TheInfoList

The Gregorian calendar is the
calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day within such a system. A calendar is al ...

used in most of the world. It was introduced in October 1582 by
Pope Gregory XIII Pope Gregory XIII ( la, Gregorius XIII; 7 January 1502 – 10 April 1585), born Ugo Boncompagni, was head of the Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the , with 1.3 billion Catholics . ...

as a modification of the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman people, Roman general and statesman who played a critical role in Crisis of the Roman Republic, the events that led to the d ...
, 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 calendar spaces
leap year A leap year (also known as an intercalary year or year) is a that contains an additional day (or, in the case of a , a month) added to keep the calendar year synchronized with the or . Because astronomical events and seasons do not repeat in a ...

s to make its average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is: There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

es.See Wikisource English translation of the (Latin) 1582 papal bull ''
Inter gravissimas ''Inter gravissimas'' (English: "Among the most serious...") was a papal bull issued by Pope Gregory XIII on 24 February 1582.
''.
Second, in the years since the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynia Bithynia (; Koine Greek Koine Greek (, , Greek approximately ;. , , , lit. "Common Greek"), also known as Alexandrian dialec ...
in AD 325, the excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the (Northern) spring equinox was occurring well before its nominal 21 March date. This date was important to the Christian churches because it is fundamental to the calculation of the date of Easter. To reinstate the association, the reform advanced the date by 10 days: Thursday 4 October 1582 was followed by Friday 15 October 1582. In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter, because astronomical new moons were occurring four days before the calculated dates. It is notable that whilst the reform introduced minor changes, that the calendar continued to be fundamentally based on the same geocentric theory as its predecessor. The reform was adopted initially by the
Catholic The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptised Baptism (from the Greek language, Greek noun βάπτισμα ''báptisma'') is a Christians, Christian r ...
countries of Europe and their overseas possessions. Over the next three centuries, the
Protestant Protestantism is a form of that originated with the 16th-century , a movement against what its followers perceived to be in the . Protestants originating in the Reformation reject the Roman Catholic doctrine of , but disagree among themselves ...
and
Eastern Orthodox The Eastern Orthodox Church, also called the Orthodox Church, is the second-largest Christian church, with approximately 220 million baptised members. It operates as a communion Communion may refer to: Religion * The Eucharist (also cal ...
countries also moved to what they called the ''Improved calendar'', with Greece being the last European country to adopt the calendar (for civil use only) in 1923. To unambiguously specify a date during the transition period (in contemporary documents or in history texts), both notations were given, tagged as 'Old Style' or 'New Style' as appropriate. During the 20th century, most non-
Western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that ide ...
countries also adopted the calendar, at least for civil purposes.

# Description

The Gregorian calendar, like the
Julian calendar The Julian calendar, proposed by Julius Caesar Gaius Julius Caesar (; 12 July 100 BC – 15 March 44 BC) was a Roman people, Roman general and statesman who played a critical role in Crisis of the Roman Republic, the events that led to the d ...
, is a
solar calendar A solar calendar is a calendar A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A calendar date, date is the designation of a single, specific day with ...
with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a
leap day February 29, also known as leap day or leap year day, is a date added to leap years. A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in most ...
being added to February in the
leap year A leap year (also known as an intercalary year or year) is a that contains an additional day (or, in the case of a , a month) added to keep the calendar year synchronized with the or . Because astronomical events and seasons do not repeat in a ...

s. The months and length of months in the Gregorian calendar are the same as for the Julian calendar. The only difference is that the Gregorian reform omitted a leap day in ''three'' centurial years every 400 years and left the leap day unchanged. A leap year normally occurred every four years, and the leap day was historically inserted by doubling 24 February. However, it is now customary to number the days of February sequentially with no gaps, and 29 February is typically considered the leap day. Before the 1969 revision of its
General Roman Calendar The General Roman Calendar is the liturgical Liturgy is the customary public worship Worship is an act of religion, religious wikt:devotion, devotion usually directed towards a deity. For many, worship is not about an emotion, it is mor ...
, the Catholic Church delayed February feasts after the 23rd by one day in leap years; Masses celebrated according to the previous calendar still reflect this delay. Gregorian years are identified by consecutive year numbers. A calendar date is fully specified by the year (numbered according to a
calendar era #REDIRECT Calendar era 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 as per the Gregorian calendar, which numbers its years in the Western Christ ...
, in this case ''
Anno Domini The terms (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , r ...
'' or
Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar The Gregorian calendar is the used in most of the world. It was introduced in October 1582 by as a modification of the , reducing the average year from 365.2 ...
), the month (identified by name or number), and the day of the month (numbered sequentially starting from 1). Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar (see the "beginning of the year" section below). Calendar cycles repeat completely every 400 years, which equals 146,097 days. Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. During intervals that do not contain any century common years (such as 1900), the calendar repeats every 28 years, during which
February 29 February 29, also known as leap day or leap year day, is a date added to leap year A leap year (also known as an intercalary year or wikt:bissextile, bissextile year) is a calendar year that contains an additional day (or, in the case of a ...
will fall on each of the seven days of the week once and only once. All other dates of the year fall on each day exactly four times, each day of the week having gaps of 6 years, 5 years, 6 years, and 11 years, in that order.

