Dec 21, 2012 - The Mesoamerican Long Count calendar.

The Mesoamerican Long Count calendar, notably used by the Maya civilization among others of pre-Columbian Mesoamerica, completes its thirteenth b'ak'tun cycle since the calendar's mythical starting point (equivalent to 3114 BC August 11 in the proleptic Gregorian calendar, according to the "GMT-correlation" JDN= 584283).[7] The Long Count b'ak'tun date of this starting point (13.0.0.0.0) is repeated, for the first time in a span of approximately 5,125 solar years. The significance of this period-ending to the pre-Columbian Maya themselves is unclear, and there is an incomplete inscription (Tortuguero Stela 6) that records this date. It is also to be found carved on the walls of the Temple of Inscriptions in Palenque, where it functions as a base date from which other dates are computed.[8] However, it is conjectured that this may represent in the Maya belief system a transition from the current Creation world into the next. The December solstice for 2012 also occurs on this day.

Dec 23, 2012

The alternative date for the completion of the thirteenth b'ak'tun cycle in the Maya calendar, using a version of the GMT-correlation based on a JDN of 584285 (a.k.a. the "Lounsbury correlation"), which is supported by a smaller number of Mayanist researchers.

There were two Mayan Calendars which later influenced the Aztec Calendars. One had 260 days and was a sacred calendar used for worship. The other one consisted of 365 days and was a seasonal calendar used for farming and normal daily life.
Another chronological measure used by the Mayans, but not the Aztecs, was the Long Count. This was a count of the number of days since they believed that the world had begun.

The Mayan Numbering System

Before looking at the calendars it will be helpful to understand a little of the Mayan numbering system. They used a vigesimal numbering system, that is one to the base of 20. They had the concept of zero long before it was understood in Europe. They only had two other symbols. Zero was represented by a shell, one by a dot and five by a bar. The symbols were placed together to make an icon, for example 13 (decimal) was represented by two bars with three dots over to make a single icon. A positional system, similar to ours, was used with an icon representing 0 - 19 in the right most position, then one representing (1x20) - (19x20) to the left of it.

As 20 x 20 = 400 which is more than the number of days in a solar year the numbering system was modified for the calender such that the second icon had a radix of 18 instead of 20.
The remaining icons had a radix of 20.

The Tzolkin - The Mayan Sacred Calendar

The sacred year is made up of four quarters each of 65 days. Each quarter is then sub-divided into five groups of 13 days. Each day has both a number and a symbol and both are needed to define the date. The numbers run from 1 to 13 and there are 20 different symbols. Day one is defined by number 1 and symbol 1. Day two is defined by number 2 and symbol 2. This continues until day 13 which is defined by number 13 and symbol 13. Day 14 is defined by number 1 and symbol 14, day 15 by number 2 and symbol 15. Day 20 is defined by number 7 and symbol 20. Day 21 by number 8 and symbol 1. As 13 is not a factor of 20 the same pair do not re-occur for 260 days and then a new sacred year starts.

The Haab - The Mayan Seasonal Calendar

The year was made up of 18 months of 20 days each with five additional days to bring the total to 365. This calendar defines a year of 18 months, each of 20 days, and five extra days, 365 days in total. These extra days were considered unlucky and so very little was done on them. Each year had a name and number combination as did the days in the The Tzolkin but this time there were only 52 such combinations before repeating. This series was known as a 'bundle' or a 'Calendar Round'.
The Mayan Year Cycles

The two calendars of 260 days and 365 days run simultaneously and after a period of 52 years they will once more start on the same day. This is one of many cycles which the Mayans kept track of. They were great astronomers and recorded the sun-spot cycle (11.3 years) and various planetary cycles. The longest cycle they discovered was the orbit of our solar system around Pleiades, a cycle of 26,000 years.

The Maya Long Count

It was usual to quote dates by giving them in both calendars. This meant that each combination would only be unique for the period of a Calendar Round. To record dates for longer periods the Long Count was used. This is a count of the number of days since they believed that the world had begun. This has been calculated to be 12 August 3113 BC by the Gregorian Calendar.
The long count is normally written in two parts, the first being the count of days and the second being the current date according to both calendars.

The days are counted using the following system :

1 day    = 1 kin                  
20 kins   = 1 Uinal  (20 days)      
18 Uinals = 1 Tun    (360 days)    
20 Tuns   = 1 Katun  (7200 days)    
20 Katuns = 1 Baktun (144,000 days)

The number is written as 5 groups of digits like this : Baktuns / Katuns / Tuns / Uinals / kins

Most of the recorded dates which have been found begin with '9' which means between (9 x 144000) days and (10 x 144000) days since the start of the long count which would equate to 436 AD to 829 AD. It is thought that when the long count reaches 13.0.0.0.0 it is reset to 0.0.0.0.0 thus giving a period of 5125.37 years.
This brings us to 22 Dec 2012 AD when either time ends or we start a new Maya Era.
By combining the long count and the Tzolkiun and Haab dates it is possible to quote a date which will be unique for a period of 374,152 years, or 73 Maya Eras.
So perhaps we will be safe after 2012 AD after all.

Examples of dates using the Maya Long Count :

             Long Count      Calendar Round
Gregorian                     Txolkiun     Haab
===========   ==============  ===========  ======
14 Nov 1539 - 11.16. 0. 0. 0  13 Ahau       8 Xul
14 Aug 1995 - 12.19. 2. 7. 0   8 Ahau       8 Uo
14 Feb 1996 - 12.19. 2.16. 5  11 Chikehan  13 Pax