The Anno Domini Dating System

This was devised in 525 by Dionysius Exiguus, who used it to work out the date of Easter but did not use it to date any historical event. He designed his system to replace the Diocletian era that had been used in old Easter tables because he did not want to use the system of a tyrant who persecuted Christians. The last year of the old table, Diocletian 247, was immediately followed by the first year of his table, AD 532. When he first devised his table, Julian calendar years were identified by naming the consuls who held office that year - he himself stated that the 'present year' was 'the consulship of Probus Junior', which was 525 years 'since the incarnation of our Lord Jesus Christ'. Thus Dionysius implied that the year Jesus was born occurred 525 years earlier, without stating the specific year during which his birth or conception occurred.

Two Systems

In both Julian and Gregorian Calendars, there are two systems of numbering years BC.

Historical Year Numbering System.

Bede (672-735) and other early historians wrote in Latin which does not have a Roman numeral 0. They did not like to use the Latin word zero nulla as a year, so the year preceding AD 1 is 1 BC. We can call this the historical year numbering system.

Astronomical Year Numbering System.

Mathematically, it is more convenient to include a year zero and represent earlier years as negative to make it easier to calculate the number of years between a negative year and a positive year. So, the astronomical year numbering system includes a year zero and represents earlier years as negative.