A complicated and accurate calendar using three simultaneous independent systems. It consisted of the Long Count, Tzolkin, and Haab. The Long Count is related to precession of the equinoxes, and will end in 2012 when the winter solstice will be very near the point at which the ecliptic plane and galactic plane intercept (Jenkins).

The Long Count is a mixed base 20/base 18 representation of the number of days since the beginning of the "Mayan Era." The starting point is written as 13.0.0.0.0, which corresponds to Aug 13 3114 BC, Aug 11 3114 BC, or Oct 15 3374 BC Gregorian (according to different authorities). Mayan dates are written in the form baktun.katun.tun.unial.kin, where these lengths are defined below.

Unit	Definition	Days	Years
unial	20 kin	20 days	--
tun	18 unial	360 days	year
katun	20 tun	7,200 days	years
baktun	20 katun	144,000 days	years

The Tzolkin was a combination of a numbered "week" of 13 days and a named "week" of 20 days.

The Haab was a civil calendar consisting of 18 "months" of 20 days each (numbered 0-19), followed by 5 extra days (called Uayeb and observed as days of prayer and mourning), giving total year length of 365 days.

For example, my birthday (March 18, 1969 AD Gregorian) would be written 12.17.15.10.14, 8 Muluc 17 Kayab.

The Mayan calendar was adopted by other central American cultures including Aztecs and Toltec. Mayans were aware that the year was slightly longer than 365 days, and computed the tropical year as 365.242036 days.

Calendar