[Lynne]

I think you want
Original post date is at least X days ago 1095 days ago.

That would be posts started 1095 days ago, and 1096 days ago, etc. The other would be only posts until 1095 days ago. You may want to think about using Last Post Date instead (what if a thread was started 9 years ago but is still active?).

When you click on Prune Threads, the next screen will give you the option to just Prune All Threads, or Prune Threads Selectively. If you hit the second option, then a list of all threads will be presented to you and you can uncheck any you don't want pruned.