Proleptic Gregorian calendar

The proleptic Gregorian calendar extends the Gregorian calendar backward, proleptically, to dates before its 1582 introduction, creating a consistent dating system across history. For example, when U.S. President George Washington was born on 11 February 1731, Great Britain and its colonies still used the Julian calendar, also referred to as Old Style dating. When Britain finally adopted the Gregorian calendar in September 1752, Washington's birth date became 22 February 1732. During his life, both dates were observed and commemorated, but the latter is the one used and recognized since then.