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The Babylonians probably invented the first calendar, at a time when the heavenly bodies were regarded as gods, and their cycles and phenomena as divine manifestations. The divine scribe Thoth monitored the calendar, and this task was traditionally a sacred office among all peoples.
The year was established as 365 days, and the lunar month as 29 days, by the Chinese, as early as the 14th century BC, but the earliest datable example of a lunar calendar is the Gezar calendar, from the age of Solomon, 10th century BC. The Mayan calendar is difficult to date, but is probably the most highly evolved and sophisticated of any early civilisation. The length of a tropical or solar year is now established as 365.24219647 days, which is how long the earth takes to orbit the sun. The Gregorian calendar, now used throughout the world, has a calendar year of 365.2425 days; a discrepancy with the solar year of minute each year or 1 day every 3,300 years, which is almost as accurate as it is possible to practically devize. This calendar also united the dating of religious festivals based on the Moon with seasonal activities based on the Sun. Most early calendars were collections of months, which were not integrated with the solar year and so years tended to vary in length. Often a year
would be only ten months long, followed by an interim period of "empty time" lasting two or three months, disregarded by the calendar as it occurred while work in the fields and gardens ceased.


There were more nefasti, or public holidays, in the Roman Calendar than in any other, and by the time Claudius had added a further six there were 159 holidays in their 365-day year. Marcus Aurelius returned the business year to 230 days, but this was short-lived. In the manuscript of the 354 AD calendar of Philocalcus there were over 200 public holidays. The Roman Calendar evolved from a lunar year of 10 months totalling 304 days running from March to December, the remaining period being a "dark time", outside the calendar. This calendar was introduced by Romulus, according to Ovid, with Numa Pompilius, the second of the kings, traditionally ascribed with adding January (the month of the Janus festival) and February (month of purification) in 700 BC. 
The Roman Republican Calendar was introduced around 304 BC, incorporating all the indigenous Roman festivals. This calendar had 365 days, each month containing 29 or 30 days, with an additional 27-28 day month following February 23 to make up the total required when necessary. The date of the New Year was fixed as 15 March in 222 BC until 153 BC when it was transferred to the Kalends of January, where it remains.
The month was divided according to the Kalends (Kalendae), the first, the Nones (Nonae) the fifth or seventh, and the Ides (from iduare, to divide), a method which was retained after the Julian reform. On the first of each month it was the duty of the priests to announce whether the the Nones would fall on the fifth or seventh day, and on which days feasts were to be observed. This calendar fell into chaos following political tinkerings by pontifices, who adjusted the lengths of months for their own electoral ends.
Julius Caesar attempted a major reform in 44 BC, by which time the calendar had become three months behind the seasons, and imported the Alexandrian astronomer Sosigenes, who proposed to substitute an Egyptian solar year of 365 years. He made two intercalations, adding 23 days to follow February 23, as was normal, and inserting two months totalling 67 days between the end of November and the beginning of December so that, in Rome at least, the beginning of March 44 BC (45 BC astronomically) in the Roman Republican calendar became 1 January of the 1st year in the new Julian Calendar instead of year 709 in the old Roman calendar (commemorating the foundation of Rome). This had the advantage that the new year
started shortly after the Winter Solstice. An extra day was added between February 23 and 24, or six days before the Kalends of March, every fourth year, and this was known as a punctum temporis (point of time) by Caesar. Those years in which 23 February occurred as two days running were called 'bisextile' years.
These revisions failed because pontifices misunderstood the calendar edict and added leap years every three years instead of four. This error persisted undetected for 35 years when Augustus corrected it by adding extra days between 8 BC and 4 AD. To improve numbering, Martius (March), May (Maius) and Quintilis (renamed Julius in honour of Julius Caesar after his assassination in 44 BC) were left with 31 days each, October was reduced by one day to 30 days, February was increased to 29 days (30 in a leap year), and all other 29-day months became 30-day, creating a neat alternation between 30 and 31 days throughout the year:
Martius 31         September 31
Aprilis 30          October 30
Maius 31           November 31
Junius 30          December 30
Julius 31          Januarius 31
Sextilis 30        Februarius 29/30

In 8 BC the Senate changed the name of the sixth month from Sextilis to Augustus, in honour of the then emperor, Augustus Caesar. Unfortunately they chose a month of only 30 days and not wishing his name to go to one of the shorter months, while the late Julius Caesar had a 31-day month named after him, rather than pick another month they then transposed the lengths of the months from Augustus to December; this produced an extra day, which they cancelled out by changing Februarius to 28/29 days, creating the hotch-potch of month lengths that continues to this day:
Martius 31        September 30
Aprilis 30         October 31
Maius 31          November 30
Junius 30         December 31
Julius 31          Januarius 31
Augustus 31      Februarius 28/29

Gradually the adoption of Januarius as the first month became universal, giving the pattern familiar in Christian countries today.