# Gregorian reform

The Gregorian calendar was a reform of the Julian calendar. It was instituted by
papal bull A papal bull is a type of public decree, letters patent upLetters patent transferring a predecessor of the Nancy Nancy may refer to: Places France * Nancy, France, a city in the northeastern French department of Meurthe-et-Moselle a ...
''
Inter gravissimas ''Inter gravissimas'' (English: "Among the most serious...") was a papal bull issued by Pope Gregory XIII on 24 February 1582.
'' dated 24 February 1582 by Pope Gregory XIII, after whom the calendar is named. The motivation for the adjustment was to bring the date for the celebration of Easter to the time of year in which it was celebrated when it was introduced by the early Church. The error in the Julian calendar (its assumption that there are exactly 365.25 days in a year) had led to the date of the equinox according to the calendar drifting from the observed reality, and thus an error had been introduced into the calculation of the date of Easter. Although a recommendation of the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynia Bithynia (; Koine Greek Koine Greek (, , Greek approximately ;. , , , lit. "Common Greek"), also known as Alexandrian dialec ...
in 325 specified that all Christians should celebrate Easter on the same day, it took almost five centuries before virtually all Christians achieved that objective by adopting the rules of the Church of Alexandria (see
Easter Easter,Traditional names for the feast in English are "Easter Day", as in the ''Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'' and Samuel Pepys''The Diary of Samuel Pe ...
for the issues which arose).

## Background

Because the
date of Easter As a moveable feast A moveable feast or movable feast is an observance in a Christian liturgical calendar, borrowed from the Hebrew Lunisolar calendar, which therefore occurs on a different date (relative to the Roman Civil calendar, civil or s ...
is a function the ''
computus As a moveable feast, the date of Easter is determined in each year through a calculation known as ''computus'' (Latin for 'computation'). Easter is traditionally celebrated on the first Sunday after the Paschal full moon, being the first full moo ...
'' of the date of the (northern hemisphere) spring
equinox An equinox is traditionally defined as the time when the plane In mathematics, a plane is a flatness (mathematics), flat, two-dimensional surface (mathematics), surface that extends infinitely far. A plane is the two-dimensional space, two-di ...

, the
Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian church, with 1.3 billion baptised Catholics Catholic Church by country, worldwide . As the wo ...

considered unacceptable the increasing divergence between the canonical date of the equinox and observed reality. Easter is celebrated on the Sunday after the ecclesiastical full moon on or after 21 March, which was adopted as an approximation to the March equinox. European scholars had been well aware of the calendar drift since the early medieval period.
Bede Bede ( ; ang, Bǣda , ; 672/326 May 735), also known as Saint Bede, The Venerable Bede, and Bede the Venerable ( la, Beda Venerabilis), was an English Benedictine The Benedictines, officially the Order of Saint Benedict ( la, Ordo Sanc ...

, writing in the 8th century, showed that the accumulated error in his time was more than three days.
Roger Bacon Roger Bacon (; la, Rogerus or ', also '' Rogerus''; ), also known by the Scholastic accolades, scholastic accolade ''Doctor Mirabilis'', was a medieval England, medieval English philosopher and Franciscans, Franciscan friar who placed consider ...
in c. 1200 estimated the error at seven or eight days.
Dante Dante Alighieri (), probably baptized Durante di Alighiero degli Alighieri and often referred to Mononymous person, simply as Dante (, also ; – 1321), was an Italian poetry, Italian poet, writer and philosopher. His ''Divine Comedy'', origina ...

, writing c. 1300, was aware of the need for calendar reform. An attempt to go forward with such a reform was undertaken by
Pope Sixtus IV Pope Sixtus IV (21 July 1414 – 12 August 1484), born Francesco della Rovere, was head of the Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the List of Christian denominations by number of memb ...

