Proleptic Gregorian calendar Proleptic Gregorian calendar The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582. In countries that adopted the Gregorian calendar later, dates occurring in the interim (between 1582 and the local adoption) are sometimes "Gregorianized" as well. For example, George Washington was born on February 11, 1731 (Old Style), as Great Britain and its possessions were using the Julian calendar with English years starting on March 25 until September 1752. After the switch, that day became February 22, 1732, which is the date commonly given as Washington's birthday. The proleptic
Adoption of the Gregorian calendar old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During and for some time after the change between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them. The Gregorian calendar was decreed in 1582 by the papal bull "Inter gravissimas" by Pope Gregory XIII, to correct a divergence in the canonical date of the [northern] spring equinox from observed reality (due to an error in the Julian system) that affected the calculation