Time to push the calendar back by a day in order to bring the seasons back in line again! We use a calendar which models the year as 365.2425 Solar days. In order to add up to a whole number of days, you need to look at a period of 400 years. If that bothers you, never fear! The Earth's rotation is slowing down at a rate somewhere from 1.7 to 2.3 ms per century. By my calculations, in a mere 2.5 million to 3.4 million years, we will no longer need leap days -- the year will be exactly 365 Solar days long! That is, assuming, that the Earth's orbit hasn't changed significantly enough to render the Gregorian calendar useless -- and that there will be anyone around to care.