How far back in time could you go and still understand English?

If you're like me at all you spend like 40% of your day daydreaming about going back in time. It doesn't always have to be A Connecticut Yankee in King Arthur's Court kind of scenario, sometimes its something as simple as imagining the look on 1982 Steve Wozniak's face as you fire up Game of Thrones on your smartphone.

But lets say you really wanted to get your joust on and you did go back in time like 1,00 years. Would you be able to understand English? I mean if you already read Timeline by Michael Crichton you would know the answer is no, but if you haven't read timeline, here's a clever little youtube video telling you the same thing.