Since the years of the Common Era are labeled "AD," standing for anno Domini or “in the year of the lord” in Latin, one might assume that Jesus was born in the Year 0. Specifically, he is commonly believed to have been born eight days before the New Year on December 25, 1 B.C.E. But this is very unlikely.
For one thing, there is no year 0. The day following December 31, 1 B.C.E. is January 1, 1 C.E. And it isn't as though this system of accounting can be trusted anyway. It was devised by Dionysius Exiguus, a monk who hailed from what is now Bulgaria, in the year 525 C.E. (according to himself).
Dionysius evidently thought he was writing 525 years after the birth of Jesus but nowhere does he explain how he came to this conclusion. Since there is no reason to think he had any information that we don’t possess today, let’s examine the facts ourselves.
When Tiberius reigned
The only record of the life and ministry of Jesus are the four Canonical Gospels: Matthew, Mark, Luke, and John. Modern historians take these narratives with a grain of salt when examining historical Jesus, as the authors of these works were clearly more interested in theology than history; they were written quite a bit after the fact; and they contradict one another, themselves and other historic documents in both matters big and small.
Still, these are the only documents that we, and indeed, Dionysius, have to go by.
Luke tells us, somewhat indirectly, that Jesus was born during the reign of King Herod or shortly thereafter (1:5). Matthew tells us this directly: “Jesus was born in Bethlehem of Judaea in the days of Herod the king” (2:1).
Since Herod died in 4 B.C.E., it seems that we can assume that Jesus was born that year or before.
But modern historians aren’t convinced that this is necessarily so. It seems that Matthew set Jesus’ birth during the reign of Herod to allow for his account of the Massacre of the Innocents, an infanticide allegedly ordered by Herod (2:14), but lacking any historical corroboration, nor any mention in the other gospels. According to Matthew this leads Jesus’ family to flee to Egypt.
So Jesus may or may not have been born during the reign of Herod the Great (37 B.C.E. to 4 B.C.E.). What other evidence do we have?
Luke tells us “Jesus, when he began his ministry, was about thirty years of age” (3:23) and says that this was “In the fifteenth year of the reign of Tiberius Caesar” (3:1).
Since we know that Tiberius ascended the throne in 14 C.E., it is simple arithmetic to calculate when Jesus was born: Subtract 15 from 30, you get 15; now subtract this from 14, the year of Tiberius’ coronation, and you get 1 B.C.E. This is probably what led Dionysius to determine his accounting of the years.
But Luke, who never met Jesus and likely didn’t know when Jesus was born anyway, doesn’t tell us that Jesus was 30 when he began his ministry, he says he was "about" 30, which means we could add or subtract a few years anyway.
Luke also tells us (2:1-2) that Jesus was born in Bethlehem, not in the family home in Nazareth, because of the Census of Quirinius (governor of Syria), which required Joseph to return to his ancestral home.
According to the Jewish historian Josephus, this census took place in 6 C.E.. That would mean that Jesus was born that year, or the next. Most historians think Luke got his facts wrong here.
Bethlehem in winter
All four of the canonical gospels agree that Jesus was crucified during the governorship of Pontius Pilate (26 C.E. to 36 C.E.). Since all four gospels agree on this, and since Pilate is an historic figure whose existence is corroborated by outside evidence, this can be taken as an historical fact.
The problem is we are not told how old Jesus was when he died. The closest thing we have is Luke’s statement that he was about 30 when he started preaching.
If we subtract 30 from the years of Pilate’s governorship we get a range from 7 B.C.E. to 6 C.E, which is probably the closest estimate we can reach.
So we don’t know exactly what year Jesus was born, but at least we know he was born on December 25, right? Wrong. The gospels don’t provide us with a date nor even the season of Jesus’ birth. The only hint is a passage in Luke that at the time of the birth “in the same region there were shepherds out in the field, keeping watch over their flock by night” (2:8).
Some scholars have suggested that this means that Jesus could not have been born in the winter since in the region of Bethlehem, sheep would have been kept indoors during the cold winter nights. This claim is disputed by other scholars.
The earliest indication that Jesus was born on December 25 is a calendar from 354 C.E., more than 300 years after his time.
If early Christians knew the date of Jesus’ birth, it is somewhat strange that no record exists of such a thing before that date. It also seems rather convenient that Jesus’ birthday was December 25, which just happened to have been a festival day in the late Roman Empire for the sun god Sol Invictus.
It seems most likely that this festival was transformed from a pagan holiday celebrating the rebirth of the sun to a festival celebrating the birth of Jesus. This is completely in keeping with the practices of the early church, which often chose to co-opt and reinterpret pagan practices rather than fight them.
So it seems that we do not know when Jesus was born, not the date nor the year, but we can say it was probably sometime during the first decade C.E. or B.C.E.