Some 10 years after becoming the Pope, Gregory XIII acting on the advice of two renowned astronomers, issued a papal edict directing that the Julian calendar be corrected “to allow the calendar to catch up with the Lord’s time”.
The correction was made by making October 4, 1582 be October 15, 1582. The new calendar was called the Gregorian calendar.
The Gregorian calendar introduced 29 days in February every 4 years which would be called leap years with the exception of centennial years which would not be leap years unless they are divisible by 400.
Therefore a year would be a leap year when:
Modulus (year , 4) = 0 and Modulus (year , 100) <> 0
with the following exception:
a year would be a leap year when Modulus (year , 400) = 0
More analysis and programming of leap years are under Library:
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