Leap years

Even though the year 2100 is exactly divisible by 4 is not a leap year. Why?

This year 2012  is exactly divisible by 4.
Therefore it a leap year having 29 days in February.

Year 2100 is exactly divisible by 4 .
2100 divided by 4 is 525 ( exactly divisible by 4 )
But it not a leap year , Why ?
2100 is a century year.

Any century year should be exactly divisible by 400 to be a leap year.
If 2100 divided by 400, the answer  is 5.25.
Therefore it is not exactly divisible by 400, hence,it is  not a leap year.

2400 is a century year, it is exactly divisible by 400.    2400/400 =6
Therefore it is a leap year.
Years 2100, 2200, 2300 are century years which are not  exactly divisible by 400.
They are not leap years. But 2400 is a leap year because it is exactly divisible by 400.

The Gregorian rule is , to be a leap year , ordinary years should be exactly divisible by 4.
Century years ( ending with two zeros ) should be exactly divisible by 400 not by 4.
2500, 2600 ,2700 are not exactly divisible by 400, therefore they are not leap years.
2800 is exactly divisible by 400, therefore it is a leap year.