If I'm not mistaken, the consensus nowadays is that Jesus was at least an actual person, regardless of his perceived divinity.
So, how accurate is the calendar in regards to using his birth as a reference point?
What is the estimated deviation?
Was there some big, more well-documented event around the same time we can use to determine an accurate relation between events BC and AD? (I think that's the correct symbolism, correct me if I'm wrong)
When even was this system of counting the years first applied, and how well-informed would the institution that applied it have been?
Thanks in advance.
Okay, anyone should feel free to correct me on the details, but as I understand it, this is how it works:
Luke says that Jesus was baptised by John when he was 30, soon after John started his ministry, which was in the 15th year of Tiberius (he also gives a bunch of other time markers, such as who was governor of what). Tiberius became Emperor in 14AD, so that would make Jesus born in about 1BC or 1AD. However, there is some debate about what calendar Luke was using, whether he was counting the time Tiberius was co-princeps with Augustus (12AD), etc. Of course, Luke also could have just been wrong. But, broadly speaking, that seems okay.
The problem is that this contradicts both Matthew, and Luke himself.
As you no doubt know, Luke claims that Jesus was born in Bethlehem, which Joseph had to return to because of an Empire-wide census, carried out when Quirinius was governor of Syria. Now, an Empire-wide census almost certainly didn't happen, but Quirinius did do a census of Judea when it was integrated into Syria... in 6 AD. So if Jesus was born in 6 AD, he couldn't have been 30 in 29AD.
Matthew claims Jesus was born during the rule of Herod the Great. He says Jesus' family left until Herod's death a few years later. But Herod had died 10 years earlier than the census, in 4BC, his kingdom split between his sons, and the son who ruled in Judea was so hated that some (but not all) Jews asked the Romans to take over directly. So, since Luke's account doesn't fit his own time markers, many scholars assume Matthew is right. This of course, also doesn't quite fit with Luke's claim he started when he was 30 in the 15th year of Tiberius. Even if you count Tiberius' reign from 12AD, that still gives you a birth date a year or two after Herod's death.
Sadly, unlike the other gospel writers, Luke (whoever he was) seems to have made a good effort to date events, but unfortunately the dates just don't quite fit together properly.
One other note: I'd also point out that many scholars think that the birth narratives are myths designed to fit prophecy. The author of Luke probably was working from sources based on people who met Jesus, but it's less likely he had sources based on people who were there at Jesus' birth. So, it's possible that his statement that Jesus was baptised when he was 30, shortly after John started preaching in 29AD, is actually the most reliable date he gives (more reliable than the dates of Jesus' birth), and so in fact Jesus was born in 1AD after all. But that's not the most common view, just a possibility.
Finally, I should say that this is all rather argued about. I don't think there is any firm consensus, but a birth of about 6 AD seems to be the most common view.
As to how this date came about, it was worked out in the 6th century by a monk named Dionysius Exiguus who was working on general calendar reform, as the years were currently based on the rule of the hated Diocletian. It's not clear how he decided on Jesus' birth year, but as I've shown, the real answer isn't very clear anyway.