Yeah the past has already happened. You can't suddenly pop into it. If you're there, you've always been there. But with the concept of time being a dimension of which beings such as Q or the Prophets can sit outside, then future has already happened too so if you travel to the future the same rule applies. Time happens everywhen all at once.
Things get screwy if you use beings that can jiggle time around and create a paradox because technically, they can't.