Which calendars have leap years in them?

A year is considered a leap year if it is evenly divisible by 4, thus 2012 was evenly divisible by 4 so it was a leap year. 2013 does not evenly divide into 4 so it is not a leap year.

There are two major exceptions, a year which is divisible by 4 but not by 100 is NOT a leap year (like 1900 wasn't), but if it is also divisible by 400 then it IS a leap year (like 2000 was).