One thing that they fail to account for -- our planet and solar system and the universe are constantly moving.
The earth will be in a completely different spot even a minute from now. And six months from now, you'd be floating in space.
You'd need a spaceship.
Quite simply, each time one of the travelers decided to take trip to the futre or the past, they found themselves in deep outer space, with no way to get back to their ride, the Earth. How fast does the earth move, anyway? :-)