However, the date of December 21, 2012, is not based on Mayan tuns. It is based on the total accumulation of days since the beginning of the Mayan calendar. It is commonly accepted that the first date on the Mayan calendar is August 11, 3113 BCE on the Gregorian calendar. Therefore, we must start on that date and count forward in time.

The Mayan baktun is the equivalent of 400 Mayan tuns (years). But remember, their years are only 360 days long. 400 x 360 = 144,000. Therefore, each Mayan baktun is a total of 144,000 days long.

As of the time of posting, the Mayan date is 12.19.19.3.11. The first number represents the baktun, the second number katuns (20 Mayan years), the third number is tuns (Mayan years), the fourth is uinals (20 day “weeks”), and the final number is the day. I know that I’m making some history nut cringe at my over simplification of the Mayan calendar; I’m using “years” and “weeks” to make things easy to understand.

On December 21, 2012, the first number in the Mayan calendar will change from 12 to 13, making it 13.0.0.0.0. The last time this happened was on September 18, 1618, when the current baktun started. As you have already imagined, the Mayan date on that day was 12.0.0.0.0. Obviously, the world didn’t end.

But how do we know that 13.0.0.0.0 will happen on December 21, 2012? First, we need to calculate the total amount of days required to reach the 13th baktun: 144,000 x 13 = 1,872,000 days.