, who in 1475 invited
Regiomontanus Johannes Müller von Königsberg (6 June 1436 – 6 July 1476), better known as Regiomontanus (), was a mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) inc ...

to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing. Numerous publications over the following decades called for a calendar reform, among them two papers sent to the Vatican by the
University of Salamanca The University of Salamanca ( es, Universidad de Salamanca) is a Spanish higher education Higher education is tertiary education leading to award of an academic degree. Higher education, also called post-secondary education, third-level ...

in 1515 and 1578, but the project was not taken up again until the 1540s, and implemented only under
Pope Gregory XIII Pope Gregory XIII ( la, Gregorius XIII; 7 January 1502 – 10 April 1585), born Ugo Boncompagni, was head of the Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the , with 1.3 billion Catholics . ...

(r. 1572–1585).

## Preparation

In 1545, the
Council of Trent The Council of Trent ( la, Concilium Tridentinum), held between 1545 and 1563 in (or Trento, in northern ), was the 19th of the . Prompted by the , it has been described as the embodiment of the ."Trent, Council of" in Cross, F. L. (ed.) ''Th ...

authorised
Pope Paul III Pope Paul III ( la, Paulus III; 29 February 1468 – 10 November 1549), born Alessandro Farnese, was head of the Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the , with 1.3 billion Cathol ...

to reform the calendar, requiring that the date of the vernal equinox be restored to that which it held at the time of the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynia Bithynia (; Koine Greek Koine Greek (, , Greek approximately ;. , , , lit. "Common Greek"), also known as Alexandrian dialec ...
in 325 and that an alteration to the calendar be designed to prevent future drift. This would allow for more consistent and accurate scheduling of the feast of Easter. In 1577, a was sent to expert mathematicians outside the reform commission for comments. Some of these experts, including Giambattista Benedetti and Giuseppe Moleto, believed
Easter Easter,Traditional names for the feast in English are "Easter Day", as in the ''Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'' and Samuel Pepys''The Diary of Samuel Pe ...

should be computed from the true motions of the Sun and Moon, rather than using a tabular method, but these recommendations were not adopted. The reform adopted was a modification of a proposal made by the
Calabria it, Calabrese , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = , demographics1_title1 = , demogr ...

n doctor
Aloysius Lilius Aloysius ( ) is a given name. It is a Latinisation of the names Alois Alois (Latinized ''Aloysius'') is an Old Occitan form of the name Louis (given name), Louis. Modern variants include ''Aloys'' (German), ''Alois'' (Czech, German), ''Aloïs'' (F ...
(or Lilio). Lilius's proposal included reducing the number of leap years in four centuries from 100 to 97, by making three out of four centurial years common instead of leap years. He also produced an original and practical scheme for adjusting the epacts of the Moon when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform. Ancient tables provided the Sun's mean longitude. The German mathematician
Christopher Clavius Christopher Clavius (25 March 1538 – 6 February 1612) was a Jesuit The Society of Jesus (SJ; la, Societas Iesu) is a religious order of the Catholic Church The Catholic Church, often referred to as the Roman Catholic Church, ...

, the architect of the Gregorian calendar, noted that the tables agreed neither on the time when the Sun passed through the vernal equinox nor on the length of the mean tropical year.
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; 14 December 154624 October 1601) was a Danish , known for his accurate and comprehensive astronomical observations. He was born in , which became part of Sweden in the next century. Tycho was well known ...

