You're kidding, right? Einstein figured out the nature of time and the theories in question (special and general relatively) have been proven over and over. You time travel forward a tiny bit relative to those who do not every time you accelerate, even if you return to your starting point. Time travel is the past is probably a practical impossibility but, if not, would require stupendous quantities of every just to send a wee bit of matter backwards a tiny bit. Such a device would have grave, grave gravitational side effects and could not be hidden.
Go to the public library and check out one of the books at Dewey Decimal number 530.11. "Simply Einstein" by Richard Wolfson is good.