Here is one piece of the reasoning: If one were to look at the sky during the time period that should be 0 A.D., one would not see anything unusual. But, if one were to look between 12-6 A.D., one would see a Halley's comet, Uranus passing close to Saturn and Venus, as well as a supernova explosion. One of these is likely what was termed the Star of Bethlehem, which would date Jesus's birth. (It should be noted though that, just because there may be a scientific explanation for the star, doesn't mean that it wasn't of spiritual origins.) People have also tried to trace the dates through the ages and have come up with discrepancies just through the math as well.
But, that is not to say that I am certain of the dating. It could be that, somehow, the Mayan calendar does end on our calendar's 21/12/12. I cannot be sure, and of course, I'm not an expert. I would actually hope that it ends sooner, rather than later, because it will be very exciting one way or another. Tammy (or at least the being whom she is channeling) seems to think that 21/12/12 is indeed the correct date, which makes me much more optimistic than I was previously about this particular year.
So, I would "be prepared" for a shift in awareness this december, but I don't really think you have to do much to prepare. Just trust in Spirit - the rest will fall into place when it's time.