Dionysius Exiguus was the 6th century monk from Scythia Minor who worked in Rome to improve the calendar. He is the man who invented BC and AD (Before Christ and Anno Domini, now commonly known as BCE, Before the Common Era, and CE, Common Era).
If you read the highly technical account of his work in Wikipedia, you can see that reconciling the Gregorian and Julian calendars, not to mention the Diocletian one, was a pretty complicated task for someone who, frankly, was a bit thick. In the event, although he intended to start the AD era at the year of Jesus’ birth, he was 4 years out. This was worked out by Kepler and others in the 17th century. It all depends on Jesus being born in the reign of Herod. Now he died in what we call 4 BC; so JC must have been born in that year or earlier. Don’t loose sleep over it – Jesus’ message is just as intriguing whatever year he was born in.