1582

1582 is notable in the history of calendars because of the introduction of the Gregorian calendar. Pope Gregory XIII issued the reform through the papal bull Inter gravissimas to correct the drift of the calendar with respect to the vernal equinox and the calculation of Easter.

The reform skipped ten days: after October 4, 1582, the next day was October 15, 1582, in regions adopting the new calendar. The Gregorian calendar also introduced a refined leap-year rule: years divisible by 4 are leap years, except that century years are not leap years unless divisible by 400. This rule is more accurate over long periods than the Julian system.

Adoption varied by territory. Catholic states in Europe quickly adopted the reform in 1582, while Protestant, Orthodox, and other regions retained the Julian calendar for longer. Over subsequent centuries, the new calendar spread worldwide, with notable late adopters including the British Empire in 1752 and Russia in 1918.

Historically, both calendars were used in the same era for different regions, creating confusion in dating events across sources. The 1582 reform laid the groundwork for the modern civil calendar, providing a more stable framework for aligning civil dates with astronomical and ecclesiastical computations that continued to influence timekeeping for centuries.