View source for Julian calendar

{{Short description|Calendar introduced by Julius Caesar}}
{{About|a calendar used for civil and liturgical purposes|the day-number calendar used for astronomical and historical calculations|Julian day|day of year|Ordinal date|the terms 'Julian date' and 'Julian Period'|Julian day}}
{{Pp-vandalism|small=yes}}
[[File:Retrato de Julio César (26724093101) (cropped).jpg|thumb|upright|The [[Tusculum portrait]] of [[Julius Caesar]]]]
{{Infobox calendar date today}}

The '''Julian calendar''' is a [[solar calendar]] of 365 days in every year with an additional [[leap day]] every fourth year (without exception). The Julian calendar is still used as a religious calendar in parts of the [[Eastern Orthodox Church]] and in parts of [[Oriental Orthodox Churches|Oriental Orthodoxy]] as well as by the [[Amazigh|Amazigh people]] (also known as the Berbers).<ref>{{Cite web |url=https://www.dailysabah.com/africa/2018/01/12/berbers-mark-new-year-in-algeria-welcoming-2968 |title=Berbers mark New Year in Algeria, welcoming 2968 |website=[[Daily Sabah]] |date=12 January 2018 |access-date=25 June 2019 |quote=The Berber calendar is an agrarian system, based around the seasons and agricultural work, that was inspired by the Julian calendar.}}</ref> For a quick calculation, between 1901 and 2099 the much more common [[Gregorian calendar |Gregorian date]] equals the Julian date plus 13 days.

The Julian calendar was proposed in 46&nbsp;BC by (and takes its name from) [[Julius Caesar]], as a reform of the earlier [[Roman calendar]], which was largely a [[lunisolar calendar|lunisolar]] one.{{sfn|Richards|2013|p = 595}} It took effect on {{nowrap|1 January 45 BC}}, by his [[edict]].  Caesar's calendar became the predominant calendar in the [[Roman Empire]] and subsequently most of the [[Western world]] for more than 1,600 years, until 1582 when [[Pope Gregory XIII]] promulgated a revised calendar. Ancient Romans typically designated years by the names of ruling consuls; the ''[[Anno Domini]]'' system of numbering years was not devised until 525, and became widespread in Europe in the eighth century.

The Julian calendar has two types of years: a normal year of 365 days and a [[leap year]] of 366 days. They follow a simple cycle of three normal years and one leap year, giving an average year that is 365.25 days long. That is more than the actual solar year value of approximately 365.2419 days (the current value, which varies), which means the Julian calendar gains one&nbsp;day every 129&nbsp;years. In other words, the Julian calendar gains 3.1 days every 400 years. 

[[Gregorian calendar#Gregorian reform|Gregory's calendar reform]] modified the Julian rule by eliminating occasional leap days, to reduce the average length of the calendar year from 365.25 days to 365.2425 days and thus dramatically reducing the Julian calendar's drift against the [[Tropical year|solar year]]: the Gregorian calendar gains just 0.1 day over 400 years. For any given event during the years from 1901 through 2099, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date (for instance Julian 1 January falls on Gregorian 14 January). Most [[Catholicism|Catholic]] countries adopted the new calendar immediately; [[Protestantism|Protestant]] countries [[adoption of the Gregorian calendar|did so]] slowly in the course of the following two centuries or so; most Orthodox countries retain the Julian calendar for religious purposes but adopted the Gregorian as their [[civil calendar]] in the early part of the twentieth century.

==Table of months==
{| class="wikitable"
|-
! Months (Roman) || Lengths before 46&nbsp;BC || Length in 46&nbsp;BC || Lengths as of 45&nbsp;BC || Months (English)
|-
|[[Ianuarius]]<ref name="J">The letter [[J]] was not invented until the 16th century.</ref>|| 29 || 29 || 31 ||January
|-
|[[Februarius]]|| 28 (in common years)<br />In intercalary years:<br />23 if Intercalaris is variable<br />23–24 if Intercalaris is fixed || 28 || 28 (leap years: 29) ||February
|-
|[[Intercalaris]] (Mercedonius)<br> (only in intercalary years)|| 27 (or possibly 27–28) || 23 || — || —
|-
|[[Martius (month)|Martius]] || 31 || 31 || 31 ||March
|-
|[[Aprilis]] || 29 || 29 || 30 ||April
|-
|[[Maius]] || 31 || 31 || 31 ||May
|-
|[[Iunius (month)|Iunius]]<ref name="J" /> || 29 || 29 || 30 ||June
|-
|[[Quintilis]]<ref>The spelling ''Quinctilis'' is also attested; see page 669 of ''The Oxford Companion to the Year''.</ref> (Iulius) || 31 || 31 || 31 ||July
|-
|[[Sextilis]] (Augustus) || 29 || 29 || 31 ||August
|-
|[[September (Roman month)|September]] || 29 || 29 || 30 ||September
|-
|[[October (Roman month)|October]] || 31 || 31 || 31 ||October
|-
| 2nd intercalary || — || 33 || — || —
|-
| 3rd intercalary || — || 34 || — || —
|-
|[[November (Roman month)|November]] || 29 || 29 || 30 ||November
|-
|[[December (Roman month)|December]] || 29 || 29 || 31 ||December
|-
|'''Total'''|| 355 or 377–378 || 445 || 365–366 ||| 365–366
|}

=={{anchor|Motivation}}History==
===Motivation===
The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day [[intercalary month]], the [[Mercedonius|Mensis Intercalaris]], was sometimes inserted between February and March. This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days.<ref>T H Key, "A Dictionary of Greek and Roman Antiquities" (article ''Calendarium''), London, 1875, available at [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Calendarium.html].</ref> Some say the ''mensis intercalaris'' always had 27 days and began on either the first or the second day after the Terminalia (23 February).<ref>Blackburn, B & Holford-Strevens, L ''The Oxford Companion to the Year'', Oxford University Press, 1999, reprinted with corrections, 2003, pp. 669–70.</ref>

If managed correctly, this system could have allowed the Roman year to stay roughly aligned to a [[tropical year]]. However, since the [[Pontiff|pontifices]] were often politicians, and because a Roman magistrate's term of [[official|office]] corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.<ref>Censorinus, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Censorinus/text*.html#20.7 ''De die natali'' 20.7] {{In lang|la}} Latin)</ref>

Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. [[Varro]] used it in 37&nbsp;BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.<ref name="Varro">Varro, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Varro/de_Re_Rustica/1*.html#28 ''On Agriculture'' I.1.28.]</ref> A century later, when [[Pliny the Elder|Pliny]] dated the [[winter solstice]] to 25 December because the sun entered the 8th degree of Capricorn on that date,<ref>Pliny, ''Natural History:'' [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D18%3Achapter%3D59 ''(Book 18, LIX / LXVI / LXVIII / LXXIV)''.]</ref> this stability had become an ordinary fact of life.

=== Context of the reform ===
Although the approximation of {{frac|365|1|4}} days for the tropical year had been known for a long time,<ref>{{cite journal|first=R. A.|last=Parker|title=Ancient Egyptian Astronomy|journal=Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences|volume=276|issue=1257|date=May 1974|pages=51–65|jstor=74274}}</ref> ancient [[solar calendar]]s had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.

The [[octaeteris]], a cycle of eight [[Lunar calendar|lunar years]] popularised by [[Cleostratus]] (and also commonly attributed to [[Eudoxus of Cnidus|Eudoxus]]) which was used in some early Greek calendars, notably in [[Attic calendar|Athens]], is 1.53 days longer than eight [[Julian year (astronomy)|mean Julian years]]. The length of nineteen years in the [[Metonic cycle|cycle of Meton]] was 6,940 days, six hours longer than the mean Julian year. The mean Julian year was the basis of the [[Callippic cycle|76-year cycle]] devised by [[Callippus]] (a student under Eudoxus) to improve the Metonic cycle.

In Persia (Iran) after the reform in the [[Iranian calendars#Old Persian calendar|Persian calendar]] by introduction of the Persian Zoroastrian (i. e. Young Avestan) calendar in 503&nbsp;BC and afterwards, the first day of the year (1 Farvardin=[[Nowruz]]) slipped against the [[March equinox|vernal equinox]] at the rate of approximately one day every four years.<ref>Hartner, Willy. "The young Avestan and Babylonian calendars and the antecedents of precession." Journal for the History of Astronomy 10 (1979): 1. pp.&nbsp;1–22. [[doi:10.1177/002182867901000102]]</ref><ref>Stern, Sacha. ''Calendars in antiquity: Empires, states, and societies''. Oxford University Press, 2012., p. 178.</ref>

Likewise in the [[Egyptian calendar]], a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238&nbsp;BC ([[Decree of Canopus]]). Caesar probably experienced this "wandering" or "vague" calendar in that country. He landed in the Nile delta in October 48&nbsp;BC and soon became embroiled in the Ptolemaic dynastic war, especially after [[Cleopatra VII|Cleopatra]] managed to be "introduced" to him in [[Alexandria]].

