# 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 U.S. 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.

## Usage

ISO 8601:2004 (clause 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, especially when converting Long Count dates (1st century BC – 10th century AD).

The best practice for citation of historically 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.

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.

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, MySQL, SQLite, PHP, CIM, Delphi and Python.

## 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 accomplished by repeating 24 February (see Julian reform). Therefore, the dates between 24 and 29 February in all leap years were irregular.

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

Julian Range Proleptic Gregorian Range Gregorian Ahead By
to 1 March 100
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
Julian Range Gregorian Range Difference
From 5 October 1582
to 18 February 1700
From 15 October 1582
to 28 February 1700
10 days
From 19 February 1700
to 17 February 1800
From 1 March 1700
to 28 February 1800
11 days
From 18 February 1800
to 16 February 1900
From 1 March 1800
to 28 February 1900
12 days
From 17 February 1900
to 15 February 1923
From 1 March 1900
to 28 February 1923
13 days
Orthodox Julian Range Gregorian Range Julian Behind By
From 16 February 1923
to 14 February 2100
From 1 March 1923
to 28 February 2100
13 days