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 "Gregorian-ized" as well. For example, George Washington was born on February 11, 1731 (Old Style), as Britain was using the Julian calendar. After the switch, that day became February 22, which is the date commonly given as Washington's birthday.

The Gregorian calendar is explicitly required for all dates before 1582 by ISO 8601:2004 (clause 4.3.2.1 The Gregorian calendar) if the partners to information exchange agree. It is also used by most Maya scholars, especially when converting Long Count dates (1st century BC – 10th century).

The best practice for historians quoting contemporary documents is to give the date as in the original and to add as a note any contextual clues, conclusions about the calendar used and equivalents in other calendrical systems. This gives others the opportunity to re-evaluate the evidence.

For these calendars one can distinguish two systems of numbering years BC. Bede and later historians did not use the Latin zero, nulla, as a year (see 0 (year)), so the year preceding AD 1 is 1 BC. In this system the year 1 BC is a leap year (likewise in the proleptic Julian calendar). Mathematically, it is more convenient to include a year 0 and represent earlier years as negative, for the specific purpose of facilitating the calculation of the number of years between a negative (BC) year and a positive (AD) year. This is the convention used in astronomical year numbering and in the international standard date system, ISO 8601. In these systems, the year 0 is a leap year.

Although the nominal Julian calendar began in 45 BC, leap years between 45 BC and 1 BC were irregular (see Leap year error). Thus the Julian calendar with quadrennial leap years was only used from the end of AD 4 until 1582 or later.

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