I presume that you know what a leap year is.  A year is the amount of time the Earth takes to orbit the Sun, and is measured as 365 days, or 365 rotations of Earth.  It is, in reality, a little bit longer.  Each orbit of the Sun takes 365.25 days, or 365 days and six hours.  So what do we do with that six hours? We ignore it for 3 years.  On the fourth year we add up those extra hours and we get 24 hours, an extra day! So we add in February 29th every four years. This...