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 is only occasionally used today. North Korea from 1997 officially counts years based on the Juche era, the first year of which is 1912. The Republic of China (ROC) formally adopted the Gregorian calendar at its founding on , but China soon descended into a period of warlordism with different warlords using different calendars. With the unification of China under the Kuomintang in October 1928, the Nationalist government decreed that effective the Gregorian calendar would be used. China retained the Chinese traditions of numbering the months and a modified Era System, backdating the first year of the ROC to 1912;