Caesar imposed a peace, and a banquet was held to celebrate the event.<ref name="Book 10">Lucan, ''[[Pharsalia]]:'' [http://mcllibrary.org/Pharsalia/book10.html ''Book 10''.]</ref> [[Lucan]] depicted Caesar talking to a wise man called [[Acoreus]] during the feast, stating his intention to create a calendar more perfect than that of Eudoxus<ref name="Book 10" /> (Eudoxus was popularly credited with having determined the length of the year to be {{frac|365|1|4}} days).<ref>Émile Biémont, ''Rythmes du temps, astronomie et calendriers'', éd. De Boeck (Bruxelles), 2000 ({{ISBN|2-8041-3287-0}}), p. 224.</ref> But the war soon resumed and Caesar was attacked by the Egyptian army for several months until he achieved victory. He then enjoyed a long cruise on the Nile with Cleopatra before leaving the country in June 47&nbsp;BC.<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#52 ''Caesar'' 52.1.] {{Webarchive|url=https://archive.today/20120530163202/http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#52 |date=2012-05-30 }}</ref>

Caesar returned to Rome in 46&nbsp;BC and, according to [[Plutarch]], called in the best philosophers and mathematicians of his time to solve the problem of the calendar.<ref>Plutarch, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Caesar*.html#59 ''Lives of the Noble Grecians and Romans'': Caesar 59].</ref> Pliny says that Caesar was aided in his reform by the astronomer [[Sosigenes of Alexandria]]<ref>Pliny, ''Natural History:'' [https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D18%3Achapter%3D57 ''(Book 18, LVII)''.]</ref> who is generally considered the principal designer of the reform. Sosigenes may also have been the author of the astronomical almanac published by Caesar to facilitate the reform.<ref>Encyclopædia Britannica [https://www.britannica.com/EBchecked/topic/555018/Sosigenes-of-Alexandria Sosigenes of Alexandria].</ref> Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the {{frac|365|1|4}} days of Greek astronomy. According to Macrobius, Caesar was assisted in this by a certain Marcus Flavius.<ref>Macrobius, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#14.2 ''Saturnalia'' I.14.2] (Latin).</ref>

=== Adoption of the Julian calendar ===
{{more citations needed section|date=December 2013}}
Caesar's reform only applied to the [[Roman calendar]]. However, in the following decades many of the local civic and provincial calendars of the empire and neighbouring client kingdoms were aligned to the Julian calendar by transforming them into calendars with years of 365 days with an extra day [[intercalation (timekeeping)|intercalated]] every four years.<ref>This section is based on S. Stern, ''Calendars in Antiquity'' (OUP 2012) pp. 259–297.</ref><ref>{{Cite book|url=https://books.google.com/books?id=tiMZAI4oS-MC|title=Calendars in Antiquity: Empires, States, and Societies|last=Stern|first=Sacha|date=2012|publisher=OUP Oxford|isbn=978-0-19-958944-9|pages=259–297|language=en}}</ref> The reformed calendars typically retained many features of the unreformed calendars. In many cases, the New Year was not on 1 January, the leap day was not on the [[bissextus|traditional bissextile day]], the old month names were retained, the lengths of the reformed months did not match the lengths of Julian months, and, even if they did, their first days did not match the first day of the corresponding Julian month. Nevertheless, since the reformed calendars had fixed relationships to each other and to the Julian calendar, the process of converting dates between them became quite straightforward, through the use of conversion tables known as "hemerologia".<ref>Studied in detail in W. Kubitschek, ''Die Kalendarbücher von Florenz, Rom und Leyden'' (Vienna, 1915).</ref>

The three most important of these calendars are the [[Coptic calendar|Alexandrian calendar]] and the [[Ancient Macedonian calendar]]─which had two forms: the Syro-Macedonian and the [[Roman Asia|'Asian']] calendars. Other reformed calendars are known from [[Cappadocia]], [[Cyprus]] and the cities of (Roman) Syria and Palestine. Unreformed calendars continued to be used in [[Gaul]] (the [[Coligny calendar]]), Greece, Macedon, the Balkans and parts of Palestine, most notably in Judea.

The Asian calendar was an adaptation of the [[Ancient Macedonian calendar]] used in the [[Asia (Roman province)|Roman province of Asia]] and, with minor variations, in nearby cities and provinces. It is known in detail through the survival of decrees promulgating it issued in 8{{nbsp}}BC by the proconsul [[Paullus Fabius Maximus]]. It renamed the first month Dios as {{lang|la|Kaisar}}, and arranged the months such that each month started on the ninth day before the kalends of the corresponding Roman month; thus the year began on 23 September, Augustus's birthday.

== Julian reform ==
=== Realignment of the year ===
The first step of the reform was to realign the start of the calendar year (1 January) to the tropical year by making 46&nbsp;BC 445 days long, compensating for the intercalations which had been missed during Caesar's pontificate. This year had already been extended from 355 to 378 days by the insertion of a regular [[intercalary month]] in February. When Caesar decreed the reform, probably shortly after his return from the [[Battle of Thapsus|African campaign]] in late Quintilis (July), he added 67 more days by inserting two extraordinary intercalary months between November and December.{{efn|1=It is not known why he decided that 67 was the correct number of days to add, nor whether he intended to align the calendar to a specific astronomical event such as the winter solstice. [[Ideler]] suggested (''Handbuch der mathematischen und technischen Chronologie'' II 123–125) that he intended to align the winter solstice to a supposedly traditional date of 25 December.} The number may compensate for three omitted intercalary months (67 = 22+23+22). It also made the distance from 1 March 46&nbsp;BC, the original New Year's Day in the Roman calendar, to 1 January 45&nbsp;BC be 365 days.}}

These months are called ''Intercalaris Prior'' and ''Intercalaris Posterior'' in letters of [[Cicero]] written at the time; there is no basis for the statement sometimes seen that they were called "[[Undecimber]]" and "[[Duodecember|Duodecimber]]", terms that arose in the 18th century over a millennium after the Roman Empire's collapse.{{efn|E.g., "... we have a sidelight on what was involved in "the year of confusion" as it was called. According to Dion Cassius, the historian, there was a governor in Gaul who insisted that, in the lengthened year, two months' extra taxes should be paid. The extra months were called Undecimber and Duodecimber".<ref>P. W. Wilson, [https://books.google.com/books?id=9xcbAAAAYAAJ&q=undecimber+duodecimber ''The romance of the calendar''] (New York, 1937), 112</ref> The eponymous dating of the cited passage ''Cassius 54.21'' shows that it actually refers to an event of 15&nbsp;BC, not 46&nbsp;BC.<ref>[https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/54*.html#21 Dio Cassius 54.21]</ref>}} Their individual lengths are unknown, as is the position of the [[Nones (calendar)|Nones]] and [[Ides (calendar)|Ides]] within them.<ref>J. Rüpke, ''The Roman Calendar from Numa to Constantine: Time, History and the Fasti'', 117f., suggests, based on the ritual structures of the calendar, that 5 days were added to November and that the two intercalary months each had 31 days, with Nones and Ides on the 7th and 15th.</ref>

Because 46&nbsp;BC was the last of a series of irregular years, this extra-long year was, and is, referred to as the "last year of confusion". The new calendar began operation after the realignment had been completed, in 45&nbsp;BC.<ref>[[William Smith (lexicographer)|William Smith]], ''[[Dictionary of Greek and Roman Antiquities]]:'' [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Calendarium.html#p231 Year of Julius Caesar], following [[Ideler]], interprets Macrobius, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#14.13 ''Saturnalia'' 1.14.13] (Latin) to mean that Caesar decreed that the first day of the new calendar began with the new moon which fell on the night of 1/2 January 45&nbsp;BC. 
<br />
The new moon was on 2 January 45&nbsp;BC (in the [[Proleptic Julian calendar]]) at 00:21 UTC, according to [[Institut de mécanique céleste et de calcul des éphémérides|IMCCE]] (a branch of the [[Paris Observatory]]): [http://bugle.imcce.fr/en/grandpublic/phenomenes/phases_lune/index.php ''Phases of the moon (between −4000 and +2500)''] {{webarchive|url=https://web.archive.org/web/20110720232543/http://bugle.imcce.fr/en/grandpublic/phenomenes/phases_lune/index.php|date=2011-07-20}}. This is in agreement with the [http://astropixels.com/ephemeris/phasescat/phases-0099.html historical moon phase tables by Fred Espenak] in which the new moon was on 2 January 45&nbsp;BC at 00:43 UTC. Espenek's table assumes that the first Julian year of 45 BC was a leap year. If the first year of 45 BC was not a leap year, there would be a day offset, and the new moon would have been on 1 January 45&nbsp;BC at 00:43 UTC.
<br />
Espnek's historical moon phase tables also indicate that there was a new moon on 1 March 45&nbsp;BC at 08:39 UTC ([[Calends|Kalends]] of March), quarter moon on 8 March 45&nbsp;BC at 09:00 UTC (a day after [[Roman calendar#Days|Nones of March]]), and full moon on 15 March 45&nbsp;BC at 07:19 UTC ([[Ides of March]]). Espenak's tables of the phases of the moon are based on computational procedures described in ''Astronomical Algorithms'' by [[Jean Meeus]] (Willmann-Bell, Inc., Richmond, 1998). 
<br />
More recent studies of the Macrobius manuscripts have shown that the word on which Idler's supposition is based, which was read as ''lunam'', should be read as ''linam'', meaning that Macrobius was simply stating that Caesar published an edict giving the revised calendar – see e.g., p.99 in the translation of Macrobius by P. Davies. 
<br />
Smith gives no source or justification for his other speculation that Caesar originally intended to commence the year precisely with the winter solstice.</ref>

=== Months ===

The Julian months were formed by adding ten days to a regular pre-Julian Roman year of 355 days, creating a regular Julian year of 365 days. Two extra days were added to January, Sextilis (August) and December, and one extra day was added to April, June, September, and November. February was not changed in ordinary years, and so continued to be the traditional 28 days. Thus, the ordinary (i.e., non-leap year) lengths of all of the months were set by the Julian calendar to the same values they still hold today.

