What's interesting is that the case for this actually being Lyons and not Midwest is now very strong. And so if we are looking at Lyons BoS, then there would have to have been enough time for events to move forward to include, for example, the building of the airship, Maxson reaching advlthood (whatever his rank may be), and any change in leadership, such as Sarah Lyons succeeding her father (It is unlikely that Owen Lyons would be brought in as the Elder this time round given his age at the time of FO3). And so this sheds even more doubt on the claims that the game is set exactly "200 years" after the great war.
I am quite certain that once we get to see some kind of footage from QuakeCon, or once we at least get to read an article from a reliable source who attended, we will find out an exact date due to the in-game use of the Pip-Boy. I think it would be very hard to conceal this during a one hour demo.