it's worth emphasizing that this mesoamerican calendar (as used by several cultures -- including the Maya -- in Central and South America before European colonization) does not predict an apocalypse. It never did, despite what the movie "2012" told us. ... Archaeologists studying this fascinating culture have been able to decipher their many calendars, but their longest period calendar -- the "Long Count" -- is what set alarm bells off in the fertile minds of a few conspiracy theorists, doomsayers and guys looking to make a fast buck.

The Long Count was used by the Maya to document past and future events. Their other calendars were simply too short to document any date beyond 52 years. The 52-year calendar -- known as the "Calendar Round" -- was used as it spans a generation, or the approximate lifetime of an individual.

Using the Calendar Round meant that events in a person's life could be chronicled over 52 years -- or 52 "Haab's," spanning 18,980 unique days. ... Now, purely as a consequence of the Long Count's numerical value, many Mayan scholars agree that the calendar will "run out" after 5,126 years (or, at least, it's first cycle does).

http://news.discovery.com/space/the-2012-mayan-calendar-doomsday-date-might-be-wrong.html