Gregorian calendar

The Gregorian calendar is the calendar widely used in the Western world. A modification of the Julian calendar, it was first proposed by the Neapolitan doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 (Note: The papal bull Inter gravissimas was signed in the year 1581 for unknown reasons, but printed on 1 March in 1582. Although 1581 is often attributed to the use of years beginning on 25 March by the papacy, other contemporaneous papal bulls have years that do not agree with March years, let alone years since a pope was named or other types of years.)

Confusion with British vs. American usage

Dates of events in Great Britain prior to 1752 are usually now shown in their original Old Style form, whereas dates of events in (then British) America prior to 1752 are usually now shown in the New Style form.

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• For example, Shakespeare died on 23 April (OS), and it is rare to see this converted to 3 May (NS). But while George Washington was born on 11 February (OS), his birthday is now celebrated on 22 February (NS).

However, neither of these practices is universal in either country, so it is sometimes very unclear which calendar is being used, and this can lead to false assumptions, which can lead to dates being inaccurately converted from one calendar to the other. Since the resurgence of interest in the history of the calendar, more information about the real dates (according to various calendars) of events has been forthcoming and many previous errors have been corrected. While these changes are welcome, there is still much scope for confusion; therefore, noting the calendar being used in transitional periods would help the reader understand the dates involved.

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