His calculation went as follows: In the pre-Christian era years, the Romans counted from ab urbe condita (“the founding of the City” [Rome]). Thus 1 AUC signifies the year that Rome was founded, 5 AUC signifies the 5th year of Rome’s reign, etc. Dionysius received a tradition that the Roman emperor Augustus reigned 43 years, and was followed by the emperor Tiberius.
“Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate governor of Judea, and Herod being tetrarch of Galilee and his brother Philip tetrarch of Ituraea of the region of Trachonitis, and Lysanias the Petrarch of Abilene” (Luke 3:1)
Based on this analysis, it indicates that when Jesus turned 30 years old, it was the 15th year of Tiberius reign. If Jesus was 30 years old in Tiberius’ reign, then he lived 15 years under Augustus (placing Jesus birth in Augustus’ 28th year of reign). Augustus took power in 727 AUC. Therefore, Dionysius put Jesus birth in 754 AUC.
However, his birth was in the days of Herod, and Herod died in 750 AUC – four years before the year in which Dionysius places Jesus birth.
“Now when Jesus was born in Bethlehem of Judaea in the days of Herod the king, behold there came wise men from the east to Jerusalem” (St. Matthew 2:1)
New Testament writes about the date of Jesus’ birth, with certainty, the birth did not occur in AD 1. The Christian’s starting point for Jesus birth is based on a miscalculation introduced 533 ca. by Dionysius Exiguous. “The DePascha Computus” an anonymous document believed to have been written in North Africa around 243 CE, placed Jesus birth on March 28. Clement, a bishop of Alexandria (d. ca. 215 CE), thought Jesus was born on November 18. Based on historical records, Fitzmyer guesses that Jesus birth occurred on September 11, 3 BCE.