By 1545, when the Council of Trent was held, it had become apparent that the length of the year was not 365.25 days, but 365.24219647, as Ptolemy had been aware. Scientists such as Beda (8th century), and astronomers including Roger Bacon (13th century), Campanus and Regiomontanus (15th century) had pointed out that the calendar was slow of the true astronomical dates. This meant that Lent and Easter were being celebrated at inappropriate times. It was important that Easter was celebrated on the Sunday following the first Full Moon following the Spring Equinox.
The accumulated error of the Julian Calendar (1 days in 200 years) had by then reached 10 days and Pope Gregory XIII acted to overcome this by proclaiming in his Bull 'Inter Gravissimas' dated 24 February 1582, that 5 October that year would become 15 October, in order that 21 March would again fall on the true Spring Equinox. He also decreed that to avoid such mistakes in the future, every century year would not be a leap year unless it was exactly divisible by 400, adopting a reform that had been proposed by a medical doctor, Lilius (Aloigi Giglio, born 1552 in Spain), and brought to the Pope by his brother, ten years after the death of Lilius. Consequently, it would be 3,300 years before the calendar would be in error by one day. Had it been ruled that centuries were leap years only if divisible by 500 rather than 400 the error would have been considerably less.
Other countries, or parts of a country, adopted the Gregorian calendar at different times. Most Catholic countries adopted it soon after its institution, but Protestant countries were rather slower. The Catholic parts of Switzerland and Germany seem to have changed over between 1583 and 1584, but the Protestant areas held firm until 1700/1701. Different authorities give different dates for the changeover of some countries, such as Belgium, Bulgaria, the Netherlands and China.
When England adopted the Gregorian Calendar in 1752, 3 September became 14 September, leading to angry demonstrations from taxpayers who shouted with good reason, "Give us back our eleven days!". The USA introduced the Gregorian Calendar at the same date. France, which had adopted the Gregorian Calendar in 1582, abandoned it from 1793 to 1805 in favour of the Revolutionary Calendar.
To add to this confusion, prior to the introduction of the Gregorian calendar the new year in England was not regarded as starting until the Spring Equinox around 23 March. Therefore a date we would write as 23 February 1737 might be written as 23 February 1736, or it might not. Historians sometimes convert dates before 3 September 1752 into Gregorian, or New Style format, and dates after into Julian, or Old Style format.

General dates of changeover (NS)
Italy, Spain, Portugal 15 October 1582
Poland (except some areas to 21 March 1915) 15 October 1582
France 20 December 1582
Holland 25 December 1582
Holland (Zeeland & Holland provinces) 12 January 1583
Germany 15 October-22 December 1583
Switzerland (Catholic)(erratic to 1811) 22 January 1584
Hildesheim, Germany 1 January 1631
Denmark and Norway 1 March 1700
Switzerland (Protestant) 12 January 1701
England, the Empire and USA 14 September 1752
Sweden and Finland (erratic) 18 February-1 March 1753
Japan 1 January 1873
Egypt 1 January 1875
Albania December 1912
USSR (most of) 1 February 1914
Lithuania 1 January 1915
Bulgaria 1 April 1916
Estonia 1 April 1918
Russia 14 February 1918
Serbia, Croatia, Bosnia (Yugoslavia) 2 March 1920
Romania 1 April 1920
Bulgaria 1 October 1920
Eastern Orthodox Church (Russia and the Balkans) 1 May 1923
Greece (erratic) 29 July 1916-23 March 1924
Turkey 1 January 1927
China (some sources give 12 January 1912) 1 January 1929

Time Conversion Table (OS to NS)

In the Gregorian calendar the year 0 AD becomes 1 AD, so a correction of 1 year has to be made for years BC. Centenary years not divisible by 400 were leap years in the Julian Calendar, but are not leap years in the Gregorian calendar.
Dates are converted as follows:
5.3. 500 BC - 3.3. 300 BC                             - 5 days
4.3. 300 BC - 2.3. 300 BC                             - 4 days
3.3. 200 BC - 1.3. 100 BC                             - 3 days
2.3. 100 BC - AD 29.2. 100                           - 2 days
AD 1.3. 100 - AD 29.2. 200                           - 1 days
AD 1.3. 200 - AD 29.2. 300                              0 days
AD 1.3. 300 - AD 29.2. 500                           + 1 days
AD 1.3. 500 - AD 29.2. 600                           + 2 days
AD 1.3. 600 - AD 29.2. 700                           + 3 days
AD 1.3. 700 - AD 29.2. 900                           + 4 days
AD 1.3. 900 - AD 29.2.1000                          + 5 days
AD 1.3.1000 - AD 29.2.1100                          + 6 days
AD 1.3.1100 - AD 29.2.1300                          + 7 days
AD 1.3.1300 - AD 29.2.1400                          + 8 days
AD 1.3.1400 - AD 29.2.1500                          + 9 days
AD 1.3.1500 - AD 29.2.1700                          + 10 days
AD 1.3.1700 - AD 29.2.1800                          + 11 days
AD 1.3.1800 - AD 29.2.1900                          + 12 days
AD 1.3.1900 - AD 29.2.2100                          + 13 days

Copyright 2002 [Laurence Upton]. All rights reserved.
Last updated: May 06, 2003