also noticed discrepancies. The Gregorian leap year rule (97 leap years in 400 years) was put forward by Petrus Pitatus of Verona in 1560. He noted that it is consistent with the tropical year of the
Alfonsine tables 350px, Alfonsine tables The ''Alfonsine tables'' ( es, Tablas alfonsíes, la, tabulae alphonsinae), sometimes spelled ''Alphonsine tables'', provided data for computing the position of the Sun, Moon The Moon is Earth's only proper natur ...
and with the mean tropical year of Copernicus (''
De revolutionibus ''De revolutionibus orbium coelestium'' (; English translation: ''On the Revolutions of the Heavenly Spheres'') is the seminal work on the heliocentric theory Heliocentrism is the astronomical model in which the Earth Earth is the third ...
'') and
Erasmus Reinhold Erasmus Reinhold (22 October 1511 – 19 February 1553) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany ...
(''''). The three mean tropical years in Babylonian sexagesimals as the excess over 365 days (the way they would have been extracted from the tables of mean longitude) were 0;14,33,9,57 (Alfonsine), 0;14,33,11,12 (Copernicus) and 0;14,33,9,24 (Reinhold). In decimal notation, these are equal to 0.24254606, 0.24255185, and 0.24254352, respectively. All values are the same to two sexagesimal places (0;14,33, equal to decimal 0.2425) and this is also the mean length of the Gregorian year. Thus Pitatus' solution would have commended itself to the astronomers. Lilius's proposals had two components. First, he proposed a correction to the length of the year. The mean
tropical year A tropical year (also known as a solar year or tropical period) is the time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to event ...
is 365.24219 days long. A commonly used value in Lilius's time, from the Alfonsine tables, is 365.2425463 days. As the average length of a Julian year is 365.25 days, the Julian year is almost 11 minutes longer than the mean tropical year. The discrepancy results in a drift of about three days every 400 years. Lilius's proposal resulted in an average year of 365.2425 days (see
Accuracy In a set of measurements, accuracy is closeness of the measurements to a specific value, while precision is the closeness of the measurements to each other. ''Accuracy'' has two definitions: # More commonly, it is a description of ''systematic err ...
). At the time of Gregory's reform there had already been a drift of 10 days since the Council of Nicaea, resulting in the vernal equinox falling on 10 or 11 March instead of the ecclesiastically fixed date of 21 March, and if unreformed it would have drifted further. Lilius proposed that the 10-day drift should be corrected by deleting the Julian leap day on each of its ten occurrences over a period of forty years, thereby providing for a gradual return of the equinox to 21 March. Lilius's work was expanded upon by Christopher Clavius in a closely argued, 800-page volume. He would later defend his and Lilius's work against detractors. Clavius's opinion was that the correction should take place in one move, and it was this advice that prevailed with Gregory. The second component consisted of an approximation that would provide an accurate yet simple, rule-based calendar. Lilius's formula was a 10-day correction to revert the drift since the Council of Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. The proposed rule was that "years divisible by 100 would be leap years only if they were divisible by 400 as well". The 19-year cycle used for the lunar calendar required revision because the astronomical new moon was, at the time of the reform, four days before the calculated new moon. It was to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years that are no longer leap years (i.e. 1700, 1800, 1900, 2100, etc.) In fact, a new method for computing the date of Easter was introduced. The method proposed by Lilius was revised somewhat in the final reform. When the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days. The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).

### First printed Gregorian calendar

A month after having decreed the reform, the pope (with a brief of 3 April 1582) granted to one Antoni Lilio the exclusive right to publish the calendar for a period of ten years. The was printed by Vincenzo Accolti, one of the first calendars printed in Rome after the reform, notes at the bottom that it was signed with papal authorization and by Lilio (''Con licentia delli Superiori... et permissu Ant(onii) Lilij''). The papal brief was revoked on 20 September 1582, because Antonio Lilio proved unable to keep up with the demand for copies.

Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church (of which he was the supreme religious authority) and the
Papal States The Papal States ( ; it, Stato Pontificio), officially the State of the Church ( it, Stato della Chiesa, ; la, Status Ecclesiasticus; also '), were a series of territories in the Italian Peninsula The Italian Peninsula (Italian Ital ...
(which he personally ruled). The changes that he was proposing were changes to the civil calendar, over which he had no authority. They required adoption by the civil authorities in each country to have legal effect. The bull ''Inter gravissimas'' became the law of the Catholic Church in 1582, but it was not recognised by
Protestant Churches Protestantism is a form of Christianity Christianity is an Abrahamic religions, Abrahamic Monotheism, monotheistic religion based on the Life of Jesus in the New Testament, life and Teachings of Jesus, teachings of Jesus, Jesus of Nazareth. I ...
,
Eastern Orthodox Churches The Eastern Orthodox Church, also called the Orthodox Church, is the , with approximately 220 million baptised members. It operates as a of churches, each governed by its bishops in local . The church has no central doctrinal or governme ...
,
Oriental Orthodox Churches The Oriental Orthodox Churches are a group of Eastern Christian Eastern Christianity comprises Christian traditions and church families that originally developed during classical and late antiquity in Western Asia Western Asia, also We ...
, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged. On 29 September 1582,
Philip II of Spain Philip II) in Spain , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = , national_anthem = , image_map = , map_caption ...

decreed the change from the Julian to the Gregorian calendar. This affected much of Roman Catholic Europe, as Philip was at the time ruler over
Spain and Portugal , image_flag = Bandera de España.svg , image_coat = Escudo de España (mazonado).svg , national_motto = , national_anthem = , image_map = , map_caption = , image_map2 ...
as well as much of Italy. In these territories, as well as in the
Polish–Lithuanian Commonwealth The Polish–Lithuanian Commonwealth, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, and, after 1791, the Commonwealth of Poland, was a country and bi-federation A federation (also known as a federal state) is ...
(ruled by
Anna Jagiellon Anna Jagiellon ( pl, Anna Jagiellonka, lt, Ona Jogailaitė; 18 October 1523 – 9 September 1596) was Queen of Poland and Grand Duchess of Lithuania from 1575 to 1587. Daughter of Polish King Sigismund I the Old and Italian duchess Bona ...

