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### HomeEnergy says

simple

normal year has 365 full days

one full circle of Earth around the Sun is near (but not exactly) 365.25 days

Therefore, every 4 years, we add 1 day to February

But because the 0.25 day discrepancy is a bit less, every 100 years we don't add a day in February

To make it even more exact we do add one day in February every 400 years, last time was year 2000, next time will be 2400 (I believe many won't live to see year 2400 though...)

In short, every 400 years we add 4x24+1 day = 97 days, this add 97/400=0.2425 day on average every year

### paxwill says

What exactly do you mean by "calculate leap years" ?

If you want to know which years are leap years, there is a simple set of rules:

All years that are divisible by 4 are leap years, except years that are divisible by 100. But if the year is divisible by 400, then it is a leap year.

Examples:

1988 -- leap year because 1988 is divisible by 4

1900 -- not a leap year because although 1900 is divisible by 4, it is divisible by 100 but not by 400

2000 -- leap year because it is divisible by 400

I hope that explains what you were asking about.

### ackman1465 says

Go on to any reasonable Web search engine and you can get a thorough explanation.....