The Haab

The Haab is a 365-day, solar-year calendar, divided into 18 months of 20 days each.

The five extra days were considered “unlucky” and made a shorter 19th month.

Consider how the Long Count works as a fancy odometer: It has five slots separated by decimals, using hieroglyphs (where we would use numerals). Basically, Dec. 21, 2012 (approximately) is when almost all the numbers in this cycle of the odometer reach their highest point and they all go back to zero: It will read 13.0.0.0.0.