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
Celtic calendar made up of bronze fragments, in a single huge plate. It is inscribed in Gaulish with Latin characters and uses Roman numerals. The Coligny Calendar is an attempt to reconcile the cycles of the moon and sun, as is the modern Gregorian calendar. However, the Coligny calendar considers the phases of the moon to be important, and each month always begins with the same moon phase. The calendar uses a mathematical arrangement to keep a normal 12-month calendar in sync with the moon and keeps the whole system in sync by adding an intercalary month every 2 years. The Coligny