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
Proleptic Gregorian calendar Julian and proleptic Gregorian calendar dates are as follows: The table below assumes a Julian leap day of 29 February, but the Julian leap day (the bissextile day) was in Latin or 24 February (see Julian reform), so dates between 24 and 29 February in all leap years were irregular. Note: When converting a date in a year which is leap in one calendar but not the other, include 29 February in the calculation when the conversion crosses the border between February and March. Proleptic Gregorian calendar The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to