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

From March 1, 200 to February 28, 300 it corresponds to the Julian calendar. [What about the fact that the year 300 was a leap year in the Julian Calendar, but not in the Gregorian 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.

## Difference between Julian and proleptic Gregorian calendar dates

Before the introduction of the Gregorian calendar, the difference between Julian and proleptic Gregorian calendar dates has increased as follows:

Julian range Proleptic Gregorian range Difference
From February 29, 300
to February 27, 500
From March 1, 300
to February 28, 500
1 day
From February 28, 500
to February 26, 600
From March 1, 500
to February 28, 600
2 days
From February 27, 600
to February 25, 700
From March 1, 600
to February 28, 700
3 days
From February 26, 700
to February 24, 900
From March 1, 700
to February 28, 900
4 days
From February 25, 900
to February 23, 1000
From March 1, 900
to February 28, 1000
5 days
From February 24, 1000
to February 22, 1100
From March 1, 1000
to February 28, 1100
6 days
From February 23, 1100
to February 21, 1300
From March 1, 1100
to February 28, 1300
7 days
From February 22, 1300
to February 20, 1400
From March 1, 1300
to February 28, 1400
8 days
From February 21, 1400
to February 19, 1500
From March 1, 1400
to February 28, 1500
9 days
From February 20, 1500
to October 4, 1582
From March 1, 1500
to October 14, 1582
10 days