Also known as the Maya Long Count Calendar, it basically calculates present day by counting forward from an established start date. In the Maya culture, the start date of their calendar is equal to August 11, 3114 BC in the calendar we all know and use today, the Gregorian calendar.
At the time, the two calendars that were most used to calculate a day date did not account for the years, and so the Maya developed the Long Count calendar.
How to make a date calculation
The date is calculated by using a series of placeholders to indicate the number of days passed since the start of the cycle, or rather, the creation of the current successful world. According to Maya prophecy, success of this world will be defined at the end of this cycle, or December 21st, 2012.
A base-20 system is used to define the placeholders, except in the third placeholder which uses base-18. This differs from the base-10 system that the world uses today.
For the first placeholder, a date of 0.0.0.0.19 equals 19 days, but then the second placeholder gets involved at 0.0.0.1.0 to indicate 20 days (number multiplied by 20), and so on. However, the third placeholder is base-18, so 0.0.1.0.0 equals 2,160 days.
The calculation:
0.0.0.0.0 or (0 x 20 x 20 x 18 x 20) + (0 x 20 x 18 x 20) + (0 x 18 x 20) + (0 x 20) + the number.
A date of 4.6.3.9.15 = (4 x 20 x 20 x 18 x 20) + (6 x 20 x 18 x 20) + (3 x 18 x 20) + (9 x 20) + 15 = 576,000 + 43,200 + 1,080 + 180 + 15 = 620,475 days, or 1,1699 years, 11 months, 3 days (approx.) since the start of the calendar on August 11th, 3014BC. This would equate to some time in the second week of September, 1314BC, by the current Gregorian calendar.
The presence of the number zero for a placeholder is also one of the first instances of the concept of zero, and used as a placeholder.
The mystery behind the Mayan Doomsday Prophesy
According to the Maya, the Gods attempted to create the world four times and failed with the first three, and thus we are now living in that final fourth world. The doomsday warning is based soley on the fact that the previous unsuccessful creation ended on a long count of 12.19.19.17.19, with the next cycle starting on 13.0.0.0.0 – the current Mayan Long Count cycle ends (12.19.19.17.19) on December 20th 2012, essentially ending this world as a failed world, and a new cycle / world (13.0.0.0.0) would begind on December 21st, 2012.