The earth orbits the sun once every 365.256 days. The calendar, however, only has 365 honest-to-God slots. This means that there is 0.256 days left over each year which after 4 years add up to 4 x 0.256 = 1.024 days. Every four years, we decide to add a 'Leap Day' at the end of February which normally got short-changed and only had 28 days in a standard year. This still leaves us at the end of 4 years with 0.024 days or 34.4 minutes extra. This extra bit of unaccounted time in our calendar is made up for every two centuries by adding a second day. 1600 was a leap year, 1700, 1800 and 1900 were not, but in 2000 we will have 366 days with February 29th added. This follows the rule that century years divisible by 400 ARE leap years.