Proleptic Gregorian calendar

The proleptic Gregorian calendar is created by extending the Gregorian calendar to dates before its introduction in 1582. In countries that adopted the Gregorian calendar after its introduction, dates occurring between then and when said countries adopted the calendar are sometimes "Gregorianized". For example, U.S. President George Washington's birthday was originally dated February 11, 1731. This was because Great Britain was using the Julian calendar at the time. When Great Britain switched to the Gregorian calendar, the date was changed proleptically to February 22, 1732. This is still the present-day dating of his birthday.