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A calendar defines, among many other things, the length of each year. Calendars with different year lengths must use different numbering systems.
However within a single calendar, it is possible to have several year numbering systems. This occurs for both the Gregorian calendar currently in common use and also the Julian calendar which preceded it.
Calendars with identical year lengths can share a numbering system, as has been proposed for several proposed reformed calendars based on Gregorian years.
The average year length within the Gregorian calendar is almost exactly one solar year, and continuing corrections will keep this true. There are several systems of year numbering, and several different names for the most common system.
- The original numbering system is still the most common, and is based on the supposed year of the birth of Jesus, although it is now believed that he was born four to six years earlier than traditionally supposed.
- AD stands for Anno Domini, Latin for In the year of the Lord. The letters AD were originally placed before the year number (eg AD 1960), but now are increasing placed after the year number (eg 1960 AD).
- BC stands for Before Christ marking the period before the birth year of Christ. The letters BC are placed after the year number (eg 10 BC). The years are numbered in reverse order, 2 BC preceding 1 BC.
- There is no year 0 in this numbering system and the year AD 1 immediately follows 1 BC.
- ACN and aCn stand for Ante Christum Natum, and are identical in meaning to BC whose adoption they preceded.
- CE stands for Common Era and nowadays is being accepted as a replacement of AD. It and BCE are preferred by those, Christian or otherwise, who think the link between the standard calendar and a particular religion is inappropriate. The letters CE are placed after the year number (eg 1960 CE).
- BCE stands for Before Common Era and is identical to BC. The letters BCE are placed after the year number (eg 10 BCE). The numbering is again identical to BC.
- BCE has also been claimed to stand for Before Christian Era, but this is generally regarded as a religiously motivated attempt to defeat the intention of the use of CE and BCE.
- NS stands for New Style and identifies Gregorian dates for events that took place while the Julian calendar was in use, which continued in some places into the 20th century. See Julian calendar below, also Proleptic Gregorian Calendar.
- Astronomical year numbering is the same as AD for positive year numbers, but includes a year zero immediately preceding AD 1 to simplify arithmetic calculations, so there is a year difference for years BC.
- BP stands for Before Present (i.e., before 1950), and is used for rounded figures where the roughly 2000 years difference between BC and BP is not significant, eg 50,000 BP.
See Anno Domini for a discussion of the arguments for and against various terms.
The Julian calendar was in use from 45 BC to AD 1582, and in many parts of the world until AD 1752, Russia until AD 1918, Greece until AD 1924 and Turkey until AD 1926.
It has a very slightly longer average year length than the Gregorian calendar, and therefore falls slowly out of step with the solar year. It is still used by Orthodox Churches for reckoning the date of easter, but no longer for year numbering.
- AUC stands for Ab Urbe Condita, Latin for from the foundation of the city (meaning Rome), and was the original Julian numbering system. 1 AUC is 753 BC.
- Other numbering systems were based on the reign of the emperor of the time. That of Diocletan survived for some years after his reign.
- OS stands for Old Style and marks a Julian date, as opposed to NS for New Style which marks a Gregorian date.
- AD, BC and aCn are authentic for Julian OS dates, and their synonyms will also be encountered. This system was first proposed in AD 525 and quickly spread. The meanings are the same as for similarly-marked Gregorian year numbers.
In the period in which the Julian calendar was in official use, the similarly-marked Julian and Gregorian year numbers for a particular date will either be equal or the Julian year number will be one less than the Gregorian (and of course the day of the month and perhaps the month will differ, but the day of the week will be the same).
Julian dates will also sometimes be encountered for events outside the period of the calendar's use, see Proleptic Julian calendar.
Within this period, there are two reasons for the difference. Firstly, as Gregorian dates were set to be identical to Julian in 45 BC, the differing year length meant that by AD 1582 a ten day difference had accumulated. Secondly and more significantly, while the Julian year originally started on January 1, early in church history was moved to March 25, the supposed date of the Resurrection, where it remained until the introduction of the Gregorian calendar. This is extremely confusing, for example March 30, AD 501 is almost a year before March 20, AD 501 in this system. This led to the use of separate ecclesiastical and legal year ends, the legal year ending on December 31 preceding the ecclesiastical end of the same year. Sometimes these two will be seen combined, for example Francis Bacon's birth-date could be given as January 22nd, 1560-61 (OS). This style of year also gives a clue that a Julian date is probably intended.
- Dates of adoption of the Gregorian calendar
- Specification of the Julian calendar
- Complications of the beginnings of years
- Julian-Gregorian conversion summary
Proposed reformed calendars
New calendar proposals based on the Gregorian year length, such as the World calendar, the International Fixed Calendar and the Positivist calendar, are compatible with Gregorian numbering, or could use their own schemes.
The Islamic calendar is a lunar calendar and has a normal year length of 354 days, significantly shorter than the solar year, so there is no simple conversion between Gregorian and Islamic year numbers. Moreover, the end of each month of the Islamic calendar depends on local observations, so different countries can and do follow different calendars.
- AH stands for Anno Hegirae, Latin for in the year of the Hijra. The year AH 1 was set in AH 17 to the year during which Muhammad emigrated to the city of Medina.
- BH refers to years before AH 1. There is no year zero.
The 1st day of Muharram (the 1st month) AH 1 was the 16th of July AD 622 in the Julian calendar. The 1st of January 2000 in the Gregorian calendar corresponded to the 24th day of Ramadan (the 9th month) AH 1420 in the most widely accepted Islamic calendar. See Islamic calendar.
- AM stands for Anno Mundi meaning in the year of the world. It is placed before the number.
The Hebrew year begins in September or October of the Gregorian year, with AM 1 beginning in October, 3761 BC. Allowance must also be made for the absence of a year 0 from the normal Gregorian numbering.
French Revolutionary Calendar
The French Revolutionary Calendar was a solar calendar in force in France for only 13 years 1793-1806 and again briefly in 1871. Years were numbered with the roman numerals I-XIV, each year commencing on the Northern Autumn equinox, with year I starting September 22, AD 1792.
Summaries still to be prepared.
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