When you ask most any modern day scientist to detail the origin of our time chronology, they cannot give you an honest answer. They don't know, but they do agree on various historical facts.
Our present Gregorian calendar, which is named after Pope Gregory XIII who instituted it in March of 1582, is not the most accurate that has been used by civilization. Although our calendar is quite "refined" it is not as accurate as the one that the Ancient Mayans used thousands of years ago.
The Gregorian year is a bit too long. The error amounts to three days in ten thousand years. The Ancient Mayans year was too short, but the error was only two days in ten thousand years. However, the Ancient Mayans were able to calculate much more accurately than many of our present day scientists think possible. In fact, their mistake was nil in ten thousand years!
Such a revolutionary science is absolutely baffling. It sheds a totally different light on the Ancient Mayan's civilization. The Mayan's reckoning of time was covered with a magical mysticism. They were of the opinion that events moved in a circle, represented by returning cycles of service. Days, months, and years were all members of shift-relief-teams marching through eternity. The priests were able to calculate the combined influence of all the marchers, and in this way they were able to predict the fate of mankind.
It was expected by the Ancient Mayans that every 260 years, history would repeat itself in certain circles. This mixing of intuition with time may appear to be strange to us. Yet quite intelligent ideas were behind it.