Dionysius Exigius (aka Dennis the Short), a monk from Russia who died about 544, was asked by Pope John I to set out the dates for Easter from the years 527 to 626. It seems that the Pope was keen to produce some order in the celebration of Easter. Dionysius decided to begin with what he considered to be the year of Jesus' birth. He chose the year in which Rome had been founded and determined, from the evidence known to him, that Jesus had been born 753 years later. He did have an error in that because one emperor changed his name during his reign, Dionysius counted him twice.
He was almost certainly acquainted with a suggestion by Hippolytus (170–236) that the date of Jesus' birth was December 25, but the trouble was that Hippolytus had not backed up this claim with sound arguments. Dionysius, however, had just the argument: His contemporaries claimed that God created the earth on March 25. It was inconceivable that the son of God could have been in any way imperfect. Therefore Jesus must have been conceived on March 25. This meant that he must have been born nine months later—December 25. Dionysius also concluded that, as a perfect being, Jesus could not have lived an incomplete life so he must have died on March 25 as well!
December 25 was an auspicious choice. In 274, in Rome, the Emperor Aurelian declared December 25 a civic holiday in celebration of the birth of Mithras, the sun god. By 336, in that same city, Christians countered by celebrating the birth of Jesus, the son of God, on December 25. Christians in Antioch in 375 celebrated the birth of Jesus on January 6. Christians in Alexandria did not begin to celebrate Christmas at all until 430. So until Dionysius came along there was confusion over dates, and debates raged, even over the usefulness of celebrating the birth of Jesus at all. What had been universally important for all Christians—the pre-eminent event—was the celebration of Easter.
When, in 527, he formalized the date of Jesus' birth, Dionysius put Christmas on the map. Jesus was born, he declared, on December 25 in the Roman year 753. Dionysius then suspended time for a few days, declaring January 1, 754—New Year's day in Rome—as the first year in a new era of world history.
With a stroke of ingenuity Dionysius had managed to shift the attention of the church from Easter to Christmas. From this point in time it seemed only logical to celebrate the birth of Jesus before his death. If Jesus' death by crucifixion had made possible salvation for all people everywhere, so the argument went, then his birth was the sign that God was identifying with human kind by taking human form.
But Dionysius made a mistake in his calculations. Perhaps he had never read the gospel account of the birth of Jesus. In Matthew Jesus is said to have been born while Herod was still King (2:1). That would translate into 4 BC (or even earlier) according to the calculations of Dionysius. As a consequence, for Christians the year 2000 is not two thousand years after the birth of Jesus, but more like 2004.
That was not his only mistake. Dionysius followed the convention of his times and, as the Roman calendar moved from the year 753 to 754, he called the latter "year one" of the New World order—anno domini, the year of our Lord. The concept of naught (zero) didn't come into Europe from Arabia and India until about two hundred years later. As a result, centuries end with naught and begin with the digit one. So for us the year 2000 was the end of one millennium but it was not the beginning of the next: that occurred in 2001.
Later, when Pope Gregory tidied up the calendar on 24 February 1582, the calendar lost eleven days. To synchronise the calendar of Dionysius with the movement of the sun, October 4 became October 15, and to avoid having to make further adjustments a leap year was introduced. Pope Gregory must also have known of the mistakes made by Dionysius but all he did was to confirm them, perhaps hoping that no one would notice.
There is one other problem. Bishop Ussher (1581–1656) worked out the precise year of creation as 4004 BC (he knew about Dionysisus getting the date of Jesus birth wrong). But he also advanced the view that the earth had a total life span of six thousand years. In order to come up with this conclusion he based his calculations on all the generations mentioned in the Bible.
In reality we do not know when Jesus was born—neither the year, the month, nor the day. The chronology of our western calendar is based on mythology masquerading as theology. We do well to treat it all with the humour it deserves.
https://www.westarinstitute.org/resources/the-fourth-r/dionysius-exiguus/
