It really is way easier than everyone is making out to be. Three words puts this all back in the realm of plausibility for the GM: rare spell components. That's it! Do you wanna travel back to the stone age or into the techno heavy future? Fine gather up a Tarrasque eye & and a god's tear. Viola! Now for spells of a smaller scope make the reagents easier to get. It opens up a whole new realm of arcane magic. Can't deal with paradox effects? No problem time snaps back and the PC ends up transported to some Olympian styled extra-planar court.