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The "Christian calendar" is the term traditionally used to designate the calendar commonly in use, although it originated in pre-Christian Rome. This calendar is used by the United States, and most countries in the world. This section presents historical information about the Christian calendar. The Christian calendar has years of 365 or 366 days. It is divided into 12 months that have no relationship to the motion of the moon. In parallel with this system, the concept of weeks groups the days in sets of 7. Two main versions of the Christian calendar have existed in recent times: The Julian calendar and the Gregorian calendar. The difference between them lies in the way they approximate the length of the tropical year and their rules for calculating Easter.

The Julian calendar was a reform of the Roman calendar which was introduced by Julius Caesar in 46 BC and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years (including years divisible by 100, unlike the modern Gregorian calendar.) Hence the Julian year is on average 365.25 days long.

The Julian calendar remained in use into the 20th century in some countries as a national calendar, but it has generally been replaced by the modern Gregorian calendar. It is still used by the Berber people of North Africa and by many national Orthodox churches. Orthodox Churches no longer using the Julian calendar typically use the Revised Julian calendar rather than the Gregorian calendar.

The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar.

The Julian calendar, introduced by Julius Caesar in -45, was a solar calendar with months of fixed lengths.

Motivation

The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 days after the first 23 or 24 days of February, the last five days of February becoming 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.

According to the later writers Censorinus and Macrobius, the ideal intercalary cycle consisted of ordinary years of 355 days alternating with intercalary years, alternately 377 and 378 days long. On this system, the average Roman year would have had 366¼ days over four years, giving it an average drift of one day per year relative to any solstice or equinox. Macrobius describes a further refinement wherein, for 8 years out of 24, there were only three intercalary years, each of 377 days. This refinement averages the length of the year to 365¼ days over 24 years. In practice, intercalations did not occur schematically according to these ideal systems, but were determined by the pontifices. So far as can be determined from the historical evidence, they were much less regular than these ideal schemes suggest. They usually occurred every second or third year, but were sometimes omitted for much longer, and occasionally occurred in two consecutive years.

If managed correctly this system allowed the Roman year, on average, to stay roughly aligned to a tropical year. However, if too many intercalations were omitted, as happened after the Second Punic War and during the Civil Wars, the calendar would drift rapidly out of alignment with the tropical year. Moreover, since intercalations were often determined quite late, the average Roman citizen often did not know the date, particularly if he were some distance from the city. For these reasons, the last years of the pre-Julian calendar were later known as years of confusion. The problems became particularly acute during the years of Julius Caesar's pontificate before the reform, 63 BC to 46 BC, when there were only five intercalary months, whereas there should have been eight, and none at all during the five Roman years before 46 BC.

The reform was intended to correct this problem permanently, by creating a calendar  that remained aligned to the sun without any human intervention.

Julian reform

The first step of the reform was to realign the start of the calendar year (1 January) to the tropical year by making 46 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 African campaign in late Quintilis (July), he added 67 (=22+23+22) more days by inserting two extraordinary intercalary months between November and December. 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 "Unodecember" and "Duodecember". Their individual lengths are unknown, as is the position of the Nones and the Ides within them. Because 46 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 first year of operation of the new calendar was 45 BC.

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 Ianuarius, Sextilis (Augustus) and December, and one extra day was added to Aprilis, Iunius, September and November, setting the lengths of the months to the values they still hold today:

Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established Roman fasti (days prescribed for certain events) relative to the start of the month. However, since Roman dates after the Ides of the month counted down towards the start of the next month, the extra days had the effect of raising the initial value of the count of the day after the Ides. Romans of the time born after the Ides of a month responded differently to the effect of this change on their birthdays. Mark Antony kept his birthday on the 14th day of Ianuarius, which changed its date from a.d. XVII Kal. Feb. to a.d. XIX Kal. Feb., a date that had previously not existed. Livia kept the date of her birthday unchanged at a.d. III Kal. Feb., which moved it from the 28th to the 30th day of Ianuarius, a day that had previously not existed. Augustus kept his on the 23rd day of September, but both the old date (a.d. VIII Kal. Oct.) and the new (a.d. IX Kal. Oct.) were celebrated in some places.

The old intercalary month was abolished. The new leap day was originally inserted following February 24, a.d. VI Kal. Mar. by Roman reckoning, since this is the point at which intercalary months were inserted in the pre-Julian calendar. It was considered as extending that day to 48 hours, so it was dated as "a.d. VI bis Kal. Mar.", and is called the bissextile day. During the late Middle Ages when days in the month came to be numbered in consecutive day order, the Leap Day was considered to be the last day in February in leap years, i.e. February 29.

