Long Count

Detail showing three columns of glyphs from 2nd century CE La Mojarra Stela 1. The left column gives a Long Count date of 8.5.16.9.7, or 156 CE. The two right columns are glyphs from the Epi-Olmec script.

Since Calendar Round dates can only distinguish in 18,980 days, equivalent to around 52 solar years, the cycle repeats roughly once each lifetime, and thus, a more refined method of dating was needed if history was to be recorded accurately. To measure dates, therefore, over periods longer than 52 years, Mesoamericans devised the Long Count calendar.

The Maya name for a day was k'in. Twenty of these k'ins are known as a winal or uinal. Eighteen winals make one tun. Twenty tuns are known as a k'atun. Twenty k'atuns make a b'ak'tun.

The Long Count calendar identifies a date by counting the number of days from the Mayan creation date 4 Ahaw, 8 Kumk'u (August 11, 3114 BC in the proleptic Gregorian calendar or September 6 in the Julian calendar). But instead of using a base-10 (decimal) scheme like Western numbering, the Long Count days were tallied in a modified base-20 scheme. Thus 0.0.0.1.5 is equal to 25, and 0.0.0.2.0 is equal to 40. As the winal unit resets after only counting to 18, the Long Count consistently uses base-20 only if the tun is considered the primary unit of measurement, not the k'in; with the k'in and winal units being the number of days in the tun. The Long Count 0.0.1.0.0 represents 360 days, rather than the 400 in a purely base-20 (vigesimal) count.

**Table of Long Count units**
Days	Long Count period	Long Count period	Approx solar years
1	= 1 K'in
20	= 20 K'in	= 1 Winal	0.055
360	= 18 Winal	= 1 Tun	1
7,200	= 20 Tun	= 1 K'atun	19.7
144,000	= 20 K'atun	= 1 B'ak'tun	394.3

There are also four rarely-used higher-order cycles: piktun, kalabtun, k'inchiltun, and alautun.

Since the Long Count dates are unambiguous, the Long Count was particularly well suited to use on monuments. The monumental inscriptions would not only include the 5 digits of the Long Count, but would also include the two tzolk'in characters followed by the two haab' characters.

The Mesoamerican Long Count calendar forms the basis for a New Age belief, first forecast by José Argüelles, that a cataclysm will take place on or about December 21, 2012, a forecast that mainstream Mayanist scholars consider a misinterpretation, yet is commonly referenced in pop-culture media as the 2012 problem.

For example, Sandra Noble, executive director of the Mesoamerican research organization FAMSI, notes that "[f]or the ancient Maya, it was a huge celebration to make it to the end of a whole cycle". However, she considers the portrayal of December 2012 as a doomsday or cosmic-shift event to be "a complete fabrication and a chance for a lot of people to cash in."