Oh, I don’t know... haven’t thought about this in years but just off the top of my head - spacetime is grainy and bumpy and foamy at sub-sub-sub-atomic levels and microscopic wormholes open up and almost immediately close (something like within a few trillionths of a nanosecond). If you could (get ready for a real big “if” here) stabilize and widen that wormhole, and move one end of the wormhole spacially near to the other end and move it there very close to the speed of light such that relativistic time-dilation sets both ends of the wormhole at different times, then you (or perhaps merely an electronic squirt of information, if the hole can’t accomodate a human body) could step through the hole to some time in the future and then as easily step through to the past.
This would work. No reason why it wouldn’t. Don’t expect us to manage this level of technology anytime soon though. Breaking lightspeed isn’t a real big mystery either. I could tell you a thoroughly workable theory for that too. Again, I wouldn’t expect any practical results on this either - not in our lifetimes (or at least a few generations).