The world is supposed to end...again. This coming December.
The Mayan Calendar is round. Even if you carved really really really small symbols, eventually you will run out of circle. Considering how long ago this calendar was carved, it's a safe bet when the Mayan stone carvers ran out of circle, they figured it was Miller Time and they wouldn't have to worry about the calendar again in their lifetimes.
Then, the Spanish showed up and offered to smuggle them through Mexico to high paying jobs the Gringos were offering, off the books, cash..no really, the streets are paved in Gold. (Ok, I'm paraphrasing history a little there).
So, it just happens that when the Mayans ran out of space, the last date happened to land on Dec. 21, 2012.
The only calamity likely to happen is that the descendants of Mayans, still extant today, are likely to exclaim, "sunuvabitch! it's time to carve another one of those damn things!"