The whole 2012 nonsense is based on the Mayan calendar after it was converted to the Gregorian calendar that we use today. They used a calculation they call the GMT constant (3 initials for the last-known Mayan researchers). They predicted the end using this calculator, but recent evidence has shown that they are wrong on the calculation and that the "end" of the Mayan calendar could be as far as 100 yrs away.
Read this book for more info: Calendars and Years II: The Astronomy and Time in the Ancient and Medieval World
Also, the Mayan calendar doesn't necessarily "end" in a "dark rift" like some suggest. It doesn't end at all. In fact, it just starts a new cycle like the odometer on your car. The Gregorian calendar was set to end on September 18th, 1618. Nothing happened then (or on June 15th, 1224, or on March 13th, 830) so nothing is going to happen in 2012.
I'm fascinated by good end time prophesies and the Mayan one we've spun is one of the best. It plays out on a cosmic scale that appears to be naturally cyclical; so it's a good one that can appeal to the imagination of all.
Brittanytodd is correct, but we'll ignore that and have our fun anyway. I guess, after the 2013 new year I'll watch to see of the Saint Malachy prophesy comes true. Not sure what's out there past that. I don't doubt we'll find something.