# Proleptic Gregorian calendar

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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 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 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 RangeProleptic Gregorian RangeGregorian 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

## Related Research Articles

The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ".

Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0; the years before that are designated with negative numbers and the years after that are designated with positive numbers. Astronomers use the Julian calendar for years before 1582, including the year 0, and the Gregorian calendar for years after 1582, as exemplified by Jacques Cassini (1740), Simon Newcomb (1898) and Fred Espenak (2007).

ISO 8601 is an international standard covering the worldwide exchange and communication of date- and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, with updates in 1991, 2000, 2004, and 2019. The standard aims to provide a well-defined, unambiguous method of representing calendar dates and times in worldwide communications, especially to avoid misinterpreting numeric dates and times when such data is transferred between countries with different conventions for writing numeric dates and times.

The Julian calendar, proposed by Julius Caesar in AUC 708, was a reform of the Roman calendar. It took effect on 1 January AUC 709 , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandria.

A leap year is a calendar year that contains an additional day added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting an additional day or month into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected. A year that is not a leap year is a common year.

AD 1 (I), 1 AD or 1 CE is the epoch year for the Anno Domini calendar era. It was the first year of the Common Era (CE), of the 1st millennium and of the 1st century. It was a common year starting on Saturday or Sunday, a common year starting on Saturday by the proleptic Julian calendar, and a common year starting on Monday by the proleptic Gregorian calendar. In the Roman Empire, 1 AD was known as the Year of the Consulship of Caesar and Paullus, named after Roman consuls Gaius Caesar and Lucius Aemilius Paullus, and less frequently, as year 754 AUC within the Roman Empire. The denomination "AD 1" for this year has been in consistent use since the mid-medieval period when the anno Domini (AD) calendar era became the prevalent method in Europe for naming years. It was the beginning of the Christian/Common era. The preceding year is 1 BC; there is no year 0 in this numbering scheme. The Anno Domini dating system was devised in AD 525 by Dionysius Exiguus.

The proleptic Julian calendar is produced by extending the Julian calendar backwards to dates preceding AD 8 when the quadrennial leap year stabilized. The leap years that were actually observed between the implementation of the Julian calendar in 45 BC and AD 8 were erratic: see the Julian calendar article for details.

The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events.

The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. This calendar is based on the ancient Egyptian calendar. To avoid the calendar creep of the latter, a reform of the ancient Egyptian calendar was introduced at the time of Ptolemy III which consisted of the intercalation of a sixth epagomenal day every fourth year. However, this reform was opposed by the Egyptian priests, and the reform was not adopted until 25 BC, when the Roman Emperor Augustus imposed the Decree upon Egypt as its official calendar. To distinguish it from the Ancient Egyptian calendar, which remained in use by some astronomers until medieval times, this reformed calendar is known as the Coptic or Alexandrian calendar. Its years and months coincide with those of the Ethiopian calendar but have different numbers and names.

Dominical letters or Sunday letters are a method used to determine the day of the week for particular dates. When using this method, each year is assigned a letter depending on which day of the week the year starts.

A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, it is the year 2021 as per the Gregorian calendar, which numbers its years in the Western Christian era.

The Doomsday rule is an algorithm of determination of the day of the week for a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years. The algorithm for mental calculation was devised by John Conway in 1973, drawing inspiration from Lewis Carroll's perpetual calendar algorithm. It takes advantage of each year having a certain day of the week upon which certain easy-to-remember dates, called the doomsdays, fall; for example, the last day of February, 4/4, 6/6, 8/8, 10/10, and 12/12 all occur on the same day of the week in any year. Applying the Doomsday algorithm involves three steps: Determination of the anchor day for the century, calculation of the anchor day for the year from the one for the century, and selection of the closest date out of those that always fall on the doomsday, e.g., 4/4 and 6/6, and count of the number of days between that date and the date in question to arrive at the day of the week. The technique applies to both the Gregorian calendar and the Julian calendar, although their doomsdays are usually different days of the week.

Old Style (O.S.) and New Style (N.S.) indicate a dating system from before and after a calendar change, respectively. Usually this is the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and the 20th century.

The Ethiopian calendar, Eritrean calendar, or the Ge'ez calendar is the principal calendar used in Ethiopia and Eritrea, which also serves as the liturgical year for Ethiopian and Eritrean Christians belonging to the Orthodox Tewahedo Churches, Eastern Catholic Churches, and Eastern Protestant Christian P'ent'ay Churches. The Ethiopian calendar is a solar calendar that has more in common with the Coptic calendar of the Coptic Orthodox Church of Alexandria and Coptic Catholic Church, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on August 29 or August 30 in the Julian calendar. A gap of seven to eight years between the Ethiopian and Gregorian calendars results from an alternative calculation in determining the date of the Annunciation.

A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar ; in this system, the year 1 BC is followed directly by year AD 1. However, there is a year zero in both the astronomical year numbering system, and the ISO 8601:2004 system, the interchange standard for all calendar numbering systems,. And there is a year zero in most Buddhist and Hindu calendars.

Dual dating is the practice, in historical materials, to indicate some dates with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen, a slash or are placed one above the other. The need for dual dating arose from the transition from an older calendar to a newer one. For example, in "10/21 February 1750/51", the dual day of the month is due to the correction for excess leap years in the Julian calendar by the Gregorian calendar, and the dual year is due to some countries beginning their numbered year on 1 January while others were still using another date.

The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the 'tropical' or 'solar' year that the inaccuracy had caused during the intervening centuries.

The tables below list equivalent dates in the Julian and Gregorian calendars. Years are given in astronomical year numbering.

The adoption of the Gregorian Calendar was an event in the modern history of most cultures and societies, marking a change from their traditional dating system to the modern dating system, the Gregorian calendar, that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During – and for some time after – the change between systems, it has been common to use the terms Old Style and New Style when giving dates, to indicate which calendar was used to reckon them.

## 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. Spathaky, Mike. "Old Style New Style Dates and the Change to the Gregorian Calendar". GENUKI. Retrieved 27 May 2021. 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 10 February 2012.
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.