The apostle Luke often gets a bad rap as a poor historian. The details of this accusation are discussed in detail in my essay But to Fulfill: The Events of the Nativity from Jesus’ Birth until the Arrival of the Magi. But we can short cut a lot of useless arguing if we simply read the Bible. According to Luke, Jesus turned 30 years old in the “fifteenth year of Tiberius Caesar,” in other words, during the fifteenth year of his reign as Roman emperor.
(1) Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judaea, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of Ituraea and of the region of Trachonitis, and Lysanias the tetrarch of Abilene,(2) Annas and Caiaphas being the high priests, the word of God came unto John the son of Zacharias in the wilderness.(3) And he came into all the country about Jordan, preaching the baptism of repentance for the remission of sins. [Luke 3:1–3 KJV]
(23) And Jesus himself began to be about thirty years of age, being (as was supposed) the son of Joseph, which was the son of Heli. [Luke 3:23 KJV]
Tiberius was the adopted son and heir of Augustus Caesar. After his father’s death, he was confirmed princeps by the Senate in the fall of A.D. 14. Princeps means “first citizen” and was associated with Roman emperors. The English word prince is a derivative of princeps. Tiberius’ reigned from A.D. 14 until A.D. 37. If he was coronated in the fall of A.D. 14, then his first year was from the fall of A.D. 14 to the fall of A.D. 15. This would make his fifteenth year from the fall of A.D. 28 to the fall of A.D. 29. All we have to do is do the math. If Jesus was “about 30 years of age,” that is, near his thirtieth birthday, in the fifteenth year of Tiberius’ reign, he must have been born in 3 B.C. The fact that Tiberius was confirmed in the fall and that Jesus was born in the fall is an added bonus that may indicate that Luke was even more accurate that we supposed.
Jesus of Nazareth: Born between approximately 6:30–8:00 PM, September 11, 3 B.C.