Does this mean I shouldn't invest my Bitcoins in Elon Musk's new company Timely? In addition to the various problems you mention, we have to add in the accelerating expansion rate of the universe, which complicates matters for all but the shortest backward hops. On a more mundane level we'd also need to calculate the Earth's perpetual oscillation (it's always moving fractionally from its inclination of 23.5 degrees). The good news: while we can never travel backward in time (not even if one day we'll actually have a meaningful definition of the word "time") we can continue to travel forward at just under 300,000kps, along with the rest of the universe.