DS9 "Time's Orphan":
DAX: It's some sort of time portal. From the chroniton signature, we think it sent Molly about three hundred years into the past.
So the Federation now has it's hands on a bonafied time portal. O'Brien says he's going to destroy it, but we don't actually see him do it in the episode. They figured out how to get it working in a short period of time.
Remember the 24th century STU knows about the temporal prime directive, the prime directive police, etc. from multiple episodes throughout the various series, so they do know that they are eventually going to use the technology of time travel.
Did they study the portal further? Did O'Brien destroy it? Would the Federation really not use and study this technology once they got their hands on it?
Writer's notes, director's notes, EU info is welcome.