Proleptic Gregorian calendar

The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occurring in the interim period of 15 October 1582 (the first date of use of Gregorian calendrical dates, being dated 5 October 1582 in the preceding Julian calendar) to the date on which the pertinent nation adopted the Gregorian calendar and abandoned the Julian calendar are sometimes 'Gregorianized' also. For example, the birthday of President George Washington was originally dated 11 February 1731 (Old Style) because Great Britain, of which he was born a subject, used (until September 1752) the Julian calendar and dated the beginning of English years as 25 March. After Great Britain switched to the Gregorian calendar, Washington's birthday was dated 22 February 1732 proleptically, according to the Gregorian calendar applied backward. This remains the modern dating of his birthday.[1]

Usage

ISO 8601:2004 (clause 4.3.2.1 The Gregorian calendar) explicitly requires use of the proleptic Gregorian calendar for all dates before the introduction of 15 October 1582, if the partners to an exchange of information agree. Most scholars of Maya civilization also use it,[2] especially when converting Long Count dates (1st century BC – 10th century AD).

The best practice for citation of historical contemporary documents is to cite the date as expressed in the original text and to notate any contextual implications and conclusions regarding the calendar used and equivalents in other calendars. This practice permits others to re-evaluate the original evidence.[3]

For these calendars one can distinguish two systems of numbering years BC. Bede and later historians did not enumerate any year as zero (nulla in Latin; see Year zero); therefore 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 numbers 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 in astronomical year numbering and the international standard date system, ISO 8601. In these systems, the year 0 is a leap year.[4]

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 (contingent on the specific nation in question).

The proleptic Gregorian calendar is sometimes used in computer software to simplify identifying pre-Gregorian dates, e. g. in PostgreSQL,[5] MySQL,[6] SQLite,[7] PHP, CIM, Delphi and Python.[8]

Difference between Julian and proleptic Gregorian calendar dates

Before the official and first introduction of the Gregorian calendar, the differences between Julian and proleptic Gregorian calendar dates are as follows:

The table below assumes a Julian leap day of 29 February, but the Julian leap day, that is, the bissextile day (ante diem bis sextum Kalendas Martias in Latin) was 24 February (see Julian reform). Therefore dates between 24 and 29 February in all leap years were irregular.

Note: When converting a date in a year which is leap in one calendar yet not in the other, include 29 February in the calculation when the conversion crosses the border of February and March.

Julian RangeProleptic Gregorian RangeGregorian Ahead By:
From 3 March AD 4
(beginning of quadrennial leap years)
to 1 March 100		 From 1 March AD 4
to 28 February 100		 −2 days
From 2 March 100
to 29 February 200		 From 1 March 100
to 28 February 200		 −1 day
From 1 March 200
to 28 February 300		 From 1 March 200
to 28 February 300		 0 days
From 29 February 300
to 27 February 500		 From 1 March 300
to 28 February 500		 1 day
From 28 February 500
to 26 February 600		 From 1 March 500
to 28 February 600		 2 days
From 27 February 600
to 25 February 700		 From 1 March 600
to 28 February 700		 3 days
From 26 February 700
to 24 February 900		 From 1 March 700
to 28 February 900		 4 days
From 25 February 900
to 23 February 1000		 From 1 March 900
to 28 February 1000		 5 days
From 24 February 1000
to 22 February 1100		 From 1 March 1000
to 28 February 1100		 6 days
From 23 February 1100
to 21 February 1300		 From 1 March 1100
to 28 February 1300		 7 days
From 22 February 1300
to 20 February 1400		 From 1 March 1300
to 28 February 1400		 8 days
From 21 February 1400
to 19 February 1500		 From 1 March 1400
to 28 February 1500		 9 days
From 20 February 1500
to 4 October 1582		 From 1 March 1500
to 14 October 1582		 10 days

See also

References
  1. "George Washington's Birthday". National Archives. n.d. Retrieved 11 May 2016.
  2. The proceedings of the Maya hieroglyphic workshop. University of Texas. 1982. p. 173.
  3. Mike Spathaky Old Style New Style Dates and the Change to the Gregorian Calendar. "Increasingly parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the New Style 1734 started even though the Old Style 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Old or the New Style reckoning. The date should either be written 20th January 1745 O.S. (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."
  4. Doggett, L. E. (1992). "Calendars". In P. Kenneth Seidelmann (ed.). Explanatory Supplement to the Astronomical Almanac. Sausalito, California: University Science Books. ISBN 0-935702-68-7. Archived from the original on 2012-02-10.
  5. "B.4. History of Units". PostgreSQL Documentation.
  6. "11.8. What Calendar Is Used By MySQL?". MySQL 5.0 Reference Manual. Retrieved 21 July 2010.
  7. "Date And Time Functions". SQL As Understood By SQLite. Retrieved 16 September 2010.
  8. "8.1.3. date Objects". Python v3.8.2 documentation.
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