The Julian reform did not change [[Roman calendar#Months|the method used to account days of the month in the pre-Julian calendar]], based on the Kalends, Nones and Ides, nor did it change the positions of these three dates within the months. Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established religious ceremonies relative to the Nones and Ides of the month.<ref>Macrobius, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#14.9 ''Saturnalia'' 1.14.9] (Latin). Exceptionally, the extra day in April was inserted as the 26th, a.d. VI Kal. Mai. in the Julian calendar, in order to avoid adding a day to the [[Floralia]], which ran from a.d. IV Kal. Mai. (27 April in the pre-Julian calendar) to a.d. V Non. Mai.</ref>

The inserted days were all initially characterised as ''dies fasti'' ('''F''' – see [[Roman calendar#Character of the day|Roman calendar]]).<ref>Macrobius, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#14.12 ''Saturnalia'' 1.14.12] (Latin).</ref> The character of a few festival days was changed. In the early Julio-Claudian period a large number of festivals were decreed to celebrate events of dynastic importance, which caused the character of the associated dates to be changed to '''NP'''. However, this practice was discontinued around the reign of [[Claudius]], and the practice of characterising days fell into disuse around the end of the first century AD: the Antonine jurist [[Gaius (jurist)|Gaius]] speaks of ''dies nefasti'' as a thing of the past.<ref>A. K. Michels, ''The Calendar of the Roman Republic'' Appendix II; J. Rüpke, ''The Roman Calendar from Numa to Constantine'' 113–114, 126–132, 147.</ref>

=== Intercalation ===
The old intercalary month was abolished. The new leap day was dated as ''ante diem bis sextum Kalendas Martias'' ('the sixth doubled day before the Kalends of March'), usually abbreviated as ''a.d. bis VI Kal. Mart.''; hence it is called in English the [[bissextile]] day. The year in which it occurred was termed ''annus bissextus'', in English the bissextile year.

There is debate about the exact position of the bissextile day in the early Julian calendar. The earliest direct evidence is a statement of the 2nd century jurist [[Publius Iuventius Celsus|Celsus]], who states that there were two-halves of a 48-hour day, and that the intercalated day was the "posterior" half. An inscription from AD&nbsp;168 states that ''a.d. V Kal. Mart.'' was the day after the bissextile day. The 19th century chronologist [[Christian Ludwig Ideler|Ideler]] argued that Celsus used the term "posterior" in a technical fashion to refer to the earlier of the two days, which requires the inscription to refer to the whole 48-hour day as the bissextile. Some later historians share this view. Others, following [[Theodor Mommsen|Mommsen]], take the view that Celsus was using the ordinary Latin (and English) meaning of "posterior". A third view is that neither half of the 48-hour "bis sextum" was originally formally designated as intercalated, but that the need to do so arose as the concept of a 48-hour day became obsolete.<ref>W. Sternkopf, "[https://books.google.com/books?id=MmPMaTG2ukYC&pg=PA718 Das Bissextum]", (JCP 41 (1895) 718–732).</ref>

There is no doubt that the bissextile day eventually became the earlier of the two days for most purposes. In 238 Censorinus stated that it was inserted after the [[Terminus (god)|Terminalia]] (23 February) and was followed by the last five days of February, i.e., a.d. VI, V, IV, III and prid. Kal. Mart. (which would be 24 to 28 February in a common year and the 25th to 29th in a leap year). Hence he regarded the bissextum as the first half of the doubled day. All later writers, including Macrobius about 430, [[Bede]] in 725, and other medieval [[computus|computists]] (calculators of Easter) followed this rule, as does the [[liturgical year|liturgical calendar]] of the Roman Catholic Church. However, Celsus' definition continued to be used for legal purposes. It was incorporated into [[Digest (Roman law)|Justinian's Digest]],<ref>Justinian, [http://webu2.upmf-grenoble.fr/Haiti/Cours/Ak/Anglica/D50_Scott.htm#XVI Digest 50.16.98] {{Webarchive|url=https://web.archive.org/web/20120208135740/http://webu2.upmf-grenoble.fr/Haiti/Cours/Ak/Anglica/D50_Scott.htm#XVI |date=2012-02-08 }}.</ref> and in the English ''[[Statute De Anno et Die Bissextili]]'' of 1236,<ref>{{cite book |chapter-url=https://books.google.com/books?id=tKZFAAAAcAAJ&pg=PA20 |chapter=The statute ''De anno et die bissextili'', made at Westminster, Anno 21 Hen. III. and Anno Dom. 1236 |title=The Statutes at Large from Magna Charta to the End of the Reign of King Henry the Sixth |volume=1 |location=London |year=1763}}</ref> which was not formally repealed until 1879.

The effect of the bissextile day on the [[nundinal cycle]] is not discussed in the sources. According to Dio Cassius, a leap day was inserted in 41&nbsp;BC to ensure that the first market day of 40&nbsp;BC did not fall on 1 January, which implies that the old 8-day cycle was not immediately affected by the Julian reform. However, he also reports that in AD&nbsp;44, and on some previous occasions, the market day was changed to avoid a conflict with a religious festival. This may indicate that a single nundinal letter was assigned to both halves of the 48-hour bissextile day by this time, so that the [[Regifugium]] and the market day might fall on the same date but on different days. In any case, the 8-day nundinal cycle began to be displaced by the 7-day [[Seven-day week|week]] in the first century AD, and [[dominical letter]]s began to appear alongside nundinal letters in the fasti.<ref>[https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/48*.html#33.4 Dio Cassius 48.33.4], [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/60*.html#24.7 60.24.7]; C. J. Bennett, "The Imperial Nundinal Cycle", ''Zeitschrift für Papyrologie und Epigraphik'' 147 (2004) 175–179.</ref>

===Year length; leap years===
The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long. Consequently, the Julian year drifts over time with respect to the [[tropical year|tropical (solar) year]] (365.24217 days).<ref name="Richards">Using value from Richards (2013, p. 587) for tropical year in mean solar days, the calculation is {{nowrap|1/(365.2425-365.24217)}}</ref>

Although Greek astronomers had known, at least since [[Hipparchus]],<ref name=":0">[[Ptolemy|Claudius Ptolemy]], tr. [[G. J. Toomer]], ''[[Almagest|Ptolemy's Almagest]]'', 1998, Princeton University Press, p. 139. Hipparchus stated that the "solar year ... contains 365 days, plus a fraction which is less than {{sfrac|1|4}} by about {{sfrac|1|300}}th of the sum of one day and night".</ref> a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed [[equinox]] times and the seasons. This discrepancy was largely corrected by the [[Gregorian calendar|Gregorian reform]] of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years<ref name=":1">[http://aa.usno.navy.mil/faq/docs/calendars.php Introduction to Calendars] {{Webarchive|url=https://web.archive.org/web/20190613115330/http://aa.usno.navy.mil/faq/docs/calendars.php |date=2019-06-13 }}. (15 May 2013). [[United States Naval Observatory]].</ref> (even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years).<ref name=Richards />

== Leap year error ==
<!-- Linked from Roman calendar: Later reforms -->
Although the new calendar was much simpler than the pre-Julian calendar, the pontifices initially added a leap day every three years, instead of every four. There are accounts of this in Solinus,<ref>Gaius Julius Solinus, ''De mirabilibus mundi'', c.3, available at [https://books.google.com/books?id=ABxNAAAAcAAJ].</ref> Pliny,<ref>Gaius Plinius Secundus, ''Natural History'', Vol. 2, 18.57, tr. J Bostock and H T Riley, London 1855, available at [https://www.perseus.tufts.edu/hopper/text?doc=Plin.+Nat.+18.57&fromdoc=Perseus%3Atext%3A1999.02.0137].</ref> Ammianus,<ref>''The Roman History of Ammianus Marcellinus'', 26.10, Loeb Classical Library vol. II, Harvard 1940, available at [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Ammian/26*.html].</ref> Suetonius,<ref>Gaius Suetonius Tranquillus, ''Life of Julius Caesar'', 40.1, Loeb Classical Library, Harvard 1913, available at [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#ref38] {{Webarchive|url=https://archive.today/20120530163202/http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#ref38|date=2012-05-30}}.</ref> and Censorinus.<ref>Censorinus, ''The Natal Day'', 20.30, tr. William Maude, New York 1900 available at [http://elfinspell.com/ClassicalTexts/Maude/Censorinus/DeDieNatale-Part2.html#chap9].</ref>

Macrobius<ref>Macrobius Ambrosius Theodosius, ''Saturnalia'', 1.14.13–1.14.14, tr. Percival Vaughan Davies, New York 1969, Latin text at [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#14.13.]</ref> gives the following account of the introduction of the Julian calendar:
{{blockquote|text=Caesar's regulation of the civil year to accord with his revised measurement was proclaimed publicly by edict, and the arrangement might have continued to stand had not the correction itself of the calendar led the priests to introduce a new error of their own; for they proceeded to insert the intercalary day, which represented the four quarter-days, at the beginning of each fourth year instead of at its end, although the intercalation ought to have been made at the end of each fourth year and before the beginning of the fifth.

