The system of numbering the years A.D. (Anni Domini, Years of the Lord) originated with Dionysius Exiguus. Dionysius drafted a 95-year table of dates for Easter beginning with the year 532 A.D. Why Dionysius chose the year that he did to number as '1' has been a source of controversy and speculation for almost 1500 years. According to the Gospel of Luke (3.1; 3.23), Jesus was baptized in the 15th year of the emperor Tiberius and was about 30 years old at the time. The 15th year of Tiberius was A.D. 29. If Jesus was 30 years old in A.D. 29, then he was born in the year that we call 2 B.C. Most ancient authorities dated the Nativity accordingly.

Alden Mosshammer provides the first comprehensive study of early Christian methods for calculating the date of Easter to have appeared in English in more than one hundred years. He offers an entirely new history of those methods, both Latin and Greek, from the earliest such calculations in the late second century until the emergence of the Byzantine era in the seventh century. From this history, Mosshammer draws the fresh hypothesis that Dionysius did not calculate or otherwise invent a new date for the birth of Jesus, instead adopting a date that was already well established in the Greek church. Mosshammer offers compelling new conclusions on the origins of the Christian era, drawing upon evidence found in the fragments of Julius Africanus, of Panodorus of Alexandria, and in the traditions of the Armenian church.