When the AD calendar system was created, by whom, and for what purpose.
The beginnings of the AD calendar system can be credited to Dionysius Exiguus. He was a sixth-century Scythian monk who lived in what is now known as Romania.
Ironically, his intent was not to create a new time-system, rather, it was expressly designed to allow all Churches throughout Christendom to celebrate Easter on the same day. Churches celebrating Easter on different days existed for centuries and was considered a major problem during his time.
In Dionysius’ description of a new calendar, he provided a graphic table, much similar to an Excel spreadsheet, with different reference systems to calculate Easter. Dionysius main concern was to fix the Easter rite correct for every year because the previous table was almost complete. The lunar cycle was of the utmost importance for calculating Easter and had to be correlated with the Julian calendar. In order to accomplish this, he had a 7-point system. Two of the more important ones to dating the birth of Christ were the indictions and the then accepted 19-year lunar cycle. The indictions were a 15-year cyclical taxation system first initiated by Julius Caesar in 48 B.C.1
One of the intents of Dionysius’ dating system was to totally eliminate the name of the Emperor Diocletian in any reference to a time chart. The AD first meant “Anni Diocletiani” which related to the beginning of his reign at 284 AD. Diocletian’s laws, persecutions and punishments against the Christian community were severe. So great was his persecution that Dionysius did not believe it was right to associate the acronym AD with his name and changed it. The following is found in his work, Liber de Paschate Praefatio.
Verily the holy Cyril began the first cycle from the 143rd year of Diocletian and ended in the 247th year, rather than commence by the leader in the 248th year from the Tyrant, we did not wish to include in our circle the memory of an impious and persecutor, but we chose above to mark the time from the year of the incarnation of our Lord Jesus Christ.2
The table devised by Dionysius began at 513 AD which was 229 years after the start reign of Diocletian. He marked the first 229 years as “Anno Diocletiani.” Then at 532 AD he changed the acronym to mean “Anni Domini nostri Jesu Christi.” His table ends at 626 AD with no reference to the reign of Diocletian.
It is not an easy read, as the variables he used are not contemporary ones.
The best place to start reading and understanding the Dionysius English text from especially a mathematical perspective is Michael Decker’s article, Nineteen Year Cycle of Dionysius. It is not the prettiest web page but full of important information.
For more information
- There is an excellent article along with an analysis found at Wikipedia on Dionysius_Exiguus.
- The Venerable Bede brought in a new system that was close to but not quite the AD system we have today: Bede on the Problem of 1 AD.
- For a full range of articles on Christian calendars, especially as it relates to the birth of Christ, see: Christian Calendar Systems