This error continued for thirty-six years by which time twelve intercalary days had been inserted instead of the number actually due, namely nine. But when this error was at length recognised, it too was corrected, by an order of Augustus, that twelve years should be allowed to pass without an intercalary day, since the sequence of twelve such years would account for the three days which, in the course of thirty-six years, had been introduced by the premature actions of the priests.}}
So, according to Macrobius,
# the year was considered to begin after the Terminalia (23 February),<ref>Marcus Terentius Varro, ''On the Latin Language'', 6.13, tr. Roland Kent, London 1938 available at [https://ryanfb.github.io/loebolus-data/L333.pdf].</ref>
# the calendar was operated correctly from its introduction on 1 January 45&nbsp;BC until the beginning of the fourth year (February 42&nbsp;BC) at which point the priests inserted the first intercalation,
# Caesar's intention was to make the first intercalation at the beginning of the fifth year (February 41&nbsp;BC),
# the priests made a further eleven intercalations after 42&nbsp;BC at three-year intervals so that the twelfth intercalation fell in 9&nbsp;BC,
# had Caesar's intention been followed there would have been intercalations every four years after 41&nbsp;BC, so that the ninth intercalation would have been in 9&nbsp;BC,
# after 9&nbsp;BC, there were twelve years without leap years, so that the leap days Caesar would have had in 5&nbsp;BC, 1&nbsp;BC and AD&nbsp;4 were omitted and
# after AD&nbsp;4 the calendar was operated as Caesar intended, so that the next leap year was AD&nbsp;8 and then leap years followed every fourth year thereafter.<ref>Nautical Almanac Offices of the United Kingdom and the United States. (1961). ''Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac'', London: Her Majesty's Stationery Office. p. 410–11.</ref>

Some people have had different ideas as to how the leap years went. The above scheme is that of Scaliger (1583) in the table below. He established that the Augustan reform was instituted in 8&nbsp;BC. The table below shows for each reconstruction the implied proleptic Julian date for the first day of Caesar's reformed calendar and the first Julian date on which the Roman calendar date matches the Julian calendar after the completion of Augustus' reform.

{|class="wikitable"
|-
! Scholar !! Date !! Triennial leap years (BC) !! First<br> Julian day !! First<br> aligned day !! Quadriennial<br> leap year<br> resumes
|-
| [[Chris Bennett (egyptologist)|Bennett]]<ref>C. J. Bennett, "The Early Augustan Calendars in Rome and Egypt", Zeitschrift fűr Papyrologie und Epigraphik 142 (2003) 221–240 and "The Early Augustan Calendars in Rome and Egypt: Addenda et Corrigenda", Zeitschrift fűr Papyrologie und Epigraphik 147 (2004) 165–168; see also Chris Bennett, [https://web.archive.org/web/20120219041722/https://tyndalehouse.com/Egypt/ptolemies/chron/roman/024bc.htm A.U.C. 730 = 24 B.C. (Egyptian papyrus)].</ref>
| 2003
| 44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11, 8 || 31 December 46 BC || 25 February 1 BC || AD 4
|-
| Soltau<ref>W. Soltau, [https://books.google.com/books?id=dVs-AAAAYAAJ&pg=PA170 ''Römische Chronologie''] (Freiburg, 1889) 170–173. He accepted Matzat's phase of the triennial cycle but argued that it was absurd to suppose that Caesar would have made the second Julian year a leap year and that the 36 years had to be accounted from 45&nbsp;BC.</ref>
| 1889
| 45, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11 || 2 January 45 BC || 25 February AD 4 || AD 8
|-
| Matzat<ref>H. Matzat, [https://archive.org/details/rmischechronolo02matzgoog/page/n197 <!-- pg=13 --> ''Römische Chronologie'' I] (Berlin, 1883), 13–18. His argument rested on [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/48*.html#33.4 Dio Cassius 48.33.4] which mentions a leap day inserted in 41&nbsp;BC, "contrary to the (i.e., Caesar's) rule", in order to avoid having a market day on the first day of 40&nbsp;BC. Dio stated that this leap day was compensated for "later". Matzat proposed this was done by omitting a scheduled leap day in 40&nbsp;BC, rather than by omitting a day from an ordinary year.</ref>
| 1883
| 44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11 || 1 January 45 BC || 25 February 1 BC || AD 4
|-
| [[Ludwig Ideler|Ideler]]<ref>C. L. Ideler, [https://books.google.com/books?id=ql56SzvCeJMC&pg=PA130 ''Handbuch der mathematischen und technischen Chronologie''] (Berlin, 1825) II 130–131. He argued that Caesar would have enforced the bissextile day by introducing it in his first reformed year. T. E. Mommsen, [https://archive.org/details/bub_gb_qsspAAAAYAAJ/page/n281 <!-- pg=282 --> ''Die Römische Chronologie bis auf Caesar''] (Berlin, 1859) 282–299, provided additional circumstantial arguments.</ref>
| 1825
| 45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, 9 || 1 January 45 BC || 25 February AD 4 || AD 8
|-
| [[Johannes Kepler|Kepler]]<ref>J. Kepler, ''De Vero Anno Quo Æternus Dei Filius Humanan Naturam in Utero Benedictæ Virginis Mariæ Assumpsit'' (Frankfurt, 1614) Cap. V, repub. in F. Hammer (ed.), ''Johannes Keplers Gesammelte Werke'' (Berlin, 1938) V 28.</ref>
| 1614
| 43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10 || 2 January 45 BC || 25 February AD 4 || AD 8
|-
| [[Thomas Harriot|Harriot]]<ref name="Harriot" />
| After 1610
| 43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10 || 1 January 45 BC || 25 February 1 BC || AD 4
|-
| [[Heinrich Bünting|Bünting]]<ref name="Harriot">For the list of triennial leap years proposed by Bünting, Christmann and Harriot, see [http://www.hermetic.ch/cal_stud/cassidy/shspk1zc.pdf Harriot's comparative table reproduced by Simon Cassidy] (Fig. 6). The table numbers years as Julian years, where Julian year 1 = 45&nbsp;BC. Thus, Scaliger and Clavius (col. 7) resume intercalation in Julian year 53 = AD 8, while Bünting (col. 8) and Harriot (col. 3) resume it in Julian year 49 = AD 4 and Christmann (col. 9) in year 52 = AD 7.</ref>
| 1590
| 45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12 || 1 January 45 BC || 25 February 1 BC || AD 4
|-
| [[Jakob Christmann|Christmann]]<ref name="Harriot" /><ref>J. Christmann [https://books.google.com/books?id=NTc6AAAAcAAJ&pg=PA173 ''Muhamedis Alfragani arabis chronologica et astronomica elementa''] (Frankfurt, 1590), 173. His argument assumed that the triennial cycle started in the third Julian year.</ref>
| 1590
| 43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10 || 2 January 45 BC || 25 February AD 4 || AD 7<ref name="Harriot" />
|-
| [[Joseph Scaliger|Scaliger]]<ref>J. J. Scaliger, ''De emendatione temporum'' (Paris, 1583), 159, 238.</ref>
| 1583
| 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, 9 || 2 January 45 BC || 25 February AD 4 || AD 8
|}

By the systems of Scaliger, Ideler and Bünting, the leap years prior to the suspension happen to be BC years that are divisible by 3, just as, after leap year resumption, they are the AD years divisible by 4.

[[Pierre Brind'Amour]]<ref>Pierre Brind'Amour, ''Le calendrier romain'', Ottawa 1983, pp. 45–46.</ref> argued that "only one day was intercalated between 1/1/45 and 1/1/40 (disregarding a momentary 'fiddling' in December of 41)<ref>Dio Cassius 48.33.4, tr. Earnest Cary, Loeb Classical Library, 9 vol., Harvard 1914–1927, available at [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Cassius_Dio/48*.html#33.4].</ref> to avoid the nundinum falling on Kal. Ian."<ref>Refutation of Brind'Amour's theory by John Ward, ''Re: Intercalation in 45BC to 8AD'', East Carolina University Calendar discussion List CALNDR-L, April 1998.</ref>

Alexander Jones says that the correct Julian calendar was in use in Egypt in 24&nbsp;BC,<ref name="papyrus" /> implying that the first day of the reform in both Egypt and Rome, {{nowrap|1 January 45 BC}}, was the Julian date 1&nbsp;January if 45&nbsp;BC was a leap year and 2&nbsp;January if it was not. This necessitates fourteen leap days up to and including AD&nbsp;8 if 45&nbsp;BC was a leap year and thirteen if it was not.
In 1999, a papyrus was discovered which gives the dates of astronomical phenomena in 24&nbsp;BC in both the Egyptian and Roman calendars. From {{nowrap|30 August 26 BC (Julian)}}, Egypt had two calendars: the old Egyptian in which every year had 365 days and the new Alexandrian in which every fourth year had 366 days. Up to {{nowrap|28 August 22 BC (Julian)}} the date in both calendars was the same. The dates in the Alexandrian and Julian calendars are in one-to-one correspondence except for the period from 29 August in the year preceding a Julian leap year to the following 24 February.<ref>Dieter Hagedorn, ''Zum aegyptischen Kalender unter Augustus'', Zeitschrift fűr Papyrologie und Epigraphik 100 (1994) 211–222, available at [http://www.uni-koeln.de/phil-fak/ifa/zpe/downloads/1994/100pdf/100211.pdf].</ref> From a comparison of the astronomical data with the Egyptian and Roman dates, Alexander Jones<ref name="papyrus">Alexander Jones, ''Calendrica II: Date Equations from the Reign of Augustus'', Zeitschrift fűr Papyrologie und Epigraphik 129 (2000) 159–166, available at [http://www.uni-koeln.de/phil-fak/ifa/zpe/downloads/2000/129pdf/129159.pdf].</ref> concluded that the Egyptian astronomers (as opposed to travellers from Rome) used the correct Julian calendar.