) and in the Papal States, the new calendar was implemented on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582. The
Spanish Spanish may refer to: * Items from or related to Spain: **Spaniards, a nation and ethnic group indigenous to Spain **Spanish language **Spanish cuisine Other places * Spanish, Ontario, Canada * Spanish River (disambiguation), the name of several ...

and
Portuguese colonies The Portuguese Empire ( pt, Império Português), also known as the Portuguese Overseas (''Ultramar Português'') or the Portuguese Colonial Empire (''Império Colonial Português''), was composed of the overseas Colonialism, colonies, Factory ( ...
followed somewhat later because of delay in communication. Many
Protestant Protestantism is a form of that originated with the 16th-century , a movement against what its followers perceived to be in the . Protestants originating in the Reformation reject the Roman Catholic doctrine of , but disagree among themselves ...
countries initially objected to adopting a Catholic innovation; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. For example, the British could not bring themselves to adopt the Catholic system explicitly: the Annexe to their
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), "An act for regulating the commencement of the year; and for correcting the calendar now in use", also known as Chesterfield's Act or (in American usage) the British Calendar ...
established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him.
Britain Britain usually refers to: * United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use Britain as a synonym for the United ...

and the
British Empire The British Empire was composed of the dominions, Crown colony, colonies, protectorates, League of Nations mandate, mandates, and other Dependent territory, territories ruled or administered by the United Kingdom and its predecessor states. ...

(including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752.
Sweden Sweden ( sv, Sverige ), officially the Kingdom of Sweden ( sv, links=no, Konungariket Sverige ), is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that ...
followed in 1753. Prior to 1917, Turkey used the lunar
Islamic calendar The Hijri calendar ( ar, ٱلتَّقْوِيم ٱلْهِجْرِيّ '), also known as the Lunar Hijri calendar and (in English) as the Islamic, Muslim or Arabic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 o ...
with the Hegira era for general purposes and the Julian calendar for fiscal purposes. The start of the fiscal year was eventually fixed at 1 March and the year number was roughly equivalent to the Hegira year (see
Rumi calendar The ''Rumi'' calendar ( tr, Rumi takvim, lit. "Roman calendar"), a specific calendar based on the Julian calendar was officially used by the Ottoman Empire after Tanzimat (1839) and by its successor, the Republic of Turkey until 1926. It was adopt ...
). As the solar year is longer than the lunar year this originally entailed the use of "escape years" every so often when the number of the fiscal year would jump. From 1 March 1917 the fiscal year became Gregorian, rather than Julian. On 1 January 1926 the use of the Gregorian calendar was extended to include use for general purposes and the number of the year became the same as in most other countries.

# Difference between Gregorian and Julian calendar dates

This section always places the intercalary day on even though it was always obtained by doubling (the (twice sixth) or bissextile day) until the late
Middle Ages In the history of Europe The history of Europe concerns itself with the discovery and collection, the study, organization and presentation and the interpretation of past events and affairs of the people of Europe since the beginning of ...
. The Gregorian calendar is proleptic before 1582 (calculated backwards on the same basis, for years before 1582), and the difference between Gregorian and Julian calendar dates increases by three days every four centuries (all date ranges are inclusive). The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the "secular difference" between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar. :$D = \left\lfloor\right\rfloor - \left\lfloor\right\rfloor - 2$ where $D$ is the secular difference and $Y$ is the year using
astronomical year numbering Astronomical year numbering is based on AD/ CE year numbering, but follows normal decimal The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing ...
, that is, use for BC years. $\left\lfloor\right\rfloor$ means that if the result of the division is not an integer it is rounded down to the nearest integer. Thus during the 1900s, 1900/400 = 4, while during the −500s, −500/400 = −2. The general rule, in years which are leap years in the Julian calendar but not the Gregorian, is: Up to 28 February in the calendar being converted , add one day less or subtract one day more than the calculated value. Give February the appropriate number of days for the calendar being converted . When subtracting days to calculate the Gregorian equivalent of 29 February (Julian), 29 February is discounted. Thus if the calculated value is −4 the Gregorian equivalent of this date is 24 February.

# Beginning of the year

The year used in dates during the
Roman Republic The Roman Republic ( la, Rēs pūblica Rōmāna ) was a state of the classical Roman civilization, run through public In public relations Public relations (PR) is the practice of managing and disseminating information from an indiv ...
and the
Roman Empire The Roman Empire ( la, Imperium Rōmānum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post-Republican Republican can refer to: Political ideology * An advocate of a republic, a type of governme ...

was the consular year, which began on the day when consuls first entered office—probably 1 May before , 15 March from and 1 January from . The Julian calendar, which began in , continued to use 1 January as the first day of the
new year New Year is the time Time is the continued of and that occurs in an apparently succession from the , through the , into the . It is a component quantity of various s used to events, to compare the duration of events or the interva ...

