A neatly overlooked flaw for time travel is that there are 4 dimensions of space/time, and time travel only accounts for moving though one. Say you travel 100 years into the future, relative to your present position in space. Meantime the earth, the solar system and the galaxy has moved on 100 along the other three space dimensions, leaving you alone in cold, deep space.
This makes the assumption that the time travel mechanism also removes inertia, which may not be the case. But is there any reason to think gravity, or any other external force, would be felt by the time traveler? Time travel would require a warp if space/time much greater than the feeble amount of the earths gravity, or the sum for that matter. So if inertia remains the same, then the time travel device is simply going to sail off at a tangent to the earth and the sun in their respective orbits.
For short time jaunts, this probably wont be too much of a problem, assuming there is some method the device can use to travel in space to the future or past location of the earth. But for very long trips, predicting the space location is going to be problematic to say the least. My guess is would require a number of shorter trips, stopping to re-orientate and realign along the way.
I am guessing a lot of the semi-sentience of a TARDIS goes into solving just this problem.