Due to the confusion about this period, we cannot be sure exactly what day (e.g. [[Julian day number]]) any particular Roman date refers to before March of 8&nbsp;BC, except for those used in Egypt in 24{{nbsp}}BC which are secured by astronomy.

An inscription has been discovered which orders a [[Paullus Fabius Maximus#A New Calendar in the Province of Asia|new calendar]] to be used in the [[Province of Asia]] to replace the previous Greek lunar calendar.<ref>[http://epigraphy.packhum.org/inscriptions/main?url=oi%3Fikey%3D252886%26region%3D8%26subregion%3D29%26bookid%3D520%26caller%3Dsearch%26start%3D1193%26end%3D1202 OGIS 458] (Greek).</ref> According to one translation

{{Blockquote|text=Intercalation shall commence on the day after 14 Peritius [a.d. IX Kal. Feb, which would have been 15 Peritius] as it is currently constituted in the third year following promulgation of the decree. Xanthicus shall have 32 days in this intercalary year.<ref>B A Buxton and R Hannah in ''Studies in Latin Literature and Roman History'' (ed. C Deroux), XII 290.</ref> }}

This is historically correct.{{citation needed|date=August 2024}} It was decreed by [[Paullus Fabius Maximus|the proconsul]] that the first day of the year in the new calendar shall be Augustus' birthday, a.d. IX Kal. Oct. Every month begins on the ninth day before the kalends. The date of introduction, the day after 14 Peritius, was 1 Dystrus, the next month. The month after that was Xanthicus. Thus Xanthicus began on a.d. IX Kal. Mart., and normally contained 31 days. In leap year, however, it contained an extra "Sebaste day", the Roman leap day, and thus had 32 days. From the lunar nature of the old calendar we can fix the starting date of the new one as 24&nbsp;January, {{nowrap|a.d. IX Kal. Feb 5 BC}} in the Julian calendar, which was a leap year. Thus from inception the dates of the reformed Asian calendar are in one-to-one correspondence with the Julian.

Another translation of this inscription is

{{Blockquote|Intercalation shall commence on the day after the fourteenth day in the current month of Peritius [a.d. IX Kal. Feb], occurring every third year. Xanthicus shall have 32 days in this intercalary year.<ref>U. Laffi, "Le iscrizioni relative all'introduzione nel 9 a.c. del nuovo calendario della provincia d'Asia", Studi Classici e Orientali 16 (1967) 5–99.</ref>}}

This would move the starting date back three years to 8&nbsp;BC, and from the lunar synchronism back to 26 January (Julian). But since the corresponding Roman date in the inscription is 24 January, this must be according to the incorrect calendar which in 8&nbsp;BC Augustus had ordered to be corrected by the omission of leap days. As the authors of the previous{{which|date=October 2019}} paper point out, with the correct four-year cycle being used in Egypt and the three-year cycle abolished in Rome, it is unlikely that Augustus would have ordered the three-year cycle to be introduced in Asia.

==Month names==
The Julian reform did not immediately cause the names of any months to be changed. The old [[Mercedonius|intercalary month]] was abolished and replaced with a single intercalary day at the same point (i.e., five days before the end of February).<ref>{{Cite web |last=Genealogy |first=History & |title=LibGuides Home: Colonial Records & Topics: 1752 Calendar Change |url=https://libguides.ctstatelibrary.org/hg/colonialresearch/calendar |access-date=2023-10-26 |website=libguides.ctstatelibrary.org |language=en}}</ref><ref>{{Cite web |title=Intercalation |url=https://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/intercalation.html |access-date=2023-10-26 |website=penelope.uchicago.edu}}</ref>

===Roman===
The Romans later renamed months after Julius Caesar and Augustus, renaming Quintilis as "Iulius" (July)<ref name=J /> in 44&nbsp;BC and Sextilis as "Augustus" (August) in 8&nbsp;BC. Quintilis was renamed to honour Caesar because it was the month of his birth.<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#76 ''Caesar''] {{Webarchive|url=https://archive.today/20120530163202/http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Julius*.html#76 |date=2012-05-30 }} 76.1.</ref> According to a {{lang|la|[[senatus consultum]]}} quoted by Macrobius, Sextilis was renamed to honour Augustus because several of the most significant events in his rise to power, culminating in the fall of Alexandria, occurred in that month.<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Augustus*.html#31.2 ''Augustus'' 31.2]; Macrobius, [https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Macrobius/Saturnalia/1*.html#12.35  ''Saturnalia'' 1.12.35] (Latin)</ref>

Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. In AD&nbsp;37, [[Caligula]] renamed September as "Germanicus" after his [[Germanicus|father]];<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Caligula*.html#15.2 ''Caligula'' 15.2].</ref> in AD&nbsp;65, [[Nero]] renamed April as "Neroneus", May as "Claudius" and June as "Germanicus";<ref>Tacitus, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Tacitus/Annals/15C*.html#74 ''Annals'' 15.74], [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Tacitus/Annals/16*.html#12 16.12].</ref> and in AD&nbsp;84 [[Domitian]] renamed September as "Germanicus" and October as "Domitianus".<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Domitian*.html#13.3 ''Domitian'' 13.3].</ref> [[Commodus]] was unique in renaming all twelve months after his own adopted names (January to December): "Amazonius", "Invictus", "Felix", "Pius", "Lucius", "Aelius", "Aurelius", "Commodus", "Augustus", "Herculeus", "Romanus", and "Exsuperatorius".<ref>[https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/73*.html#72-15.3 Dio Cassius 73.15.3].</ref> The emperor [[Marcus Claudius Tacitus|Tacitus]] is said to have ordered that September, the month of his birth and accession, be renamed after him, but the story is doubtful since he did not become emperor before November 275.<ref>Historia Augusta, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Historia_Augusta/Tacitus*.html#13.6 ''Tacitus'' 13.6]. On the chronology see [http://www.roman-emperors.org/tacitus.htm R. McMahon, ''Tacitus''.]</ref> Similar honorific month names were implemented in many of the provincial calendars that were aligned to the Julian calendar.<ref>Surveyed in K. Scott, ''Honorific Months'', Yale Classical Studies 2 (1931) 201–278.</ref>

Other name changes were proposed but were never implemented. [[Tiberius]] rejected a senatorial proposal to rename September as "Tiberius" and October as "Livius", after his mother Livia.<ref>Suetonius, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Suetonius/12Caesars/Tiberius*.html#26.2 ''Tiberius'' 26.2].</ref> [[Antoninus Pius]] rejected a senatorial decree renaming September as "Antoninus" and November as "Faustina", after [[Faustina the Elder|his empress]].<ref>Historia Augusta, [https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Historia_Augusta/Antoninus_Pius*.html#10 ''Antoninus Pius'' 10.1].</ref>

===Charlemagne===
Much more lasting than the ephemeral month names of the post-Augustan Roman emperors were the [[Germanic calendar|Old High German names]] introduced by [[Charlemagne]]. According to his biographer, Einhard, Charlemagne renamed all of the months agriculturally in German.<ref>Einhard, [http://www.fordham.edu/halsall/basis/einhard.asp#Reforms ''Life of Charlemagne'', 29].</ref> These names were used until the 15th century, over 700 years after his rule, and continued, with some modifications, to be used as "traditional" month names until the late 18th century. The names (January to December) were: ''Wintarmanoth'' ("winter month"), ''Hornung'',{{efn|This name of February, the only name in the list without the "month" suffix, is explained by König, ''Festschrift Bergmann'' (1997), pp. 425 ff. as a collective of ''[[:wikt:horn|horn]]'', taken to refer to the antlers shed by [[red deer]] during this time. Older explanations compare the name with Old Frisian ''horning'' (Anglo-Saxon ''hornung-sunu'', Old Norse ''hornungr'') meaning "bastard, illegitimate son", taken to imply a meaning of "disinherited" in reference to February being the shortest of months.}} ''Lentzinmanoth'' ("spring month", "[[:wikt:Lent|Lent]] month"), ''Ostarmanoth'' ("[[:wikt:Easter|Easter]] month"), ''Wonnemanoth'' ("[[:wikt:Appendix:Proto-Germanic/wunjō|joy]]-month", a corruption of ''Winnimanoth'' "pasture-month"), ''Brachmanoth'' ("[[:wikt:Brache|fallow]]-month"), ''Heuuimanoth'' ("hay month"), ''Aranmanoth'' ("[[:wikt:earnian|reaping]] month"), ''Witumanoth'' ("wood month"), ''Windumemanoth'' ("vintage month"), ''Herbistmanoth'' ("harvest month"), and ''Heilagmanoth'' ("holy month").

===Eastern Europe===
The calendar month names used in western and northern Europe, in Byzantium, and by the [[Berber calendar#The months|Amazigh (Berbers)]], were derived from the Latin names. However, in eastern Europe older seasonal month names continued to be used into the 19th century, and in some cases are still in use, in many languages, including: [[Belarusian months|Belarusian]], [[Bulgarian months|Bulgarian]], [[Croatian months|Croatian]], [[Czech months|Czech]], Finnish,<ref>{{cite web |url=http://wordinfo.info/unit/3236/ip:23 |title=Calendar, Finnish |website=English-Word Information}}</ref> [[Georgian calendar|Georgian]], [[Lithuanian calendar#Names of the months|Lithuanian]], [[Macedonian months|Macedonian]], [[Polish months|Polish]], [[Romanian calendar#Traditional month names|Romanian]], [[Slovene months|Slovene]], [[Ukrainian months|Ukrainian]]. When the Ottoman Empire adopted the Julian calendar, in the form of the Rumi calendar, the [[Rumi calendar#History|month names]] reflected Ottoman tradition.

