Proleptic Gregorian calendar


The proleptic Gregorian calendar extends the Gregorian calendar backward, proleptically, to dates before its 1582 introduction, creating a consistent dating system across history.
For example, when U.S. President George Washington was born on 11 February 1731,
Great Britain and its colonies still used the Julian calendar, also referred to as Old Style dating.
When Britain finally adopted the Gregorian calendar in September 1752, Washington's birth date became 22 February 1732.
During his life, both dates were observed and commemorated, but the latter is the one used and recognized since then.

Usage

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 ; therefore the year preceding AD 1 is 1 BC. In this system, the year 1 BC is a leap year. 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 year and a positive 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. Thus the Julian calendar with quadrennial leap years was only used from the end of AD 4 until 1582 or later.
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.

Julian calendar differences

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 was accomplished by repeating 24 February. 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, 29 February is included in the calculation when the conversion crosses the border of February and March.
Julian rangeProleptic Gregorian rangeGregorian ahead by
From 3 March AD 4

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
Julian rangeGregorian rangeDifference
From
to 		From
to 		10 days
From
to 		From
to 		11 days
From
to 		From
to 		12 days
From
to 		From
to 		13 days
Orthodox Julian rangeGregorian rangeJulian behind by
From
to 		From
to 		13 days