. Even though the year used for dates changed, the civil year always displayed its months in the order January to December from the Roman Republican period until the present. During the Middle Ages, under the influence of the Catholic Church, many Western European countries moved the start of the year to one of several important Christian festivals—25 December (supposed
Nativity of Jesus The nativity of Jesus, nativity of Christ, birth of Christ or birth of Jesus is described in the biblical gospels of Luke and Matthew. The two accounts agree that Jesus Jesus, likely from he, יֵשׁוּעַ, translit=Yēšūa ...
), 25 March (
Annunciation The Annunciation (from Latin '), also referred to as the Annunciation to the Blessed Virgin Mary, the Annunciation of Our Lady, or the Annunciation of the Lord, is the Christian celebration of the announcement by the Archangel Gabriel to the ...

), or Easter (France), while the Byzantine Empire began its year on 1 September and Russia did so on 1 March until 1492 when the new year was moved to 1 September. In common usage, 1 January was regarded as New Year's Day and celebrated as such, but from the 12th century until 1751 the legal year in England began on 25 March (
Lady Day In the Western liturgical year The liturgical year, also known as the church year or Christian year, as well as the kalendar, consists of the cycle of liturgical seasons in Christian churches that determines when feast days, including c ...

). So, for example, the Parliamentary record lists the execution of
Charles ICharles I may refer to: Kings and emperors * Charlemagne (742–814), numbered Charles I in the lists of French and German kings * Charles I of Anjou (1226–1285), also king of Albania, Jerusalem, Naples and Sicily * Charles I of Hungary (1288 ...

on 30 January as occurring in 1648 (as the year did not end until 24 March), although later histories adjust the start of the year to 1 January and record the execution as occurring in 1649. Most Western European countries changed the start of the year to 1 January before they adopted the Gregorian calendar. For example, Scotland changed the start of the Scottish New Year to 1 January in 1600 (this means that 1599 was a short year). England, Ireland and the British colonies changed the start of the year to 1 January in 1752 (so 1751 was a short year with only 282 days). Later in 1752 in September the Gregorian calendar was introduced throughout Britain and the British colonies (see the section
Adoption Adoption is a process whereby a person assumes the parenting Parenting or child rearing promotes and supports the physical Physical may refer to: *Physical examination, a regular overall check-up with a doctor *Physical (album), ''Physical'' ...
). These two reforms were implemented by the
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), "An act for regulating the commencement of the year; and for correcting the calendar now in use", also known as Chesterfield's Act or (in American usage) the British Calendar ...
.Nørby, Toke
The Perpetual Calendar
/ref> In some countries, an official decree or law specified that the start of the year should be 1 January. For such countries, a specific year when a 1 January-year became the norm can be identified. In other countries, the customs varied, and the start of the year moved back and forth as fashion and influence from other countries dictated various customs. Neither the papal bull nor its attached canons explicitly fix such a date, though it is implied by two tables of
saint In religious belief, a saint is a person who is recognized as having an exceptional degree of Q-D-Š, holiness, likeness, or closeness to God. However, the use of the term ''saint'' depends on the context and Christian denomination, denominatio ...

's days, one labelled 1582 which ends on 31 December, and another for any full year that begins on 1 January. It also specifies its epact relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. The old date was derived from the Greek system: the earlier specified it relative to 1 January.

# Dual dating

During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double-dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of
John Dee John Dee (13 July 1527 – 1608 or 1609) was an Anglo-Welsh mathematician, astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe as ...
(1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.

## Old Style and New Style dates

"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which calendar reference system is used for the date given. In Britain and its colonies, where the
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), "An act for regulating the commencement of the year; and for correcting the calendar now in use", also known as Chesterfield's Act or (in American usage) the British Calendar ...
altered the start of the year, and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the Julian year has been adjusted to start on 1 January (NS) even though contemporary documents use a different start of year (OS); or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS).Death warrant of Charles I
web page of th
UK National Archives
A demonstration of New Style meaning Julian calendar with a start of year adjustment.

