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The Common Era, also known as the Christian Era, is the name of the stretch of time from somewhere around the (miscalculated) birth of Jesus until now, and is used in writing dates by designating the year "CE" ("of the Common Era") or "BCE" ("'Before the Common Era"). While this style of dating is actually quite old, having originated in the seventeenth century, only recently have scholars and historians recognized the privileging of Christianity in the AD/BC method, and re-adopted the more neutral Common Era designations.
Anno Domini and Before Christ
The use of A.D. originally came about during the 5th century C.E., when a Christian monk called Dionysius Exiguus wanted to record and predict religious holidays such as Easter. Historians of the time had followed a variety of previous dating schemes, including the dominant Roman one of "A.U.C." ("Ab Urbe Condita" or "After the Founding of the City") or a shorthand method that simply named the reigning consuls of that year. In Exiguus' time, the dominant system was Anno Diocletiani - years since the beginning of the reign of the Emperor Diocletian. But Exiguus did not like that, since Diocletian launched one of the greatest persecutions of Christians in history.
As Christianity became increasingly dominant, it eventually began to encompass almost the whole of the literate community in Europe, with religious communities becoming the main bastions of histories during the Middle Ages. It is no surprise, then, that they began to use the same method of dating for both their religious timekeeping and secular events. All years began to be designated A.D. for the most part, with various local varieties of B.C. - including the clumsy A.V.I.D.T. ("Ante Vero Incarnationis Dominicae Tempus" or "the Time Before the Lord's True Incarnation"). Alternate local methods largely dropped away.
There are several arguments against the usage of the Common Era system. One argument is an appeal to plain honesty — that since everyone knows what the epoch is really about, it is a hollow token gesture, serving no purpose beyond political correctness, to alter the labels.
Support for the BC/AD system over BCE/CE due to it being based on the birth of Jesus is strange, considering that AD and BC are inaccurate themselves. The birthdate of Jesus was definitely not in 1 AD, but around 4 years prior[notes 1], and definitely not around December 25th. Therefore, since the BCE/CE is technically an arbitrary date, which is set due to the historical precedent of BC/AD in the Gregorian calendar, and doesn't shift, it will always remain accurate.
Another criticism of the Common Era system is that it leaves much of history in BCE dates, and for added confusion lacks a year zero.[notes 2] There have been several suggestions to pick an earlier starting point, which would enable a single system to address all of history. Joseph Scaliger proposed Julian Era dates, from 4713 BCE. Another proposal, by Cesare Emiliani, is the Holocene Era, with an epoch of 10,000 BCE. This has the advantage that the arithmetic for conversion is far more straightforward than Julian Era: 2022 CE = 12022 HE and the simple fact it is based on actual science, and not solely on an arbitrary date to appease Christians while still not "being Christian" (to convert BCE to HE, subtract the year from 10001, as 1 BCE is 10000 HE).
Astronomical year numbering avoids the problems of year zero and the question of whether to use AD or CE by using positive numbers for years from 1 CE, 0 for 1 BCE, and negative numbers before that (the Julian calendar is used before 1582); hence -1000 is 1001 BCE.
The names of the days of the week and several of the months of year (in English and other Indo-European languages) also have religious origins in the deities of antiquity. Yet there has been no substantial movement to rename these more secularly, other than the French Republican Calendar (often called the French Revolutionary Calendar), used when the French were feeling particularly secular and revolutionary, and a short-lived renaming within Turkmenistan in the early 21st century, under the dictator Saparmurat Niyazov.
- Or 6 years prior, if you believe the story in the Gospel of Luke about a Roman census taking place the same year. Not that the actual Roman censuses were anything like the ridiculous "everyone go back to the city of their birth" census in Luke.
- As for year zero, there is a year zero defined by ISO and astronomers also use a year zero for calculating purposes. In both cases year zero takes the place of 1 BC or 1 BCE so that the dates before that are shifted back. Historians never use a year zero, so people will have to deal with that. In any case, a year is an interval, not a point. This means that the historical lack of year zero seems perfectly logical at first. The year 2 AD/CE began after a year (year 1 AD/CE) had elapsed from the original point of departure. However, this is assuming that Jesus was born at the exact point between the years -1 and 1, so that the intervals start from the point of his birth. Considering that he was most likely born during the course of a particular year, that year would have to be called year 0, because neither 1 nor -1 would be applicable names for it.