The Mayan calendar consists of many different cycles that work together. These include the long count, the calendar round, the short count, and the 819 day cycle among others. The dating system that doomsday entrepreneurs use is the long count. The long count is made of several cycles that work like an odometer. For Mayan era dates, as well as modern dates, five cycles need to be used. From the largest to the smallest they are -

• B'aktun = 20 k'atun
  K'atun = 20 tun
  Tun = 18 winal
  Winal = 20 k'in
  K'in = 1 day

An example of a long count would be 9.15.10.0.0. There a 9 b'aktuns, 15 k'atuns, 10 tuns, and no winals or k'ins.

The long count that is supposed to be the end of the world is 13.0.0.0.0. The day before is 12.19.19.17.19. The main significance of 13.0.0.0.0 is of course the number of zeroes.

However, the long count does not end at that point. There are many cycles beyond the b'aktun that can be used to describe any date. The are several instances of extremely large dates used. Coba Stela 1 has a long count that adds up to 41,943,040,000,000,000,000,000,000,000 years. Also, Macanxoc Stela 1 has a long count in the initial series that is 13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13.13 .13.13.13.0.0.0.0. Their is no limit to the long count other than the size of the writing surface used and the patience of the scribe.

The other problem with 13.0.0.0.0 is determining the equivalent Gregorian calendar date. To do this you need a correlation between the two systems. Most Mayanists use the astronomical (584285) or the GMT (584283) correlation. Using the astronomical correlation places 13.0.0.0.0 on December 21, 2012 and the GMT gives a date of December 23. However, there is some question as to how accurate these correlations are.

The following paper describes some of the problems with the GMT correlation and points out that GMT may be off by 260 years!

K'in in the Hieroglyphic Record: Implications of a Pattern of Dates at Copan, Honduras by Gerardo Aldana.

Other correlations place 13.0.0.0.0 anywhere from December 11, 1614 (Willson 438906) to August 12, 2532 (Weitzel 774078).

I should also point out that classical Mayan civilization collapsed around 900 AD (using GMT) and the last long count inscriptions were from around the same time. That would be somewhere around 10.3.15.X.X, which I don't think was predicted.

Scott