AD (Anno Domini Nostri Jesu Christi) and AC (Ante Christum/BC: Before Christ)

Dionysius Exiguus (c. 470 - c. 544) introduced the use of the Christian Era (see CHRONOLOGY) according to which dates are reckoned from the Incarnation, which he assigned to 25 March, in the year 754 from the foundation of Rome (A. U. C.). By this method of computation he intended to supersede the "Era of Diocletian" previously employed, being unwilling, as he tells us, that the name of an impious persecutor should be thus kept in memory. The Era of the Incarnation, often called the Dionysian Era, was soon much used in Italy and, to some extent, a little later in Spain; during the eighth and ninth centuries it was adopted in England. Charlemagne is said to have been the first Christian ruler to employ it officially. It was not until the tenth century that it was employed in the papalchancery (Lersch, Chronologie, Freiburg, 1899, p. 233). Dionysius also gave attention to the calculation of Easter, which so greatly occupied the early Church. To this end he advocated the adoption of the Alexandrian Cycle of nineteen years, extending that of St. Cyril for a period of ninety-five years in advance. It was in this work that he adopted the Era of the Incarnation.

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