This has to do with history. Which is older, the Pyramids in Egypt or the Mayan Pyramids?
The Egyptian pyramids are older, by over 1,000 years. I believe the Sumerians came up with the first calendar. The Mayan culture emerged relatively recently, the classic Mayan monuments date essentially from the period of 250 – 900 CE. The Aztecs were even more recent, arising after the collapse of Mayan civilization, so their calendar and much of the rest of their culture was heavily influenced by Mayan models. Although the Mayan culture emerged much more recently, for unknown reasons the Mayan calendar begins around 3100 BCE.
