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Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years.
Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e., the Annunciation on March 25" (Annunciation style).Although Anno Domini was in widespread use by the 9th century, the term "Before Christ" (or its equivalent) did not become common until much later.Bede the Venerable used the expression "anno igitur ante incarnationem Dominicam" (so in the year before the Incarnation of the Lord) twice.According to the Catholic Encyclopedia, popes continued to date documents according to regnal years for some time, but usage of AD gradually became more common in Roman Catholic countries from the 11th to the 14th centuries.Eastern Orthodox countries only began to adopt AD instead of the Byzantine calendar in 1700 when Russia did so, with others adopting it in the 19th and 20th centuries.When the reckoning from Jesus' incarnation began replacing the previous dating systems in western Europe, various people chose different Christian feast days to begin the year: Christmas, Annunciation, or Easter.
Thus, depending on the time and place, the year number changed on different days in the year, which created slightly different styles in chronology: but the date is estimated through two different approaches - one by analyzing references to known historical events mentioned in the Nativity accounts in the Gospels of Luke and Matthew, and the second by working backwards from the estimation of the start of the ministry of Jesus.
"Anno an xpi nativitate" (in the year before the birth of Christ) is found in 1474 in a work by a German monk.
In 1627, the French Jesuit theologian Denis Pétau (Dionysius Petavius in Latin), with his work De doctrina temporum, popularized the usage ante Christum (Latin for "Before Christ") to mark years prior to AD.
but this date had already passed in the time of Dionysius.
The Anglo-Saxon historian the Venerable Bede, who was familiar with the work of Dionysius Exiguus, used Anno Domini dating in his Ecclesiastical History of the English People, completed in 731.
However, this would mean that the approximate 33 years commonly associated with the life of Jesus would not be included in either of the BC and the AD time scales.