A.D.,  B.C.  &  B.C.E.

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ANNO DOMINI - In the year of our Lord. Inscription at Klagenfurt Cathedral, Austria.

How do we measure time? Whether going backwards in time (history), or forward for planning important events, it helps us to know where we are in time. Historians use time to plot the development of man, record battles, births and deaths. Scientists need to be able to measure time accurately to know where the planets are, and earth in relation to the sun. The Global Positioning System sets new demands on the micro measurement of time, to enable accurate navigation.

Most people use diaries and/or calendars to plan our daily routines and appointments. But wHere do we begin? How did we get our calendars?

THE JULIAN CALENDAR

The Julian calendar, proposed by Julius Caesar in AUC 708 (46 BC), was a reform of the Roman calendar. It took effect on 1 January AUC 709 (45 BC), by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandria.

The 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 minor modification to reduce the average length of the year from 365.25 days to 365.2425 days and thus corrected the Julian calendar's drift against the solar year. Worldwide adoption of this revised calendar, which became known as the Gregorian calendar, took place over the subsequent centuries, first in Catholic countries and subsequently in Protestant countries of the Western Christian world.

The Julian calendar is still used in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Berbers.

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 365.24219 days (the current value, which varies), which means the Julian calendar gains a day every 128 years. For any given event during the years from 1901 to 2099 inclusive, its date according to the Julian calendar is 13 days behind its corresponding Gregorian date. 

GREGORIAN CALENDAR

The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.

There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes. Second, in the years since the First Council of Nicaea in AD 325, the excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the (Northern) spring equinox was occurring well before its nominal 21 March date. This date was important to the Christian churches because it is fundamental to the calculation of the date of Easter. 

ANNO DOMINI

An Eastern-Roman monk named, Dionysius Exiguus, was the inventor of Anno Domini (AD) dating system, which is used to number the years of both the Gregorian calendar and the (Christianised) Julian calendar, introduced by Julius Caesar. Almost all churches adopted Exiguus's 'computus' for the dates of Easter. 

Dionysius used it to identify the several Easters in his Easter table, but did not use it to date any historical event. When he devised his table, 'Julian' calendar years were identified by naming the consuls who held office that year; he himself stated that the "present year" was "the consulship of Probus Junior", which he also stated was 525 years "since the incarnation of our Lord Jesus Christ". 

CE AND BCE

Alternative names for the Anno Domini era include vulgaris aerae (found 1615 in Latin), "Vulgar Era" (in English, as early as 1635), "Christian Era" (in English, in 1652), "Common Era" (in English, 1708), and "Current Era". Since 1856, the alternative abbreviations CE and BCE (sometimes written C.E. and B.C.E.) are sometimes used in place of AD and BC.

The "Common/Current Era" ("CE") terminology is often preferred by those who desire a term that does not explicitly make religious references but still uses the same estimated date of Christ's birth as the dividing point. For example, Cunningham and Starr (1998) write that "B.C.E./C.E. do not presuppose faith in Christ and hence are more appropriate for interfaith dialog than the conventional B.C./A.D." Upon its foundation, the Republic of China adopted the Minguo Era but used the Western calendar for international purposes. The translated term was 西元 (xī yuán; 'Western Era'). Later, in 1949, the People's Republic of China adopted 公元 (gōngyuán; 'Common Era') for all purposes domestic and foreign. 

BCE, along with CE, were said to have been developed so as not to be connoted purely on Christian origins. The usage of BCE is thus a show of respect to non-Christians, who don’t believe in Christ, or who do not know who or what Christ is.

YEAR ZERO

In the AD year numbering system, whether applied to the Julian or Gregorian calendars, AD 1 is immediately preceded by 1 BC, with nothing in between them (there was no year zero). There are debates as to whether a new decade, century, or millennium begins on a year ending in zero or one.

For computational reasons, astronomical year numbering and the ISO 8601 standard designate years so that AD 1 = year 1, 1 BC = year 0, 2 BC = year −1, etc.[note 3] In common usage, ancient dates are expressed in the Julian calendar, but ISO 8601 uses the Gregorian calendar and astronomers may use a variety of time scales depending on the application. Thus dates using the year 0 or negative years may require further investigation before being converted to BC or AD. 

Where the use of the term BC was actually coined by Dionysius Exiguus in 525BC (Before Christ), this was the undisputed notation only until recently, when several movements came forth that challenged the BC - AD notation. The reason why they started to question the original notation, is because it was already scientifically measured, using the most accurate technologies, that Christ was actually born sometime in the BC era (7-4 BC). Therefore, why was Christ already born when the supporters of the BC AD notation claim that Christ’s birth represents the year one of AD? This actually renders the meaning of AD (birth of Christ) obsolete.

REFERENCES

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THE INTERNATIONAL DATING SYSTEM RECORDING HISTORY: AD - ANNO DOMINI - IN THE YEAR OF OUR LORD

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