Leap year error

Within two years of the start of these reforms Julius Caesar was dead, assassinated on the steps of the Senate in Rome on 15 March 44BC (710AUC). To honour him, the Senate decreed that the seventh month, called Quintilis, should be renamed Julius. But Caesar was gone before he could see how his reforms were working and before the first leap year (not a term the Romans used) was due in 41BC (713AUC). And perhaps that is why, with no-one to correct them, the priests or Pontifices who were supposed to keep track of the calendar misunderstood Caesar's decree and added the extra day to February every three years instead of every four. The Romans counted inclusively so to them every fourth year meant 1 2 3 4 5 6 7 8 9 10. As a result the first leap year was 42BC instead of 41 and they carried on with this error every three years until 9BC. It is strange that his instructions should have been misunderstood - he had been elected to the college of Pontifices  himself two decades before his murder.

The mistake went unnoticed until around 9BC when Julius's successor as Emperor - his great-nephew usually known now as Augustus Caesar - called for further changes. The Pontifices's error had gone unchecked for 36 years meaning that 12 extra days had been added instead of 9. To correct this Augustus ordered a halt in the leap years until the Earth had caught up with the calendar. In 8 BC the Senate also decided to honour the Emperor by renaming the month Sextilis as Augustus. In the past it was believed that the month lengths were then changed to their present form. Under this view, Sextilis had 30 days and the argument is that Augustus could not have fewer days than Julius so it was lengthened to 31. That would have lft three 31-day months in succession as September was also believed to be 31 days long since Julius Caesar's reforms. So September and November were shortened to 30 days and October and December lengthened to 31. That left one too manydays in theyear so February was returned to its traditional length of 28 days to compensate. But nowadays this theory has fallen out of favour and it is though that the month lengths had already been set in their modern form by Julius Caesar so August was a 31 day month already and no further change was needed when Augustus was honoured. It was quite a small honour; after his death in 14AD the Senate pronounced him a God. But we can say with certainty that from 8BC there were the twelve familiar months with the same number of days they have now.

JANUARIUS, FEBRUARIUS, MARTIUS, APRILIS, MAIUS, JUNIUS, JULIUS, AUGUSTUS, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER.

or as they are more correctly written as Romans did not have a 'J' or a 'U' in their alphabet

IANVARIVS, FEBRVARIVS, MARTIVS, APRILIS, MAIVS, IVNIVS, IVLIVS, AVGVSTVS, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER.

To get the calendar back on track, Augustus decreed that the first leap year after his reforms would be 8AD and they would occur every four years thereafter. And so they did. The calendar was to remain unchanged for more than one and half millennia and apart from a minor adjustment which began to be introduced in 1582, it remains substantially the same today.

Leap

There is one more little complexity. What to do about February? It has a variable number of days. Today we cope with a leap year by adding an extra day at the end of the month - February 29th. The Romans did it differently. Julius added the extra day after the 23rd day of the month - the same place that the bi-annual extra month Mercedonius had been inserted. So the leap day was like a very short Merecedonius and familiar to Romans of the time. But if you are naming days by counting backwards from the next event, adding an extra day affects the names of the previous days as well. In February the ides fell on 13th day of the month. The next day was 16 days before the 1st of March in a normal 28 day February. If an extra day was added, then the day after the ides of February would be 17 days before the 1st of March. But that did not happen. Because - like Mercedonius - the extra day was not part of February. The counting missed a beat, like this.

In other words the extra day did not alter the calendar until after the 24th day - a time when it was always hard to tell the date in Rome. Because the 24th was normally called the sexto kalendae the extra day was called the bis-sexto-kalendae  meaning the double, twice or again sixth day before the start of March. Sometimes a Leap Year is referred to as a bissextile for that reason. This odd way of doing things has led some scholars into saying that the extra day was added after the 24th, when in fact logic - and history - determines that it was added after the 23rd. This name and system also implies that February did indeed have 28 and 29 days after Julius Caesar's reforms.

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 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 Russia in AD 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.

During the Middle Ages 1 January retained the name New Year's Day (or an equivalent name) in all Western European countries (affiliated with the Roman Catholic Church), since the medieval calendar continued to display the months from January to December (in twelve columns containing 28 to 31 days each), just as the Romans had. However, most of those countries began their numbered year on 25 December (the Nativity of Jesus), 25 March (the Incarnation of Jesus), or even Easter, as in France  (see the Liturgical year article for more details).

In England  before 1752, 1 January was celebrated as the New Year festival, but the "year starting 25th March was called the Civil or Legal Year, although the phrase Old Style was more commonly used." To reduce misunderstandings on the date, it was not uncommon in parish registers for a new year heading after 24 March for example 1661 had another heading at the end of the following December indicating "1661/62". This was to explain to the reader that the year was 1661 Old Style and 1662 New Style.

Most Western European countries shifted the first day of their numbered year to 1 January while they were still using the Julian calendar, before they adopted the Gregorian calendar, many during the sixteenth century. The following table shows the years in which various countries adopted 1 January as the start of the year. Eastern European countries, with populations showing allegiance to the Orthodox Church, began the year on 1 September from about 988. 

