Turbulance

Time travel is always going to be difficult. Not so much the location of Earth however long ago, as so many people feared. That was just a matter of making a jump, triangulating and making a second jump. The problem was turbulence. Every action, word and even movement made vortices. As was said on a Star Trek episode 'incursion factor'.

 Fortunately most things were swept up in whatever current was happening at the time. Also, there was a certain amount of excess energy required before changes in an event would 'leak' into the rest of the timestream. As well, enough energy in the opposite intent could contain the changes indefinitely. 

Do it too much, however, and memories would get blurry. Not that it was really a problem. Most people didn't have perfect recollection – and despite the selfie/social media buzz, most things still weren't recorded. Outside of a few areas under high surveillance, most of the world was unobserved and unremarkable.