[jjmarr]

Nobody actually knows what year Jesus was born, so setting "1 AD = Jesus' first full year of life" wouldn't be accurate. It's more or less the same puzzle as COBOL's alleged default.

https://en.wikipedia.org/wiki/Date_of_the_birth_of_Jesus#Acc...

The current year numbering is because a monk called Dionysius Exiguus thought the existing system of numbering the years since Emperor Diocletian was stupid as Diocletian persecuted Christians. Dionysius decided the year in which he invented his calendar was 525 years since the birth of Jesus and created the Anno Domini system.

Dionysius didn't really explain why he thought Jesus was born 525 years ago (in December of 1 BC). Many historiographers have tried to understand his logic. Thankfully, unlike COBOL programmers, Dionysius documented his reasoning for picking 1 BC as the reference year, so we only have to argue about whether he was correct.