Note that as a consequence of change of New Year, 1 January 1751 to 24 March 1751 were non-existent dates in England.

From Julian to Gregorian

The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar. Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year. As a result, the calculated date of Easter gradually moved out of phase with the moon. While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance at the time of the Julian reform. However, it accumulated significantly over time: the Julian calendar gained a day about every 134 years. By 1582, it was ten days out of alignment.

 The Gregorian Calendar was soon adopted by most Catholic countries (e.g. Spain, Portugal, Poland, most of Italy). Protestant countries followed later, and the countries of Eastern Europe even later. In the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. For 12 years from 1700 Sweden used a modified Julian Calendar, and adopted the Gregorian calendar in 1753, but Russia remained on the Julian calendar until 1917, after the Russian Revolution (which is thus called the 'October Revolution' though it occurred in Gregorian November), while Greece  continued to use it until 1923. During this time the Julian calendar continued to diverge from the Gregorian. In 1700 the difference became 11 days; in 1800, 12; and in 1900, 13, where it will stay till 2100.

Although all Eastern Orthodox countries (most of them in Eastern or Southeastern Europe) had adopted the Gregorian calendar by 1927, their national churches had not. A revised Julian calendar was proposed during 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 Orthodox churches refused to accept the lunar part, so almost all Orthodox churches continue to celebrate Easter according to the Julian calendar (the Finnish Orthodox Church uses the Gregorian Easter).

The solar part of the revised Julian calendar was accepted by only some Orthodox churches. Those that did accept it, with hope for improved dialogue and negotiations with the Western denominations, were the Ecumenical Patriarchate of Constantinople, the Patriarchates of Alexandria, Antioch, the Orthodox Churches of Greece, Cyprus, Romania, Poland, Bulgaria (the last in 1963), and the Orthodox Church in America (although some OCA parishes are permitted to use the Julian calendar). Thus these churches celebrate the Nativity on the same day that Western Christians do, 25 December Gregorian until 2800. The Orthodox Churches of Jerusalem, Russia, Macedonia, Serbia, Georgia, Ukraine, and the Greek Old Calendarists continue to use the Julian calendar for their fixed dates, thus they celebrate the Nativity on 25 December Julian (which is 7 January Gregorian until 2100).

In Northern Africa, the Julian calendar (the Berber calendar) is still in use for agricultural purposes, and is called فلاحي fellāhī "peasant" or sاﻋﺠﻤﻲ acjamī "not Arabic". The first of yennayer currently corresponds to January 14 and will do until 2100.

Converting pre-Julian dates

The fact that we use the same month names as the Romans encourages us to assume that a Roman date occurred on the same Julian date as its modern equivalent. This assumption is not correct. Even early Julian dates, before the leap year cycle was stabilised, are not quite what they appear to be. For example, it is well known that Julius Caesar was assassinated on the Ides of March in 44 BC, and this is usually converted to 15 March 44 BC. While he was indeed assassinated on the 15th day of the Roman month Martius, the equivalent date on the modern Julian calendar is probably 14 March 44 BC.

Finding the exact Julian equivalent of a pre-Julian date can be very hard. Since we have an essentially complete list of the consuls, it is not difficult to find the Julian year that generally corresponds to a pre-Julian year. However, our sources very rarely tell us which years were regular, which were intercalary, and how long an intercalary year was. Nevertheless, we do know that the pre-Julian calendar could be substantially out of alignment with the Julian calendar. Two precise astronomical synchronisms given by Livy show that in 168 BC the two calendars were misaligned by more than 2 months, and in 190 BC they were 4 months out of alignment.

We have a number of other clues to help us reconstruct the Julian equivalent of pre-Julian dates. First, we know the precise Julian date for the start of the Julian calendar (although there is some uncertainty even about that), and we have detailed sources for the previous decade or so, mostly in the letters and speeches of Cicero. Combining these with what we know about how the calendar worked, especially the nundinal cycle, we can accurately convert Roman dates after 58 BC relative to the start of the Julian calendar. Also, the histories of Livy give us exact Roman dates for two eclipses in 190 BC and 168 BC, and we have a few loose synchronisms to dates in other calendars which help to give rough (and sometimes exact) solutions for the intervening period. Before 190 BC the alignment between the Roman and Julian years is determined by clues such as the dates of harvests mentioned in the sources.

Combining these sources of data, we are able to estimate approximate Julian equivalents of Roman dates back to the start of the First Punic War in 264 BC. However, while we have enough data to make such reconstructions, the number of years before 45 BC for which we can convert pre-Julian Roman dates to Julian dates with certainty is very small, and several reconstructions of the pre-Julian calendar are possible.