Isn't it possible people back then missed a year or two, when counting? Isn't it possible that errors in counting have accumulated over the years? The fact that the way of counting years, not to mention calendars, differed in the past makes it even more difficult to ascertain the accuracy of historical dates doesn't it?--something I've always wondered. (this is a cross between showerthoughts and askhistorians material)
It is possible that some dates are incorrect, but at the same time, there are certain constants that allow us to "check" if they are correct.
For example, if we have a document detailing a comet at a certain date, and we check how often the comet is supposed to come around and it doesn't match up with the date stated, then that means that the historical document might have some mistake in its date, or that it may be referring to a different comet.
Not only that, but for anything other than ancient history, there are often multiple sources, written at around the same time about the same thing, so if the dates don't match up that's another indication.
Ultimately this cross checking of different sources, and predictable natural phenomenons lets us have a pretty good idea on whether or not the dates stated are accurate.