# Proleptic Gregorian calendar

Extending the Gregorian calendar backwards to dates preceding its official introduction produces a proleptic calendar, which should be used with some caution. For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents. For example, the Battle of Agincourt is universally considered to have been fought on 25 October 1415 which is 's Day. Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events that happened before the introduction of the Gregorian calendar. But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains in English language histories. Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar. For example, the
Battle of Blenheim The Battle of Blenheim (german: Zweite Schlacht bei Höchstädt, link=no; french: Bataille de Höchstädt, link=no; nl, Slag bij Blenheim, link=no) fought on , was a major battle of the War of the Spanish Succession The War of the Spanish ...

is always given as 13 August 1704. Confusion occurs when an event affects both. For example,
William III of England William III (William Henry; ; 4 November 16508 March 1702), also widely known as William of Orange, was the sovereign Prince of Orange from birth, Stadtholder of County of Holland, Holland, County of Zeeland, Zeeland, Lordship of Utrecht, Utrecht, ...

set sail from the Netherlands on 11 November 1688 (Gregorian calendar) and arrived at
Brixham Brixham is a fishing town and civil parish In England, a civil parish is a type of administrative parish used for local government. It is a territorial designation which is the lowest tier of local government below districts and counti ...

in England on 5 November 1688 (Julian calendar).
Shakespeare William Shakespeare (bapt. 26 April 1564 – 23 April 1616) was an English playwright, poet and actor, widely regarded as the greatest writer in the English language and the world's greatest dramatist. He is often called England's national p ...

and
Cervantes Miguel de Cervantes Saavedra (; 29 September 1547 (assumed)22 April 1616 NS) was a Spanish writer widely regarded as the greatest writer in the Spanish language Spanish () or Castilian (, ) is a Romance languages, Romance language that or ...

seemingly died on exactly the same date (23 April 1616), but Cervantes predeceased Shakespeare by ten days in real time (as Spain used the Gregorian calendar, but Britain used the Julian calendar). This coincidence encouraged
UNESCO The United Nations Educational, Scientific and Cultural Organization (UNESCO) (french: Organisation des Nations unies pour l'éducation, la science et la culture) is a specialised agency United Nations Specialized Agencies are autonomous orga ...

to make 23 April the . Astronomers avoid this ambiguity by the use of the Julian day number. For dates before the year 1, unlike the proleptic Gregorian calendar used in the international standard ISO 8601, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a year zero, year 0 and instead uses the ordinal numbers 1, 2, ... both for years AD and BC. Thus the traditional time line is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year numbering which includes a year 0 and negative numbers before it. Thus the ISO 8601 time line is , 0000, 0001, and 0002.

# Months

The Gregorian calendar continued to employ the Julian calendar, Julian months, which have Latinate names and irregular numbers of days: * January (31 days), from Latin ', "Month of Janus (mythology), Janus",. the Roman mythology, Roman god of gates, doorways, beginnings and endings * February (28 days in common year, common and 29 in
leap year A leap year (also known as an intercalary year or year) is a that contains an additional day (or, in the case of a , a month) added to keep the calendar year synchronized with the or . Because astronomical events and seasons do not repeat in a ...

s), from Latin ', "Month of the Februa", the Roman festivals, Roman festival of purgation and purification,.. cognate word, cognate with fever, the Etruscan mythology, Etruscan death god Februus ("Purifier"), and the Proto-Indo-European language, Proto-Indo-European word for sulfur * March (month), March (31 days), from Latin ', "Month of Mars (mythology), Mars",. the Roman war god * April (month), April (30 days), from Latin ', of uncertain meaning. but usually derived from some form of the verb ' ("to open") or the name of the goddess Aphrodite * May (month), May (31 days), from Latin ', "Month of Maia Maiestas, Maia",. a Roman vegetation goddess whose name is cognate with Latin ' ("great") and English ''major'' * June (month), June (30 days), from Latin ', "Month of Juno (mythology), Juno",. the Roman goddess of marriage in ancient Rome, marriage, childbirth goddess, childbirth, and rule * July (31 days), from Latin ', "Month of Julius Caesar", the month of Caesar's birth, instituted in 44BC. as part of Julian calendar, his calendrical reforms * August (month), August (31 days), from Latin ', "Month of Augustus", instituted by Augustus in 8BC in agreement with July and from the occurrence during the month of several important events during his rise to power. * September (30 days), from Latin ', "seventh month", of the ten-month Roman year of Romulus BC. * October (31 days), from Latin ', "eighth month", of the ten-month Roman year of Romulus BC. * November (30 days), from Latin ', "ninth month", of the ten-month Roman year of Romulus BC. * December (31 days), from Latin ', "tenth month", of the ten-month Roman year of Romulus BC. Europeans sometimes attempt to remember the number of days in each month by memorizing some form of the traditional verse "Thirty Days Hath September". It appears in Latin, Italian,. French and Portuguese, and belongs to a broad oral tradition but the earliest currently attested form of the poem is the English marginalia inserted into a calendar of saints :. Variations appeared in ''Mother Goose'' and continue to be taught at schools. The unhelpfulness of such involved mnemonics has been parodied as "Thirty days hath September/ But all the rest I can't remember" but it has also been called "probably the only sixteenth-century poem most ordinary citizens know by heart".. A common nonverbal alternative is the knuckle mnemonic, considering the knuckles of one's hands as months with 31 days and the lower spaces between them as the months with fewer days. Using two hands, one may start from either pinkie finger, pinkie knuckle as January and count across, omitting the space between the index finger, index knuckles (July and August). The same procedure can be done using the knuckles of a single hand, returning from the last (July) to the first (August) and continuing through. A similar mnemonic is to move up a musical keyboard, piano keyboard in semitones from an F key, taking the white keys as the longer months and the black keys as the shorter ones.

