A calendar is a way of keeping track of time. How complicated is that? Not very. Except in leap years, such as 2012, when a year has 366 days instead of the usual 365 days.
We need leap years because, in fact, it takes the Earth more than 365 days to travel around the sun. It takes the Earth about 365 days and six hours to make its trip. So every four years, one day (February 29) gets added to take that time into account.