You can't go back in time beyond the point when the first time machine is invented.
So if, or when one is ever created, we will then instantly be flooded with visitors from the future coming back to witness the very first moment of time travel. Also at that point, time will no longer be a continuum. All time will exists simultaneously…or in parallel, if you will.
I say that happens 2012. Some time in December. Laugh now…but don't forget who told ya when the time comes.

uT