The Mayan Calendar
Principals of the Mayan calendar date all the way back to the 5th century BCE.
The Mayan calendars use dates back to well into 500 B.C., but its ancient beginnings do not retract from its overall effectiveness. The Mayans had a great fascination with the stars. This subsequently leads to the creation of very detailed and accurate astrological data and also mathematics. The Mayan's have been discovered to have records of lunar cycles, Venus cycles and an intricate counting system. Through this data the Mayan's were able to develop their calendar that is actually a combination of multiple calendar systems.
The main aspect of the Mayan Calendar is a combination of a 260 day calendar, a solar calendar. The solar calendar is very similar to the modern year calendar used throughout the world. It is comprised of 52 Haabs, which can be considered the equivalent of a week. These two calendars integrated on a system based on cycles of 13 and 20 days. Some theorize that it is from this system of 13 and 20 that the 260 day calendar cycle, known as the Tzolk'in was developed. The Mayan's also used a "long count" calendar, which allowed them to refer to dates far in the future or passed. This system was mainly numerical and used a base of 20 for calculations. Some aspects of the Mayan calendar are still used in Guatemala and Oaxaca, Mexico today.
Also check out our info on the Catholic Calendar