A monk called Dionysius Exiguus in what is now Romania sat down in the year that we now call 525 a.d. to devise new tables for calculating the date of Easter in various years. In common usage at the time, people just called the years based on who was emperor at the time, so like, "the second year of Augustus" or whomever. But for Christian usage, they counted the years since the reign of the roman emperor Diocletian as the "year of the era of the martyrs". Dionysius thought that was a bit crap though because Diocletian was a Pagan who persecuted christians, so it seemed unseemly to calculate the dates of Easter based on that. So instead, he made a calculation of the years since the time of Christ, and counted that as "Anno Domini", the year (era) of the Lord.

But, it's more likely for a variety of reasons that Jesus was actually born in 2 B.C., not year 0 on Dionysius's calculation of years. It's unclear though whether he even intended that specifically, because when Jesus was "born" and when Jesus was "incarnated" are maybe different times according to obscure Christian doctrines that would have been relevant in his day.

There is no year zero (except in the French Revolution, whole other story). In western dating systems it goes 1 BC (before Christ, sometimes in European languages this is seen as AC, ante Christus, which means the same thing) then 1 AD (anno domini, year of our lord). The dating is based upon a reading of the Gospels. They describe historical events and people that the Roman and Church authorities believed they could date. This includes the reign of Herod and the census. The Gospels don’t necessarily agree with each other however, allowing for a revised date to be proposed - hence your account that Jesus was born in 4 BC.

Well, for starters, we don’t have a precise biography of Jesus’s life- everything written down about him, including the Gospels, are secondary sources written at least 30 years after his death. So even the 4 BCE date is a bit of informed guesswork, but one of the early church councils settled on 1 CE, which at the time would have been styled as something like “the 27th year of the reign of Emperor Augustus.”

Since resetting the calendar every time there is a new ruler makes understanding history difficult (especially after the fall of the Western Roman Empire), there were a few efforts undertaken by the Catholic Church to reform the calendar in the early Middle Ages. Since they were monks, they settled on the traditional date of Jesus’s birth as year 1, and since they were one of the few institutions that had continental reach, (not to mention literacy and record-keeping ability) it caught on throughout Europe, and then spread to other parts of the world via colonialism.

The supposed year of Jesus’s birth was the year 1 AD, but it is more likely that he was born in the year 4 or 6BC. It is actually fairly common for the birth years of historical figures to be uncertain.

The short answer through very fuzzy math done by monks. There is a long tradition of monks using religious records as historical records and trying to puzzle out when things happened "religously" by coming to them document historical events.

In the system we used to use "BC" and "AD" corresponded to the Birth of Jesus. Increasingly those terms are be dropped in favor of "BCE" and "CE" (Before Common Era, and Common Era) both because, as you pointed out, Jesus wasn't born in the year 1 CE *and* because the less we incorporate Iron-age mythology into our daily lives, the better.

So first off, there was no year "0", it went straight from 1 BCE to 1 CE.

It was a monk who fixed the year "1 CE" on the calendar by doing some fuzzy math and pegging Jesus's birth to be \~ 750 years *after* the founding of Rome.

0BC relates to the year which it was decided that Jesus Christ was born. But the man Jesus/Joshua (in so far as he is agreed to have actually existed as described,) was more likely alive circa. 6 to 4 BC – AD 30 or 33, based on contemporary rercords and scholarly consensus.

It's an inexact science.

The below is taken from the Wikipedia citations (Link) :

*John P. Meier writes that Jesus' birth year is c. 7 or 6 BC.\[1\] Karl Rahner states that the consensus among Christian scholars is c. 4 BC.\[2\] E. P. Sanders also favors c. 4 BC and refers to the general consensus.\[3\] Jack Finegan uses the study of early Christian traditions to support c. 3 or 2 BC.\[4\]*

This problem is the main reason people changed from saying "BC" and "AD" to saying "BCE" and "CE". (Every so often there's someone who gets outraged thinking it was some big atheist plot but really that's not it.)

Even among people who 100% believe in the events of the New Testament, there is still a realization that when the early church tried figuring out the exact year of it, they didn't have the exact date right and therefore if you keep calling them "BC" and "AD" then you get stupid-sounding statements.

Statements like "Christ was born in 4 BC" which means "Christ was born 4 years before Christ was born".

At that point you can either:

(A) Change the Epoch by 4 years. Then change literally every calendar date, every record, every historical document, to reflect the new meaning.

or

(B) Change the Epoch by 4 years. Then leave old documents alone but start using the new reckoning on new documents, with the understanding that you have to know when the document was written to know what the numbers mean.

or

(C) Leave the Epoch alone, as is. Keep the same counting system. But stop claiming it's based on when Jesus was born. Instead just change the meaning to "Years since the arbitrary year we've been commonly using for counting years." Then you don't have to change any documents, or do any weird shifting to read time-spans between written years. You just stop pretending you're measuring from Jesus's birthday.

(C) was a heck of a lot less hassle than either (A) or (B). So that's what we did. Thus we say "common era" instead of "since Christ was born".

We dont know the exact date. This is because exact measurements and precice record keeping (in Western culture anyway) didn't exist until scientific rigor became normalized. The Jewish and Christian accounts originated as word of mouth storytelling, which were inconsistent due to embellishments and approximations.

We picked an arbitrary date and made that year zero so that we could start counting the years. Obviously, time existed before our selected year zero so to account for that, we count backwards from the selected year when we need to put a date to something older.

Because religion is/was a big part of social at the time we were making up counting schemes, we picked a date that someone thought correspond to the birth of Jesus. Hundreds of years ago retroactively applying dates was never going to be an exact exercise, but we're stuck with it.

People were counting years from foundation of Roman Empire but after it fell, church decided to count years from birth of Christ that they manage to calculate. It was wrong because year 0 was missing and also Bible is contradictory to when it was.

A couple of comments seem to have touched on this already, but it really does depend on where in the world you are. I'm not an expert by any means, just a guy who reads some random stuff online from time to time. But as somebody else said, in Judaism, the year is currently 5783 (is that the years since Genesis?), and I'm pretty sure there are many Asian countries, where the predominant religion is not Christianity, that have completely different years, based on their own beliefs. However, BC/AD or BCE/CE seems to be well understood globally due to the spread of Christianity. But I may be wrong.

it was what jesus was born or something. but in jewish culture its 5783 bc we didn't start over when one random dude was born or whatever

To piggy back of of OPs question. Was this dating system something that was implemented/ named(bc/ad) after or was this something the people of the time followed ?