When was Jesus actually born?

There is no year zero, Jesus was almost certainly not born in the year 1, and 25 December is a decision rather than a record. None of that is controversial among historians, and none of it is new. It is just rarely explained.

The monk who miscounted

The numbering of our years comes from Dionysius Exiguus, a monk working in Rome in the 520s. He was asked to produce a table for calculating Easter, and rather than continue the existing practice of counting years from the reign of the emperor Diocletian — a persecutor — he counted from the Incarnation instead.

How he arrived at his figure is not recorded. What is clear is that he was a few years out. Every line of evidence puts the birth earlier than the year he chose, and the calendar has never been corrected. The count also runs straight from 1 BC to AD 1 with no year zero between them, which is why arithmetic across the divide is always one out if you are not careful.

Herod sets the latest possible date

Matthew places the birth in the reign of Herod the Great, and has the family flee to Egypt because Herod ordered the killing of the boys of Bethlehem two years old and under.

Herod's death can be dated. Josephus records a lunar eclipse shortly before it and a Passover shortly after, and the eclipse most historians identify is the one of March, 4 BC. A minority prefer a later eclipse and put the death in 1 BC, which is a real argument rather than a fringe one, but the 4 BC date remains the mainstream position.

Take 4 BC, and the birth has to be earlier still. The order to kill boys up to two years old suggests Herod was allowing for a span of time since the star first appeared. That points to a birth somewhere between about 7 and 5 BC, with 6 BC or 5 BC the most commonly named years.

The census that will not fit

Luke gives a different anchor, and it pulls the other way. He says the decree went out from Caesar Augustus, and that this taxing was first made when Quirinius was governor of Syria.

Publius Sulpicius Quirinius is a documented Roman official, and his census of Judea is documented too. It took place in AD 6, when Judea was brought under direct Roman administration after Herod's son Archelaus was removed. That is ten years after Herod the Great died. Both dates cannot be right as usually read.

Three answers are commonly offered. The Greek word rendered "first" can carry the sense of "before", which would make Luke say this registration happened before the well-known census under Quirinius. Quirinius may have held an earlier command in the region, in which case Luke is naming him by the office he is remembered for. Or Luke, writing decades later, attached the best-known census to the event.

No solution has carried the field. It is the single hardest problem in New Testament chronology, and it deserves to be stated as a problem rather than smoothed over.

A cross-check from the other end

Luke also dates the start of John the Baptist's preaching to the fifteenth year of Tiberius, which falls around AD 28 or 29, and says that Jesus was about thirty years old when he began his ministry.

Counting back thirty years from AD 29 lands near 2 or 1 BC. Counting back from a slightly later reading of the ministry's start, or reading "about thirty" with the latitude the word invites, pulls it earlier and closer to the range Matthew implies.

The check is useful but blunt. It confirms that the birth belongs in the last years before the era that bears his name. It does not narrow the year.

Why 25 December is unlikely

Luke says the shepherds were living out in the fields, keeping watch over their flock by night. Judean winters are mild, and flocks were not always housed, so this is not decisive on its own. It does, however, fit spring or autumn considerably better than late December.

The stronger point is simply that no gospel gives a date. None of the four writers mentions the day, the month, or the season. For the first two centuries of Christianity, nobody appears to have marked the birth at all; the earliest Christian calendars observe the death and resurrection, not the nativity.

Where 25 December came from

The first surviving document that puts the birth on 25 December is a Roman almanac compiled in the year 354. By then the date was already in use in the West, while much of the East kept 6 January instead, and some Eastern churches still do.

Two explanations compete, and both are respectable.

The first is borrowing. Late December carried the Roman festival of Saturnalia and, from the third century, an imperial feast of the Unconquered Sun near the solstice. Placing the birth of Christ there gave the Church a feast of its own on the busiest day in the Roman year.

The second is calculation. An old tradition held that a prophet died on the anniversary of his conception. Working from the Passion, some early writers fixed the conception at 25 March — still the date of the Annunciation — and nine months later falls on 25 December. On this reading the date was derived from the crucifixion rather than taken from a pagan calendar.

Either way, it is a date chosen for good reasons rather than one handed down. That is worth knowing, and it is not a reason to keep Christmas any less.

The Latter-day Saint 6 April tradition

Doctrine and Covenants 20 opens by dating the organisation of the Church: one thousand eight hundred and thirty years since the coming of the Lord in the flesh, on the sixth day of April.

Many Latter-day Saints have read that sentence as revealing the birth date itself, which would place it on 6 April, 1 BC. James E. Talmage argued the case at length in Jesus the Christ in 1915, and it has been repeated in lessons and talks ever since.

Others read the verse as an ordinary dating formula — a way of saying "in the year 1830" in scriptural language — and note that it would have to override the reign of Herod to work. Latter-day Saint scholars have published detailed cases for a birth in December of 5 BC and for other years besides.

The Church has never settled it. The 6 April reading is a widely held tradition with a serious argument behind it, not a doctrine, and it is fair to present it as exactly that.

The best answer available

Weighing all of it: the birth almost certainly falls between about 7 and 4 BC, most likely 6 or 5 BC, with 1 BC a minority position that some Latter-day Saint writers prefer for the reason above. The season is unknown. The day is unknown.

The gospel writers had access to none of this and were not troubled by it. What they set out to record was that the birth happened, where, and to whom. The calendar came later, and it came from us.