The Babylonians, in my opinion, were the most important ancient civilization of the ones provided. Along with the dividing of the day into twenty-four separate hours, they created a calendar with alternating 29- and 30- day months. This system required the addition of an extra month three times every eight years, and as a further adjustment, the king at the time would periodically order the insertion of an additional month to their years at irregular intervals.

Their calendar kept roughly in-step with the lunar year, yet to balance it with the solar year, the early Babylonians calculated that they needed to add an extra month three times every eight years. Still, this system did not accurately make up for the accumulated differences between the solar year and the lunar year. So, whenever the king felt that the calendar had slipped too far out of step with the seasons...