Dionysius was commissioned to produce a new set of Easter tables. The prevailing year-counting system for the tables was based on the start of Emperor Diocletian’s reign in 284, which Dionysius felt was unsuitable, since Diocletian had persecuted Christians. The irony of using that name and date as the basis for a Christian system had arisen almost by accident. Egypt had adopted this way of year-numbering, and since the mathematicians and astronomers in Alexandria were the ones who knew how to calculate the Easter dates to the required degree of precision, their tables became the touchstone for the rest of the Church.
Dionysius decided to change the ‘zero’ year to what he believed to be Jesus’s birth year. He used this in his Easter tables to date the current year as 525, and used the words ‘anni domini nostri jesu christi’ when numbering the table rows.
Use of ‘anno domini’ expanded in the early eighth century when The Venerable Bede used Dionysius’s year-numbering system, and the phrase ‘anno dominicae incarnationis’, in his book Ecclesiastical History of the English People. By the end of the century, Charlemagne had adopted the Anno Domini system for official government documents throughout his empire, and by the 14th century, the Catholic Church had largely integrated it into papal administration.
The change to ‘AD’ began to happen after Gutenberg invented movable-type printing in the 1450s. Spelling out long phrases like ‘Anno Domini’ repeatedly on a page used up valuable space, and slowed down the printing process. By the 16th century, printed English books increasingly show the ‘A.D.’ abbreviation next to year numbers.