# Weeks

In conjunction with the system of months, there is a system of weeks. A physical or electronic calendar provides conversion from a given date to the weekday and shows multiple dates for a given weekday and month. Calculating the day of the week is not very simple, because of the irregularities in the Gregorian system. When the Gregorian calendar was adopted by each country, the weekly cycle continued uninterrupted. For example, in the case of the few countries that adopted the reformed calendar on the date proposed by Gregory XIII for the calendar's adoption, Friday, 15 October 1582, the preceding date was Thursday, 4 October 1582 (Julian calendar). Opinions vary about the numbering of the days of the week. ISO 8601, in common use worldwide, starts with Monday=1; printed monthly calendar grids often list Mondays in the first (left) column of dates and Sundays in the last. In North America, the week typically begins on Sunday and ends on Saturday.

# Accuracy

The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 solar time, mean solar days long. This approximation has an error of about one day per 3,030 years with respect to the current value of the mean tropical year. However, because of the Axial precession (astronomy), precession of the equinoxes, which is not constant, and the movement of the perihelion (which affects the Earth's orbital speed) the error with respect to the ''astronomical'' vernal equinox is variable; using the average interval between vernal equinoxes near 2000 of 365.24237 days implies an error closer to 1 day every 7,700 years. By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days). In the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar with 969 leap days every 4000 years, instead of 970 leap days that the Gregorian calendar would insert over the same period. This would reduce the average year to 365.24225 days. Herschel's proposal would make the year 4000, and multiples thereof, common instead of leap. While this modification has often been proposed since, it has never been officially adopted. On time scales of thousands of years, the Gregorian calendar falls behind the astronomical seasons. This is because Earth's rotation#Changes, the Earth's speed of rotation is gradually slowing down, which makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration.

## Calendar seasonal error

This image shows the difference between the Gregorian calendar and the astronomical seasons. The ''y''-axis is the date in June and the ''x''-axis is Gregorian calendar years. Each point is the date and time of the Solstice, June solstice in that particular year. The error shifts by about a quarter of a day per year. Centurial years are ordinary years, unless they are divisible by 400, in which case they are leap years. This causes a correction in the years 1700, 1800, 1900, 2100, 2200, and 2300. For instance, these corrections cause 23 December 1903 to be the latest December solstice, and 20 December 2096 to be the earliest solstice—about 2.35 days of variation compared with the seasonal event.

# Proposed reforms

The following are Calendar reform, proposed reforms of the Gregorian calendar: * Holocene calendar * International Fixed Calendar (also called the ''International Perpetual calendar'') * World Calendar * World Season Calendar * Leap week calendars ** Pax Calendar ** Symmetry454 ** Hanke–Henry Permanent Calendar

*
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), "An act for regulating the commencement of the year; and for correcting the calendar now in use", also known as Chesterfield's Act or (in American usage) the British Calendar ...
*Calendar reform *Conversion between Julian and Gregorian calendars *Doomsday rule *French revolutionary calendar *Hebrew calendar *Dionysius Exiguus *s:Translation:Inter gravissimas, Inter gravissimas in English – Wikisource *Julian day *History of calendars * ISO 8601, an international standard for the representation of dates and times, which uses the Gregorian calendar (see Section 3.2.1). *List of adoption dates of the Gregorian calendar per country *List of calendars *Old Calendarists **Greek Old Calendarists *Revised Julian calendar (Milanković) – used in Eastern Orthodoxy Precursors of the Gregorian reform * Johannes de Sacrobosco, ''De Anni Ratione'' ("On reckoning the years"), c. 1235 *
Roger Bacon Roger Bacon (; la, Rogerus or ', also '' Rogerus''; ), also known by the Scholastic accolades, scholastic accolade ''Doctor Mirabilis'', was a medieval England, medieval English philosopher and Franciscans, Franciscan friar who placed consider ...
, ''Opus Majus'' ("Greater Work"), c. 1267

# References

* *. * * * * * * * * * * * * * * * * * * * *

*s:Translation:Inter gravissimas, Inter gravissimas in English – Wikisource *
Calendar Converter

The Perpetual Calendar Gregorian Calendar adoption dates for many countries.