Earth itself is moving around the sun at about100,000 km/h and the sun is traveling through the galaxy st about 1 million km/h.

So if Marty went back/forward just one hour then he’d be about 1,100,000 kilometers away from Earth (or 900,000 kilometers, depending on the orbital Duffin of Earth relative to the sun’s direction of travel).

And then there’s the motion and speed of the Milkyway itself.

This is all assuming that the layout of the underlying fabric of spacetime is absolute (which it seems to be, outside of expansion).