So. I'm probably just stating something that is obvious to all you smarticle ponies/foxes out there (Yes, I'm looking at you, Tyro), but.
I just noticed that Twilight aquired the knowledge of the Time Travel spells from herself. That means that, in effect, she pulled that knowledge out of thin air.
She didn't even know about the Starswirl the Bearded Wing until her future self told her, at which point she knew. But her future self learned it from her future self when her future self was her past self, and so on. Therefore, her knowledge of the Starswirl the Bearded Wing, and more importantly the Time spells, effectively came from nowhere.
I really hate discussing stuff like this, it makes my brain hurt...but at the same time it's SO INTERESTING!