Proleptic Gregorian Calendar

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It has been suggested that the section [Proleptic Gregorian calendar] from the article [Gregorian calendar] be [Merging and moving pagesmerged] into this article or section. ([Discuss])

The proleptic Gregorian calendar is produced by extending the Gregorian calendar to dates preceding its official introduction in 1582.

From 1 March 200 to 28 February 300 it corresponds to the Julian calendar.

For this calendar we can distinguish two systems of numbering years BC.

Bede and later historians did not use the Latin zero, nulla, as a 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 zero and represent earlier years as negative. 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.

Note that the Julian calendar was in actual use after AD 4, until 1582 or later (see From Julian to Gregorian), so historians and astronomers prefer to use the actual Julian calendar during that period. Likewise, the proleptic Julian calendar is used to specify dates before AD 4, its first quadrennial leap year (leap years between 45 BC and AD 4 were irregular, see Leap years error). But when seasonal dates are important, the proleptic Gregorian calendar is sometimes used, especially when discussing cultures that did not use the Julian calendar.

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