Leap Years: Our calendar is based on the premise that a year (the time it takes for the Earth to make a complete revolution about the Sun) is 365 days. That premise, however, arises from convenience, NOT actuality. There are in fact 365.242195 days in a year. That .242195-day discrepancy between the physical year and the *calendar year* causes a 365-day per year calendar to get considerably out of whack after a time. That extra .242195 days is almost .25 of a day. Hence, the bright idea arose that if an extra day was added to every fourth year, the effect of the discrepancy on the accuracy of our calendar would be greatly reduced. (Note that 4 is the reciprocal of .25.) Now, you may notice that .242195 is not exactly .25-- there's still a slight discrepancy. However, the discrepancy is now in the reverse direction; adding an extra day every four years over- compensates for the original problem (because .25 > .242195). So, the additional bright idea was conceived that the extra day would not be added to century years unless the century year is evenly divisible by 400 (i.e., the *century number* (year / 100) is evenly divisible by 4). Note that this still does not completely remove effects of the overcompensation. However, nothing can conveniently be done about that until at least the year 4000, when the rule that century years must be evenly divisible by 4000 to be considered leap can be enforced. (That of course would still not completely remove the overcompensation, just greatly reduce it. I guess in the year 40000, the rule could again be made more stringent-- if our calendar is still in use then.)