system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the year zero not exist?

I “learned” it at college in history but I had a really bad teacher who just made it more complicated every time she tried to explain it.

Edit: Damn it’s so easy. I was just so confused because of how my teacher explained it. 

Thanks guys!
gpt
Just to add some clarification here. Yes, Dionysius Exiguus invented the anno domini system in what we now call 525 AD, but he only established the AD part of it, not the BC part of it. His intention wasn't to create an entirely new calendar starting from some date, but to merely create a way of tracking and logging Easter occurrences that removed references from a Roman tyrant (at the time, they tracked years relative to Roman Emperors).

Dionysius didn't take into consideration dates before the birth of Christ. It wasn't until other scholars decided to use Dionysius' system for other things outside of merely tracking Easter that dating events before Jesus (using a system explicitly created to date things after him) had to deal with the concept.

None included a year zero, but not because they lacked the concept of a zero, but because calendars in general start from "Year 1" anyway and the AD system is "in the year of our Lord."

That means, the year that Jesus was born would be the first year under his "reign" and therefore 1 AD. Whereas the year before Jesus was born would be the first year before Jesus and therefore 1 BC. Conceptually there isn't even a room for a Year 0.