Dating using bc and ad
But if Jesus used a calendar, it would not have been the one we use.
Our calendar is called the Gregorian calendar and was instituted by Pope Gregory XIII in 1582. Quite a few societies have used calendars linked to the years their kings ruled.
Furthermore, as described above, our year reckoning was established by Dionysius Exiguus in the 6th century.
Dionysius let the year AD 1 start one week after what he believed to be Jesus’ birthday. The Gospel of Matthew tells us that Jesus was born under the reign of King Herod the Great, who died in 4 BC.
It is likely that Jesus was actually born around 7 BC.
So a person who was born in 10 BC and died in AD 10, would have died at the age of 19, not 20.
The photo above shows the date in a document signed by the Holy Roman Emperor Charles III (a.k.a. The document can be seen in the Deutsches Historisches Museum in Berlin, Germany. APR Anno Incarnat dni dccc Lxxxiii Indict i This means, “ The 4th day before Nonae of April in the year of the Incarnation of the Lord 883, Indiction 1.” " Note, however, that astronomers frequently use another way of numbering the years BC.