The sixth century abbot Dionysius Exiguus is credited with creating the year-numbering system based on Christ’s year of birth. Dionysius determined that Christ was born on December 25, in the 753rd year from the founding of the city of Rome. “From the founding of Rome” is abbreviated AUC.
New Year’s Day was celebrated on January 1st in sixth century Rome. Hence, Dionysius made January 1, 754 AUC the beginning of the Christian Era. The year 754 AUC became known as 1 AD. AD is the abbreviation of anno Domini which literally means, “in the year of the Lord.”
However, Dionysius erred in his calculation of Christ’s year of birth. Chapter two of Matthew’s Gospel indicates that Christ was born during the reign of Herod the Great. Herod tried to assassinate the baby Jesus, whom he saw as a rival, by slaughtering all male children, two years old and younger, in Bethlehem and its suburbs. Herod died in the spring of 750 AUC. Most likely, Christ was born in 749 AUC, the year before Herod’s death.
Nevertheless, the year 754 AUC was designated 1 AD. Dionysius did not allow for a year zero. When 1 BC ended, 1 AD began. Therefore, if we synchronize Christ’s actual birth with the abbot’s system of numbering years, Jesus was born in 5 BC.