LOL it is so funny that some people actually believe in this silly theory.
The Mayans didn't have only one Calendar for beginners, they had more than 5 and the one that has the December 21, 2012 as en "ending date" is named "Long Count".
Now, the thing with this calendar is that it resets to day 0 every 1,872,000 days or 7057.5 years. So the above date is not the end of the world, but probably the next reset of the calendar

The Mayan Calendar, like all calendars, has to end sometime, like the calendar we all use ends, but every 365 days and then we start counting again only changing the last number of the chronology

Its just that the Mayans, decided not to end it in every 365 days but every 7057,5 years. (I say again, in
this calendar, cause they had more and each one has a different reset date and counting system)
That's all with this "end of the world" theory^^
No one can ever predict the future.