I'm with T-Rex on this one. Star Trek made time travel too easy for it to be a compelling plot point in later episodes, and T-Rex is right to wonder why they don't do it all the time!
My own personal pet peeve with Star Trek concerns warp travel. If it takes seven (or twelve or twenty) days to get to the next starbase at warp three but only three seconds at warp nine, then why not go full speed all the time! C'mon, it makes no sense to be dawdling about the universe in a huge ship with a well-trained crew when you could be handling crises in an instant!
Captain, we need to dock at Station 33.3 for vital repairs.
Very good. Set a course. Warp five. Engage.
But Captain, that will take us 23 hours. We can get there in like ten minutes if we just go warp nine.
Oh. OK, I forgot. Let's do it then. Warp nine. Engage.
Makes sense to me.