Marty, We're Going Back in Time...
If a scientist were to ever invent a time-travel device, it would need to be a time-and-space travel device in order to be effective. If you were simply traveling through time, and not space, when you activate the device, you would emerge in space, precisely where the Earth was before you traveled in time. Since the Earth rotates and moves at an unbelievably fast rate, you would likely end up dying in the vacuum of space.