Gregorian calendar

The Gregorian calendar was first released by Pope Gregory XIII in October 1582 as a modification of the Julian calendar, with the year being reduced from 365.25 days to 365.2425 days. The leap year would occur for every year divisible by 4, except for years divisible by 100, in which they would have to be divisible by 400 to be a leap year. For example, the years 1700, 1800, and 1900 would not be leap years, but 1600 and 2000 are.