== Year numbering ==
The principal method used by the Romans to identify a year for dating purposes was to name it after the two consuls who took office in it, the eponymous period in question being the consular year. Beginning in 153&nbsp;BC, consuls began to take office on 1&nbsp;January, thus synchronizing the commencement of the consular and calendar years. The calendar year has begun in January and ended in December since about 450&nbsp;BC according to Ovid or since about 713&nbsp;BC according to Macrobius and Plutarch (see [[Roman calendar]]). Julius Caesar did not change the beginning of either the consular year or the calendar year. In addition to consular years, the Romans sometimes used the regnal year of the emperor, and by the late 4th&nbsp;century documents were also being dated according to the 15-year cycle of the [[indiction]]. In 537, [[Justinian I|Justinian]] required that henceforth the date must include the name of the emperor and his regnal year, in addition to the indiction and the consul, while also allowing the use of local [[Calendar era|eras]].

In 309 and 310, and from time to time thereafter, no consuls were appointed.<ref>{{cite web| url = http://www.tertullian.org/fathers/chronography_of_354_08_fasti.htm| title = Chronography of AD&nbsp;354.}}</ref> When this happened, the consular date was given a count of years since the last consul (called "post-consular" dating). After 541, only the reigning emperor held the consulate, typically for only one year in his reign, and so post-consular dating became the norm. Similar post-consular dates were also known in the west in the early 6th&nbsp;century. <!-- Commented out pending further research. [[User:Dojarca]] is up to 4th&nbsp;year of the consulate of [[Justinian II]] with no assurance he is the latest. --~~~~ The last known post-consular date is year&nbsp;22 after the consulate of [[Heraclius]].{{Verify source|date=August 2007}} The last emperor to hold the consulate was [[Constans II]]. --> The system of consular dating, long obsolete, was formally abolished in the law code of [[Leo VI the Wise|Leo&nbsp;VI]], issued in 888.

Only rarely did the Romans number the year from the [[founding of Rome|founding of the city (of Rome)]], {{lang|la|[[ab urbe condita]]}} (AUC). This method was used by Roman historians to determine the number of years from one event to another, not to date a year. Different historians had several different dates for the founding. The {{lang|la|[[Fasti]] Capitolini}}, an inscription containing an official list of the consuls which was published by Augustus, used an [[epoch (reference date)|epoch]] of 752&nbsp;BC. The epoch used by [[Marcus Terentius Varro|Varro]], 753&nbsp;BC, has been adopted by modern historians. Indeed, [[Renaissance]] editors often added it to the manuscripts that they published, giving the false impression that the Romans numbered their years. Most modern historians tacitly assume that it began on the day the consuls took office, and ancient documents such as the {{lang|la|Fasti Capitolini}} which use other AUC systems do so in the same way. However, Censorinus, writing in the 3rd&nbsp;century&nbsp;AD, states that, in his time, the AUC year began with the [[Parilia]], celebrated on 21&nbsp;April, which was regarded as the actual anniversary of the foundation of Rome.<ref>{{cite book |author=[[Censorinus]] |title=De Die Natali |at=21.6 |language=Latin |url=https://penelope.uchicago.edu/Thayer/L/Roman/Texts/Censorinus/text*.html#21.6}} Because the lively festivities associated with the [[Parilia]] conflicted with the solemnity of [[Lent]], which was observed until the Saturday before Easter Sunday, the early Roman church did not celebrate Easter after 21&nbsp;April.{{cite book |author=[[Bede]] |editor-first=Charles W. |editor-last=Jones |chapter=Development of the Latin ecclesiastical calendar |title=Bedae Opera de Temporibus |year=1943 |pages=1–122, esp.&nbsp;28}}</ref>

Many local eras, such as the Era of Actium and the [[Spanish Era]], were adopted for the Julian calendar or its local equivalent in the provinces and cities of the Roman Empire. Some of these were used for a considerable time.<ref>For a partial survey see {{cite book |first=A.E. |last=Samuel |year=1972 |title=Greek and Roman Chronology: Calendars and years in classical antiquity |publisher=Beck |place=Munich, Germany |pages=245&nbsp;ff}} Samuel introduces his survey by saying: "The number of eras which came into use and then expired to be replaced by yet other eras during Hellenistic and Roman times is probably not infinite, but I have not been able to find the end of them." Anatolian eras are exhaustively surveyed in {{cite book |first=W. |last=Leschhorn |year=1993 |title=Antike Ären: Zeitrechnung, Politik und Geschichte im Schwarzmeerraum und in Kleinasien nördlich des Tauros |publisher=Franz Steiner |place=Stuttgart,Germany |language=DE}}</ref> Perhaps the best known is the [[Era of Martyrs]], sometimes also called {{lang|la|Anno Diocletiani}} (after [[Diocletian]]), which was associated with the Alexandrian calendar and often used by the [[Alexandria]]n Christians to number their Easters during the 4th and 5th centuries, and continues to be used by the Coptic and [[Ethiopian Orthodox Church|Ethiopian]] churches.

In the eastern Mediterranean, the efforts of Christian chronographers such as [[Annianus of Alexandria]] to date the Biblical creation of the world led to the introduction of [[Anno Mundi]] eras based on this event.<ref>{{cite book |first=A.A. |last=Mosshammer |year=2008 |title=The Easter Computus and the Origins of the Christian Era |publisher=Oxford University Press |place=Oxford, UK |pages=27–29}}</ref> The most important of these was the [[Etos Kosmou]], used throughout the Byzantine world from the 10th century and in Russia until 1700. In the west, the kingdoms succeeding the empire initially used indictions and [[regnal year]]s, alone or in combination. The chronicler [[Prosper of Aquitaine]], in the fifth century, used an era dated from the [[Passion (Christianity)|Passion of Christ]], but this era was not widely adopted. [[Dionysius Exiguus]] proposed the system of [[Anno Domini]] in 525. This era gradually spread through the western Christian world, once the system was adopted by [[Bede]] in the eighth century.

The Julian calendar was also used in some Muslim countries. The [[Rumi calendar]], the Julian calendar used in the later years of the [[Ottoman Empire]], adopted an era derived from the lunar [[Islamic calendar|AH]] year equivalent to AD&nbsp;1840, i.e., the effective [[Rumi calendar|Rumi epoch]] was AD&nbsp;585. In recent years, some users of the [[Berber calendar#The computation of the years|Berber calendar]] have adopted an era starting in 950&nbsp;BC, the approximate date that the Libyan pharaoh {{nowrap|[[Sheshonq I]]}} came to power in Egypt.

== New Year's Day ==
The Roman calendar began the year on 1 January, and this remained the start of the year after the Julian reform. However, even after local calendars were aligned to the Julian calendar, they started the new year on different dates. The Alexandrian calendar in Egypt started on 29 August (30 August after an Alexandrian leap year). Several local provincial calendars were aligned to start on the birthday of Augustus, 23 September. The indiction caused the [[Byzantine Empire|Byzantine]] year, which used the Julian calendar, to begin on 1 September; this date is still used in the [[Eastern Orthodox Church]] for the beginning of the liturgical year. When the Julian calendar was adopted in AD&nbsp;988 by [[Vladimir I of Kiev]], the year was numbered Anno Mundi 6496, beginning on 1 March, six months after the start of the Byzantine Anno Mundi year with the same number. In 1492 (AM 7000), [[Ivan III]], according to church tradition, realigned the start of the year to 1 September, so that AM 7000 only lasted for six months in Russia, from 1 March to 31 August 1492.<ref>{{cite web| url = https://translate.google.com/translate?hl=en&sl=ru&u=http://grigam.narod.ru/kalend/kalen19.htm| title = История календаря в России и в СССР (Calendar history in Russia and in the USSR).}}</ref>

In Anglo-Saxon England, the year most commonly began on 25 December, which, as (approximately) the [[winter solstice]], had marked the start of the year in pagan times, though 25 March (the [[equinox]]) is occasionally documented in the 11th century. Sometimes the start of the year was reckoned as 24 September, the start of the so-called "western indiction" introduced by Bede.<ref>M. L. R. Beaven, "The Regnal Dates of Alfred, Edward the Elder, and Athelstan", English Historical Review 32 (1917) 517–531; idem, "The Beginning of the Year in the Alfredian Chronicle (866–87)", English Historical Review 33 (1918) 328–342.</ref> These practices changed after the Norman conquest. From 1087 to 1155 the English year began on 1 January, and from 1155 to 1751 it began on 25 March.<ref>Catholic Encyclopedia, [http://www.newadvent.org/cathen/03738a.htm#beginning General Chronology (Beginning of the Year)].</ref> In 1752 it was moved back to 1 January. (See [[Calendar (New Style) Act 1750|Calendar [New Style] Act 1750]]).

