If that was possible and the case, then they'd already have completed their goals. Time Travel to the past can only make some logical sense, if a new timeline/ branch is created at the point you travel back to, but in the timeline you left, you simply disappear and nothing changes there. Time Travel to the future would be a lot like putting yourself in stasis whilst everything else fast-forwards. But I have only ever considered Time Travel to be a fiction. Interdimensional beings, however, they are more believable.