If Jesus wasn’t born on the 25th December (see here), then when WAS Jesus born? Or, to put the question another way, when did BC turn to AD (Anno Domini, in the year of our Lord)? Surely a Christian believer would shrug and say, “He was born in AD 1.”
Well – not so fast! Scholars don’t think Jesus was born in AD 1.
There is some historical data to refer to here:
FIRST – Matthew and Luke’s gospels were written independently, and they agree that Jesus was born during the reign of the Roman appointed King, Herod the Great.
SECOND – the Roman historian Josephus places the dates of Herod’s rule from 37 BC to 4 BC. So, Jesus must have been born before 4BC.
THIRD – Matthew places Jesus birth around Herod’s reign. 
FOURTH – Luke says during the 15th year of Tiberius Ceasar, Jesus was about 30 years old. Tiberius was Ceasar between AD 14 and AD 37, so the 15th year was about AD 28. Counting backwards 30 years or so, we reach 6 BC.
Scholars today have reached a consensus that Jesus was probably born in 5 BC.
SO – the question then is – how can Jesus be born 5 years BEFORE CHRIST (BC)? That seems to make no sense at all!
Dickson points out that the reason for this is pretty straightforward. In AD 525, Pope St. John asked mathematician and theologian Dionysius Exiguus to create a chronology of events based on the limited historical records available at the time. He dated Jesus’ birth as accurately as he could, and then the Western church decided to use his chronology for the purpose of dating.
Today, we simply have more accurate historical dating of both Herod the Great and Emperor Tiberius than the ancients had. So – this lets us confidently place Jesus birth 5 years earlier than Dionysius originally thought.
It seems to me that, like before, it’s less important exactly when Jesus was born. The important thing to grasp is that he was born in the first place. Because that means we have to then grapple with the reports of his claims about himself (I can do what God does), and the reports of the miraculous things he did at that time in the first century. Crucially – his reported resurrection from the dead.
 Matthew 2:1.
 Luke 3, summarised.
 John Dickson, 12. First Noel, Undeceptions Podcast.