Today, we are living in the period called AD (ie, Anno Domini, “in the year of the Lord [Jesus Christ]). The period before AD is called BD (“Before Christ”). On the basis of the terms BC and AD, people think that Christ must be born in AD 1. This is wrong. Christ could not have been born in AD 1. Matt 2:1 tells us that Christ was born at the time when Herod was king. But by AD 1, Herod the Great was no longer living; he died in 4 BC. Moreover, Luke 2:2 tells us that Christ was born at the time when a census was being conducted by Cyrenius. This census occurred in 5 BC. Jesus thus was not and could not be born in AD 1!
Christ was born in 5 BC, and not AD 1. How did this discrepancy come about? This discrepancy was due to Dionysius’ (a Scythian monk) miscalculation when he prepared a standard calendar for the Western Church. In Dionysius’ calendar, Jan 1, 754 AUC (Anno Urbis Conditae, ie, “from the foundation of the city of Rome”) became AD 1. This became a problem because later research showed that Herod the Great (cf Matt 2:1) died in 750 AUC, ie, 4 BC. How could Jesus be born at at time when Herod was already dead? This contradicts the historical records of Scripture which tell us that Jesus was born when Herod was still alive. Thus, Jesus could not have been born in AD 1 (so Dionysius), but sometime before Herod’s death (ie, 4 BC) according to the Scriptures.
Now, exactly when was Jesus born? Jesus must have been born within 2 years prior to Herod’s death. This we gather from Matt 2:7 which tells us that after Herod had ascertained the time of the star’s appearance, he commanded the execution of all the baby boys 2 years old and below (Matt 2:16). Thus, Jesus must have been born sometime between 6-4 BC. We know that John the Baptist was conceived in the womb of Elizabeth 6 months before Mary became pregnant with Jesus (Luke 1:36). The difference in age between John and Jesus was only 6 months. Luke 3:1 tells us that John began his ministry in the 15th year of the reign of Tiberius Caesar. Tiberius became ruler in AD 11. The 15th year would bring us to AD 26 as the inaugural year of John’s baptismal ministry when he reached the age of 30. In keeping with Luke 3:23, Jesus would also be about 30 years old that year since he was only 6 months younger than John. This would thus make 5 BC the year of Christ’s birth. (Note: there is no BC or AD 0.)
(Taken from Rev. Jeffrey Khoo, The Life of Christ Part 1 (Singapore: FEBC), pg 25-6.)