Who was it that opined, “The Mayan calendar doesn’t predict The End. It’s just that whoever was compiling it got bored with the tedium and said, “That’s enough. Somebody else can take it from here.”, and no one did.
My theory is that someone did some calculations, figured out that in 2012 the would would cross the galactic plane and thought that would be a good STARTING point, and then worked the math and their calendar backwards.
As far as I'm concerned, they should have used a bigger rock.
Besides, if they couldn't even predict their own demise just a few centuries away, how can you trust their predictions this far into the future?
Kind of hard to keep making Calendars when you have Spaniards arriving on your shores and bring THEIR calendar and culture with them.