Proleptic Gregorian calendar

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