving the marked threads (using SQL)

The reason for all this is that some of "my" users are - to put it mildly - very fond of gossiping.
But after 6 months or so posts like "Have a nice holiday *hugs*" just clog the forum and I want them gone. Usually I check for threads like this every other week - but it just takes too much time.

Any idea how to make this work?