The "Mayan calendar" has been in the news a lot leading up to December 2012. If you don't already know, some think it marks December 21, 2012 as the day the world will end. Setting aside whether or not we'll still be here come 2013, it's still interesting to look at how the Mayans marked time. The singular "calendar" -- when referring to the "Mayan Calendar"-- embraced by popular culture is not technically accurate. The Mayans actually used multiple calendars.

The Mayans, an ancient civilization living in the region that falls between modern Mexico and South America, were among the first cultures to record their history and keep track of discrete blocks of time. The four most well known calendars used in Mayan culture were the Tzolk'in, Haab, Calendar Round and Long Count calendars.

The Tzolk'in calendar tracked crop rotation. It had a 260-day period for land preparation and a 260-day period for crop growth and harvest time. It used 20 day signs and 13 number signs to achieve its 260 uniquely descriptive days. (Fun tidbit: The Mayans had their own system of numbers, with special importance given to the numbers 20 -- signifying 10 fingers and 10 toes -- and 13, which stood for the 13 chief joints in the body. Hence we get the 20 day and 13 number sign multiples used by the Tzolk'in).

The Haab calendar was a bit closer to something our Gregorian-calendar world might understand. Sun-based, as is our modern calendar, the Haab had 360 days spread out over 18 20-day months.

Interestingly, although both the Haab and Tzolk'in calenders are based on periods of 20 days, called uinals, the Haab has 18 uinals instead of the 13 uinals of the Tzolk'in calendar. The advantage of this arrangement is that instead of having a year with 260 days, the year has 360 days, thereby conforming pretty closely with the solar cycle.

These calendars were fine for shorter spans of time, but the Mayans wanted to mark longer increments, so they also used the Calendar Round, lasting 18,890 days, or about 52 years. That might seem like a plenty long-enough calendar, but Mayan historians wanted to go yet further out in time -- over generations. So they needed an even longer count called, appropriately, the Long Count calendar. This calendar covered a little more than 5,125 years, which the Mayans called the Great Cycle. It's this Long Count calendar that's behind the end-of-the-world talk. Some historians peg the start of the Long Count at August 11, 3114 B.C., counting forward from there to declare December 21, 2012 the end of the Mayan calendar.