A little lightbulb went flashy-flashy in my head. No, I wrote. None of those. (Well, actually, I ticked the poll-button saying that, but give a girl some dramatic license.)
No. The calendar is the Mayans' Y2K problem.
Computer programmers were aware of the Y2K problem well before it even happened (even at the time they were programming the things) but they thought, "Oh, it is such a long time until 2000; this is the 1970s. That's 30 whole years. Surely these programs we are writing now will not last until 2000. They will have plenty of time to rewrite them, and they will replace them like they do any worn-out equipment, and meanwhile, that is two whole bytes we could be using for something else, because memory is fiendishly expensive." (I recall my father telling me that he was involved in a Y2K argument at least two decades before Y2K.)
I am sure that some of the Mayan calendar engineers said, "Look at this, why do we need to write the calendar out further than 2012, which is stupidly far in the future? How much stone do you want us to use, anyway? And do you really want to pay rock-chippers to chip out another thousand years that our grandchildren's grandchildren won't even see? Talk to us about it in a few hundred years." Tragically, their culture then ebbed and was conquered before they could update their calendars.