A calendar is a way of keeping track of the days in a year or the time it takes for the Earth to go from one place in its orbit to the same place one orbit later. The first calendars date back 5000 years to the Babylonian and Egyptian Cultures. The earliest calendars had 360 days, and this fact became the basis for our system of measuring angle and of keeping track of time, the subdivisions of an hour and of a minute. The Babylonians eventually had a calendar that was accurate to about thirty minutes in a year, or one part in 10,000. They had a leap-year; that is, they added an extra day to their calendar every fourth year to approximate a Solar year of 365 and a quarter days. These early calendars allowed the Babylonians and the Egyptians to run their systems of agriculture with good reliability.