Intercalation

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"In the Julian Calendar as well as in the Gregorian Calendar that improved it, intercalation is done by adding an extra day to February in each leap year. In the Julian Calendar this was done every four years. In the Gregorian calendar, years whose number is evenly divisible by 100 but not 400 were exempted in order to improve accuracy. Thus, 2000 was the first century year in 400 years to be a leap year."