You make a comment in the Leap Year Example that leap years are years that are divisable by 4. This is not exactly true.
In the Gregorian calendar, a normal year consists of 365 days. Because the actual length of a sidereal year (the time required for the Earth to revolve once about the Sun) is actually 365.25635 days, a "leap year" of 366 days is used once every four years to eliminate the error caused by three normal (but short) years. Any year that is evenly divisible by 4 is a leap year: for example, 1988, 1992, and 1996 are leap years.
However, there is still a small error that must be accounted for. To eliminate this error, the Gregorian calendar stipulates that a year that is evenly divisible by 100 (for example, 1900) is a leap year only if it is also evenly divisible by 400.
For this reason, the following years are not leap years: 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600 This is because they are evenly divisible by 100 but not by 400.
The following years are leap years: 1600, 2000, 2400 This is because they are evenly divisible by both 100 and 400.