It takes the Earth approximately 365.242199 days to complete a revolution around the sun and Leap years were added to keep the calendar in line with these revolutions. In 46 BC, Julius Caesar, in his aptly named Julian calendar, was the first to add Leap Years; however, he added too many. The Gregorian, or common solar calendar, corrected this some 1500 years later. Its 365 days includes Leap Years under three conditions:
- if the year is evenly divisible by 4;
- if the year can be evenly divided by 100, it is NOT a leap year, unless;
- the year is also evenly divisible by 400, then it is a Leap Year.
This means that 2000 and 2400 are Leap Years, while 1900, 2100, 2200, and 2300 are not.