There is an interesting thoery called Phantom Time http://en.wikipedia.org/wiki/Phantom_time_hypothesis related to a preiod in the middle ages that may not have existed. supposedly a king wanted to be on the throne during the millennium so faked a few hundred years of history.
If true we would only be in the 17th or 18th century. Which may explain why the world didn't end in 2012, or why there still aren't hover boards.
Your last sentence makes it clear you are joking, but just so no one is confused: Without even reading the Wikipedia article, it's trivial to imagine how that hypothesis could be readily disproven. Recorded astronomical phenomena, correspondence of cross-cultural events—even carbon dating is accurate enough to catch a discrepancy of several centuries like that.
Yes I think Halley's comet appears in the Bayeux Tapestry which I assume has been linked to an appearance in the 12th century. Better check the stitching on that to make sure it wasn't modified ;)
[EDIT] Halley's comet apparitions are roughly every century so not that conclusive... cue twilight zone music
Here's a conspiracy theory for you: Pratchett's History Monks exist, and this was their doing. But nothing much happened during those three centuries anyway, not even the horse collar, and such stretches of fallow time come in handy when you've also got to worry about centuries like the nineteenth and twentieth, where a mere hundred years doesn't start to be enough time for everything to happen in.
If true we would only be in the 17th or 18th century. Which may explain why the world didn't end in 2012, or why there still aren't hover boards.