Supposedly time travel will be limited to between the time of the invention of the time machine to any point in the future there after. As in, you could only travel as far back in the past as the invention of the original time machine...
According to Quantum Leap, you can only travel back as far as your lifetime.
Why is it when people wonder where the aliens are going to land, they always assume they will land in the US? Maybe they'd prefer a Czech Pils as opposed to a West Coast IPA.
At least I was not disappointed by a lack of a Quantum Leap reference.
Food for thought: Given the massive size of the universe in all 4 dimensions, there is a distinctly non-zero possibility that some alien civilization in the distant past once developed such a time-travel device. Just because we never got wind of it doesn't mean it didn't exist. So don't give up hope on being able to go back and drink a beer with your great-grandfather just yet. Unless great-grandpappy was a teetotaler, in which case I'd suggest drinking with either Jefferson or Socrates. No, scratch that last one.