<Li> There were confused summations of emperors' regnal years . </Li> <P> It is not known how Dionysius established the year of Jesus's birth . Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", and hence subtracted thirty years from that date, or that Dionysius counted back 532 years from the first year of his new table . It is convenient to initiate a calendar not from the very day of an event but from the beginning of a cycle which occurs in close proximity . For example, the Islamic calendar begins not from the date of the Hijra, but rather weeks prior, on the first occurrence of the month of Muharram (corresponding to 16 July 622). </P> <P> It has also been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world . At the time, it was believed by some that the Resurrection and end of the world would occur 500 years after the birth of Jesus . The old Anno Mundi calendar theoretically commenced with the creation of the world based on information in the Old Testament . It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 (or 5500 years after the world was created) with the year 6000 of the Anno Mundi calendar marking the end of the world . Anno Mundi 6000 (approximately AD 500) was thus equated with the resurrection and the end of the world but this date had already passed in the time of Dionysius . </P> <P> 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 . In this same history, he also used another Latin term, ante vero incarnationis dominicae tempus anno sexagesimo ("in fact in the 60th year before the time of the Lord's incarnation"), equivalent to the English "before Christ", to identify years before the first year of this era . 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). </P>

Why did we start using ad and bc