Getting technical with time travel is a headache since we know so little of the concept of time.
If you know enough physics, there are some theories regarding time-travel which might be viable, but none of them are particularly useful for fiction. For example, Stephen Hawking has suggested that time travel might only work in a region of spacetime which is warped in a specific way, and that the warping might only be possible with a working time machine. Thus future time-travellers would only be able to go back in time as far as the point where the first time machine was invented.
I recall seeing an article about 20 years ago in which a team from MIT developed a model which might allow backwards time travel. The process involved putting a particle into a degenerating, funnel-shaped orbit around a high-gravity point like a black hole, then accelerating the particle such that it reached light speed at the same time it crossed the event horizon. Estimates were that it would require the energy output of our entire GALAXY to do this with a single PROTON.
So, not really useful unless you're writing a space opera with a really advanced alien species.