Even before 1752, 1 January was sometimes treated as the start of the new year&nbsp;– for example by Pepys<ref>[http://www.pepysdiary.com/archive/1661/12/31/index.php Pepys Diary], "I sat down to end my journell for this year, ..."</ref> – while the "year starting 25th March was called the Civil or Legal Year".<ref name=MS>Spathaky, Mike [http://www.cree.name/genuki/dates.htm Old Style and New Style dates and the change to the Gregorian calendar].</ref> To reduce misunderstandings on the date, it was not uncommon for a date between 1 January and 24 March to be written as "1661/62". This was to explain to the reader that the year was 1661 counting from March and 1662 counting from January as the start of the year.<ref name=MS-oblique-stroke>Spathaky, Mike [http://www.cree.name/genuki/dates.htm Old Style and New Style dates and the change to the Gregorian calendar]. "An oblique stroke is by far the most usual indicator, but sometimes the alternative final figures of the year are written above and below a horizontal line, as in a fraction (a form which cannot easily be reproduced here in ASCII text). Very occasionally a hyphen is used, as 1733{{hyphen}}34."</ref> (For more detail, see [[Dual dating]]).

{| class="wikitable sortable" style="margin:auto;"
|-
! Country !! Year starting<br />1 January<ref>[[John James Bond]], [https://archive.org/details/handybookrulesa01bondgoog "Commencement of the Year"], ''Handy-book of rules and tables for verifying dates with the Christian era'', (London: 1875), 91–101.</ref><ref>Mike Spathaky ''[http://www.cree.name/genuki/dates.htm Old Style and New Style Dates and the change to the Gregorian Calendar: A summary for genealogists.]''</ref> !!Adoption of<br /> new calendar
|-
| [[Holy Roman Empire]]{{efn|The source has Germany, whose current area during the sixteenth century was a major part of the Holy Roman Empire, a religiously divided confederation. The source is unclear as to whether all or only parts of the country made the change. In general, Roman Catholic countries made the change a few decades before Protestant countries did.}}|| 1544{{efn|Previously began on 25 December, with possible exceptions}} || 1582
|-
| Spain, [[Portugal]], [[Polish–Lithuanian Commonwealth]] || 1556 || 1582
|-
| [[Prussia]], [[Denmark–Norway]]|| 1559 || 1700
|-
| Sweden || 1559 || 1753{{efn|Sweden started a conversion process in 1700, which was abandoned later that year due to the [[Great Northern War]], and in 1712 returned to the Julian calendar. In 1753 Sweden switched to the Gregorian calendar. See [[Swedish calendar]].}}
|-
|France || [[Edict of Roussillon|1567]]{{efn|Previously began on Easter Sunday, with several exceptions such as Lyon on 25 December and Vienne on 25 March}} || 1582
|-
|[[Southern Netherlands]] || 1576<ref>Per decree of 16 June 1575. Hermann Grotefend, "[http://www.manuscripta-mediaevalia.de/gaeste/grotefend/g_o.htm#Osteranfang Osteranfang]" (Easter beginning), ''[http://www.manuscripta-mediaevalia.de/gaeste/grotefend/grotefend.htm Zeitrechnung de Deutschen Mittelalters und der Neuzeit]'' (Chronology of the German Middle Ages and modern times) (1891–1898).</ref> || 1582
|-
| [[Lorraine (province)|Lorraine]] || 1579 || 1760
|-
| [[Holland]], [[Zeeland]] || 1583 || 1582
|-
| [[Dutch Republic]] except [[Holland]] and [[Zeeland]] || 1583 || 1700
|-
| [[Kingdom of Scotland|Scotland]] || 1600{{efn|Previously began on 25 March from 1155}} || 1752
|-
| Russia || 1700{{efn|Previously began on 1 March from 988 until 1492, and 1 September from 1492}} || 1918
|-
| [[Grand Duchy of Tuscany|Tuscany]] || 1750<ref>Alexandre Dumas, [https://archive.org/details/storiadelgoverno00duma/page/201 <!-- quote=toscana 1750 calendario gregoriano. --> Storia del governo della Toscana: sotto La casa de'Medici].</ref><ref>[https://www.florencewithguide.com/it/blog-it/il-calendario-fiorentino/ Il calendario fiorentino].</ref> || 1582<ref>Lorenzo Cattini, [https://books.google.com/books?id=v--l2PRjuJQC Legislazione toscana raccolta e illustrata], vol. 10, p. 208.</ref>
|-
| [[British Empire]] excluding [[Scotland]]|| 1752{{efn|Previously began on 25 March from 1155}} || 1752{{efn|1751 in England only lasted from 25 March to 31 December. The dates 1 January to 24 March which would have concluded 1751 under the old calendar became part of 1752 when the beginning of the numbered year was changed from 25 March to 1 January.}}
|-
| [[Republic of Venice]] || 1522{{efn|Previously began on 1 March}} || 1582
|-
| [[Serbia]] || 1804{{Citation needed|date=January 2008}} || 1918
|-
| [[Ottoman Empire]] || 1918 || 1917{{efn|See [[Rumi calendar]] for details. It is often stated that Turkey adopted the Gregorian calendar in 1926 or 1927: in fact this is when it adopted the AD/CE [[epoch]].}}
|}

== Replacement by the Gregorian calendar ==
{{Main|Adoption of the Gregorian calendar}}
The [[Gregorian calendar]] has [[Adoption of the Gregorian calendar|replaced]] the Julian as the [[civil calendar]] in all countries which had been using it{{snd}}Greece being the last to do so, in 1923. The [[liturgical calendar]] used by Christian denominations in the west are almost all based on the Gregorian calendar, but most [[Eastern Orthodox]] churches continue to base theirs on the Julian.

A calendar similar to the Julian one, the [[Alexandrian calendar]], is the basis for the [[Ethiopian calendar]], which is still the civil calendar of Ethiopia. Egypt converted from the Alexandrian calendar to Gregorian on 1 Thaut 1592/11 September 1875.<ref>[https://books.google.com/books?id=4llHAQAAIAAJ&dq=Beardsley+Fish+1875+1348&pg=PA1348 Papers Relating to the Foreign Relations of the United States, vol. 2, pp. 1348–1349].</ref>

During the changeover between calendars and for some time afterwards, [[dual dating]] was used in documents and gave the date according to both systems. In contemporary as well as modern texts that describe events during the period of change, it is customary to clarify to which calendar a given date refers by using an [[Old Style and New Style dates|O.S. or N.S. suffix]] (denoting Old Style, Julian or New Style, Gregorian).

=== Transition history ===
{{See also|Adoption of the Gregorian calendar|Old Style and New Style dates}}
In 1582, [[Pope Gregory XIII]] promulgated the [[Gregorian calendar]]. Reform was required as the Julian calendar year, with an average length of 365.25 days, was longer than the natural [[tropical year]]. On average, the astronomical solstices and the equinoxes advance by 10.8 minutes per year against the Julian calendar year. As a result, 21 March (which is the base date for [[Computus|calculating the date of Easter]]) gradually moved out of alignment with the March equinox.
[[File:Julian to Gregorian Date Change.png|thumb|In 1582 when Roman Catholic countries such as Spain adopted the Gregorian calendar, ten days were omitted from the month of October.]]
While Hipparchus and presumably [[Sosigenes of Alexandria|Sosigenes]] were aware of the discrepancy, although not of its correct value,{{sfn|Richards|1998|page=216}} it was evidently felt to be of little importance at the time of the Julian reform (46&nbsp;BC). However, it accumulated significantly over time: the Julian calendar gained a day every 128 years. By 1582, 21 March was ten days out of alignment with the March equinox, the date where it was reckoned to have been in 325, the year of the [[First Council of Nicaea|Council of Nicaea]].

Since the Julian and Gregorian calendars were long used simultaneously, although in different places, calendar dates in the transition period are often ambiguous, unless it is specified which calendar was being used. In some circumstances, double dates might be used, one in each calendar. The notation [[Old Style and New Style dates|"Old Style" (O.S.)]] is sometimes used to indicate a date in the Julian calendar, as opposed to [[Old Style and New Style dates|"New Style" (N.S.)]], which either represents the Gregorian date or the Julian date with the start of the year as 1 January. This notation is used to clarify dates from countries that continued to use the Julian calendar after the Gregorian reform, such as Great Britain, which did not adopt the reformed calendar until 1752, or Russia, which did not do so until 1918 (see [[Soviet calendar]]). This is why the Russian Revolution of 7 November 1917 N.S. is known as the [[October Revolution]], because it began on 25 October O.S.

== Modern usage ==
=== Eastern Orthodox ===
{{See also|New Calendarists}}[[File:Bogojavlenie.jpg|thumb|250px|[[Russian icon]] of the [[Epiphany (feast)|Theophany]] (the baptism of Jesus by [[John the Baptist]]) (6 January), the highest-ranked feast which occurs on the fixed cycle of the [[Eastern Orthodox liturgical calendar]]]]

Although most [[Eastern Orthodox Church|Eastern Orthodox]] countries (most of them in [[Eastern Europe|eastern]] or southeastern Europe) had adopted the Gregorian calendar by 1924, their national churches had not. The "[[Revised Julian calendar]]" was endorsed by a [[synod]] in [[Constantinople]] in May 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part which calculated Easter astronomically at [[Jerusalem]]. All Eastern Orthodox churches refused to accept the lunar part, so all Orthodox churches continue to celebrate Easter according to the Julian calendar, with the exception of the [[Finnish Orthodox Church]]<ref>Bishop Photius of Triaditsa, [http://orthodoxinfo.com/ecumenism/photii_2.aspx "The 70th Anniversary of the Pan-Orthodox Congress, Part II of II"]; {{cite web
|url=http://www.hs.fi/english/article/Moscow-affiliated+Russian+Orthodox+church+grows+in+Helsinki/1135230488329
|date=21 September 2007
|title=HELSINGIN SANOMAT (International edition)
|access-date=11 May 2010
|archive-date=14 July 2014
|archive-url=https://web.archive.org/web/20140714150231/http://www.hs.fi/english/article/Moscow-affiliated+Russian+Orthodox+church+grows+in+Helsinki/1135230488329
|url-status=dead
}}</ref> (and the [[Estonian Apostolic Orthodox Church|Estonian Orthodox Church]].<ref>{{cite thesis |title=THE DATE OF EASTER: CLASSICAL CONSIDERATIONS AND CONTEMPORARY CHALLENGES |date=2024 |url=https://ruor.uottawa.ca/server/api/core/bitstreams/300383f1-5612-4680-b15a-d4112434713c/content |first=R.D. ALBERT P. |last=MARCELLO  |pages=135, 136 |publisher=University of Ottawa}}</ref> from 1923 to 1945).

