Leap Year
It is a leap year if the year is exactly divisible by 4 andis not divisible by 100 unless it is divisable by 400.
The calendar year is 365 days long, unless the year is exactly
divisible by 4, in which case an extra day is added to February to
make the year 366 days long.
If the year is the last year of a century, eg. 1700, 1800, 1900,
2000, then it is only a leap year if it is exactly divisible by 400.
Therefore, 1900 wasn't a leap year but 2000 will be.
The reason for these rules is to bring the average length of the
calendar year into line with the length of the Earth's orbit around
the Sun, so that the seasons always occur during the same months each year
