system
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human
ELI5: How was the BC/AD dating system created?

These are the two things confusing me. This dating system is meant to be modelled from Jesus' birth, but many historians claim that it is actually just some time near there (suggesting 8BC for example). Also, if it is modelled after Jesus' birth, why is Christmas not on the 1st January? Shouldn't the first day of the year be the day of Jesus' birth? Thank you to anyone who can explain this to me.

Edit: Thank you to everyone who responded because it really helped a lot.

However, I don't think that I explained my exact query well enough. I'm not asking about how the BC/AD system was created (as many of you have explained). My question is, whether or not the BC/AD system is right, whether or not Jesus existed or not, why is it that Jesus' supposed birth is not on January 1st?

Let's assume that Jesus was born on December 25th (I don't want this to turn into a debate about religion, because that's not the point of my question). Then if Dionysus Exiguus the monk was modelling his calendar around Jesus' birth, why would he not Jesus' birth the first day of the year? This way, when you reference a year like 4000 BC, it is actually relative to Jesus' supposed birth.

I'm sorry if I'm explaining this really badly. I know it's a strange question to ask, but I really want to know, because it seems very interesting.
gpt
It was created by a 6th Century monk named Dionysus Exiguus.

At the time, the calendar was based on a Roman emperor who wasn't terribly kind to Christians.  Dionysus was working on an Eastern calendar, and didn't think it was fitting to link it to that particular person.  So instead he did some rough math and took a wild guess what year Jesus should have been born, and based the calendar year on that, keeping everything else the same.

He did this for his own personal calendar, and had no idea with would eventually catch on, including his incomplete scholarship.  By the time people figured out the dates had to be off, it was too late, the calendar had been too widely adopted, and being off a few years after centuries had passed didn't really matter.