Calendars & the Meaning of Anno Domini (A.D.)

Abbreviation for Anno Domini - Latin for The Year Of Our Lord - used in the Gregorian Calendar to refer to the current era. A date such as 1945 A.D. literally means 'the 1945th year of our lord', the lord in question being Jesus Christ, providing a religious context and clearly distinguishing the time from an earlier era, where B.C is used instead. The use of A.D. was popularised by Bede.
Modern historical research suggests the current A.D. date is actually wrong, as Jesus was born 4-7 years earlier than the year 1 date the Gregorian Calendar works from. However, in the modern age the actual meaning of A.D. is widely forgotten or misunderstood and the term simply signifies a different era from BC.  There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in about AD 525, but was not widely used until after AD 800.

The Anglo-Saxon historian the Venerable Bede, who was familiar with the work of Dionysius, used Anno Domini dating in his Ecclesiastical History of the English People, finished in 731. In this same history he also used another Latin term, "ante vero incarnationis dominicae tempus" ("the time before the Lord's true incarnation"), equivalent to the English "before Christ", to identify years before the first year of this era,[14] thus establishing the standard of not using a year zero,[15] even though he used zero in his computus. 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).[3]:881

On the continent of Europe, Anno Domini was introduced as the era of choice of the Carolingian Renaissance by Alcuin. Its endorsement by Emperor Charlemagne and his successors popularizing the usage of the epoch and spreading it throughout the Carolingian Empire ultimately lies at the core of the system's prevalence. 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.[16] 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.

The Gregorian calendar, and the year numbering system associated with it, is the calendar system with the most widespread use in the world today.  In the 16th century, Pope Gregory XIII (1502-1585) decided to resolve the problem of the "out-of-kilter" calendar once and for all. Pope Gregory XIII engaged several highly-regarded astronomers including the noted Jesuit Christopher Clavius (1538-1612) to devise a new calendar to overcome the deficiencies of the Julian Calendar. On 24 February 1582, he signed a Papal bull known as Inter gravissimas authorising the new Gregorian Calendar