Interesting fact:

The modern calendar that we’re familiar with now only came into popular use in the 1600s, so there were many systems of less unified date-keeping before then. The first form of calendar is said to be the solar calendar used by Egyptians circa 45BC. Later, Julius Caesar brought in the Julian calendar, which was the first to use a 12-month system. This calendar did have a major flaw though—it was off by 11 minutes.