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Which years are leap years?
In the Gregorian calendar, which is the calendar used by most modern countries, the following rules decides which years are leap years:
1. Every year divisible by 4 is a leap year.
2. But every year divisible by 100 is NOT a leap year
3. Unless the year is also divisible by 400, then it is still a leap year.
This means that year 1800, 1900, 2100, 2200, 2300 and 2500 are NOT leap years, while year 2000 and 2400 are leap years.
This actually means year 2000 is kind of special, as it is the first time the third rule is used in many parts of the world.
In the old Julian Calendar, there was only one rule: Every year divisible by 4 is a leap year. This calendar was used before the Gregorian calendar was adopted.