The Orthodox Churches of [[Eastern Orthodox Patriarchate of Jerusalem|Jerusalem]], [[Russian Orthodox Church|Russia]], [[Serbian Orthodox Church|Serbia]], [[Montenegrin Orthodox Church|Montenegro]], Poland (from 15 June 2014), [[Macedonian Orthodox Church – Ohrid Archbishopric|North Macedonia]], [[Georgian Orthodox and Apostolic Church|Georgia]], and the [[Greek Old Calendarists]] and other groups continue to use the Julian calendar, thus they celebrate the Nativity on 25 December ''Julian'' (which is 7 January ''Gregorian'' until 2100). 

The [[Orthodox Church of Ukraine]] announced in late May 2023 that they would use the Gregorian calendar to celebrate Christmas on December 25, 2023, partly in reflection to [[Russia]]'s [[Russian invasion of Ukraine|invasion of the country]] in early 2022; the church continues to celebrate Easter on the date according to the Julian tradition.<ref>{{cite news |title=Orthodox Church of Ukraine to switch to Revised Julian calendar, celebrate Christmas on Dec. 25 |date=May 24, 2023 |url=https://kyivindependent.com/orthodox-church-of-ukraine-to-switch-to-revised-julian-calendar-celebrate-christmas-on-dec-25/ |work=Kyiv Independent }}</ref>

==== Date of Easter ====
Most branches of the [[Eastern Orthodox Church]] use the Julian calendar for [[Computus|calculating the date of Easter]], upon which the timing of all the other [[moveable feast]]s depends. Some such churches have adopted the [[Revised Julian calendar]] for the observance of [[fixed feast]]s, while such Orthodox churches retain the Julian calendar for all purposes.<ref>[http://www.oikoumene.org/en/resources/documents/commissions/faith-and-order/i-unity-the-church-and-its-mission/towards-a-common-date-for-easter/index Towards a Common Date of Easter.] {{Webarchive|url=https://web.archive.org/web/20170620205601/http://www.oikoumene.org/en/resources/documents/commissions/faith-and-order/i-unity-the-church-and-its-mission/towards-a-common-date-for-easter/index |date=2017-06-20 }} (5–10 March). World Council of Churches/Middle East Council of Churches Consultation, Aleppo, Syria.</ref>

===Syriac Christianity===
The [[Ancient Assyrian Church of the East]], an [[East Syriac]] rite that is commonly miscategorised under "eastern Orthodox", uses the Julian calendar, where its participants celebrate Christmas on 7 January ''Gregorian'' (which is 25 December ''Julian''). The [[Assyrian Church of the East]], the church it split from in 1968 (the replacement of traditional Julian calendar with Gregorian calendar being among the reasons), uses the Gregorian calendar ever since the year of the [[schism]].<ref>[https://theorthodoxchurch.info/blog/news/the-quest-for-orthodox-assyrian-alliance/ The Quest for Orthodox–Assyrian Alliance] {{Webarchive|url=https://web.archive.org/web/20210502080754/https://theorthodoxchurch.info/blog/news/the-quest-for-orthodox-assyrian-alliance/ |date=2021-05-02 }} Orthodoxy Cognate PAGE</ref>

=== Oriental Orthodox ===
The [[Armenian Patriarchate of Jerusalem]] of [[Armenian Apostolic Orthodox Church]] uses Julian calendar, while the rest of Armenian Church uses Gregorian calendar. Both celebrate the Nativity as part of the Feast of [[Epiphany (holiday)|Theophany]] according to their respective calendar.<ref>{{Cite web|url=https://www.peopleofar.com/2015/01/07/armenians-celebrate-christmas-january-6th/|title=Armenian Christmas on January 6th|last=Reply|first=hairabed|date=2015-01-07|website=PeopleOfAr|language=en-US|access-date=2018-12-21}}</ref>

=== Berbers ===
The Julian calendar is still used by the [[Berbers]] of the [[Maghreb]] in the form of the [[Berber calendar]].<ref>{{cite journal|title=The manipulation of time: Calendars and power in the Sahara|first=Clare|last=Oxby|journal=Nomadic Peoples |series=New Series|volume=2|issue=1/2|year=1998|pages=137–149|doi=10.3167/082279498782384522|jstor=43123542}}</ref>

=== Foula ===
[[Foula]] in [[Shetland]], [[Scotland]], a small settlement on a remote island of the archipelago, still celebrates festivities according to the Julian calendar.<ref>{{cite news |last1=Merritt |first1=Mike |title=Remote Foula islanders finally get to celebrate Christmas |url=https://www.thetimes.com/uk/scotland/article/remote-foula-islanders-finally-get-to-celebrate-christmas-lcjlrn56m |date=27 December 2023 |language=en}}</ref>

== See also ==
* [[Byzantine calendar]]
* [[Conversion between Julian and Gregorian calendars]]
* [[Julian day]]
* [[Julian year (astronomy)]]
* [[List of adoption dates of the Gregorian calendar per country]]
* [[Mixed-style date]]
* [[Old New Year]]
* [[Proleptic Gregorian calendar]]
* [[Proleptic Julian calendar]]
* [[Revised Julian calendar]]
* [[Roman timekeeping]]
* [[Week]]

== Citations ==
{{reflist|24em}}
==Notes==
{{Notelist|2}}

==Sources ==
{{refbegin}}
* {{cite book
| publisher = Oxford University Press
| isbn = 978-0-19-286205-1
| last = Richards
| first = E. G
| title = Mapping Time: The Calendar and its History
| url = https://archive.org/details/mappingtimecalen00rich
| url-access = registration
| date = 1998
}}
* {{cite book
| last1 = Richards | first1 = E. G.
| editor-last1 = Urban | editor-first1 = Sean E.
| editor-last2 = Seidelmann | editor-first2 = P. Kenneth
| edition = 3rd
| publisher = University Science Books
| isbn = 978-1-891389-85-6
| title = Explanatory Supplement to the Astronomical Almanac
| location = Mill Valley, Calif
| date = 2013
}}
{{refend}}

== Further reading ==
{{refbegin|30em |indent=yes}}
* [https://www.webexhibits.org//calendars/index.html Calendars through the ages] on WebExhibits
* [http://penelope.uchicago.edu/~grout/encyclopaedia_romana/calendar/romancalendar.html The Roman Calendar]
* {{cite book| first1=Bonnie |last1=Blackburn |first2=Leofranc |last2=Holford-Strevens |title=The Oxford Companion to the Year |publisher=Oxford University Press |date=2003}}
* {{cite book
|title=Le Calendrier romain: Recherches chronologiques
|date=1983
|publisher=Ottawa University Press
|last1=Brind'Amour
|first1=Pierre
}}
* {{cite web | date = 2008 | url = https://ethiopianembassy.org/ethiopian-time/ | title = Ethiopian Time | publisher = Embassy of Ethiopia | location = Washington D.C.}}
* {{cite book
|title=Caesar's Calendar: Ancient Time and the Beginnings of History
|url=https://archive.org/details/caesarscalendara0000feen
|url-access=registration
|date=2007
|publisher=University of California Press
|location=Berkeley
|last1=Feeney
|first1=Dennis|isbn=9780520251199
}}
* {{cite book
|title=The Roman Calendar from Numa to Constantine: Time, History and the Fasti
|date=2011
|publisher=Wiley
|last1=Rüpke
|first1=Jörg}}
* {{cite book
|title=Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac
|author=Nautical Almanac Offices of the United Kingdom and the United States of America
|publisher=Her Majesty's Stationery Office
|date=1961
|location=London}}
* {{cite book
|last1=Stern
|first1=Sacha
|title=Calendars in Antiquity: Empires, States and Societies
|date=2012
|publisher=Oxford University Press}}
{{refend}}

== External links ==
{{Wiktionary|evenly divisible}}
{{Commons}}
* [https://www.fourmilab.ch/documents/calendar/#juliancalendar Calendar Converter] – converts between several calendars, for example Gregorian, Julian, Mayan, Persian, Hebrew
* [https://ponomar.net/cgi-bin/menologion.cgi Orthodox Calendar]

{{Calendars}}
{{Ancient Rome topics}}
{{Time Topics}}
{{Time measurement and standards}}
{{Chronology}}
{{Authority control}}

[[Category:Julian calendar| ]]
[[Category:Eastern Orthodox liturgy]]
[[Category:Liturgical calendars]]
[[Category:Roman calendar]]
[[Category:1st-century BC establishments in the Roman Republic]]
[[Category:40s BC establishments]]