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.

Written primarily in Greek, 1984 edition

The Letter and Prologue on Easter of Theophilus of Alexandria (385-412), the 95-year list of Paschal data compiled by Cyril (412-444), and the Prologue or Praefatio to that list written in Latin about 482 in the persona of Cyril are among the foundational documents for our knowledge of the Alexandrian Easter cycle. That cycle, through the Latin versions of Dionysius Exiguus, Bede, and others was the standard method for determining the date of Easter in the western churches until the end of the sixteenth century. There has been no modern critical edition of either Prologue since those of Bruno Krusch in 1880. This new edition of the texts is based on Alden A. Mosshammer's discovery or rediscovery of manuscript witnesses unknown to Krusch and overlooked by more recent scholars who have engaged these texts. The historical introduction summarizes current knowledge about the history of Easter calculations in early Christian communities, including a new hypothesis attributing the Alexandrian cycle in its final form to the mathematician and astronomer Theon of Alexandria working in the 370's. Although both texts have already been translated into English, Mosshammer's new translations are based on his new reconstruction of the texts. The commentaries address many issues currently under debate in historical scholarship, such as the origin of 21 March as the conventional date of the vernal equinox. The newly reconstructed text of the Prologue attributed to Cyril and Mosshammer's extensive commentary make that difficult text intelligible for the first time.