When did we start counting years in our current calendar? Did Jesus die and people just said “ok we are now in 33AD and there will be 12 sections every year, each with around 30 days”.
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The Gregorian calendar is, to some extent, based on a hodgepodge of earlier systems. The idea of dividing the year into 12 months of around 30 days each was actually rather new at the time Jesus was born, having been proposed by Julius Caesar and later modified by the emperor Augustus. Before then the Romans had used a 10-month calendar, which is why the names of the last few months of the year derive from the numbers 7, 8, 9, and 10 even though they’re the 9th, 10th, 11th, and 12th months.
But that calendar still counted its years from the founding of Rome, or *AUC* *(ab urbe condita)*. Switching over to the supposed year of Jesus’s birth didn’t happen until the year that we now call AD 525. Before then Christians had been counting from year that the emperor Diocletian, the last emperor to institute major persecution against Christians, began reigning (AM, *anno martyrium*). A monk named Dionysius Exiguus was working out a new system for calculating the date of Easter, and introduced the idea of calculating from the years since Jesus’s birth in order to stop celebrating the faith’s persecution. This caught on quickly, even though Dionysius’s calculations for Jesus’s birth are now thought to have been a few years off. This is one of the reasons you sometimes see the same era referred to as *CE* (Common Era): if Jesus wasn’t born that year anyway, then it makes little sense for the calendar to say that he was.
But in any event, Dionysius’s reform wouldn’t be the last. The Julian Calendar is responsible for the famous leap-year rule of adding an extra day to February every four years. the problem with this is that it’s actually too frequent: Earth takes very slightly longer to go around the Sun than this. The margin of error is about 3 days every 400 years. This doesn’t seem like a lit, but by 1582 the calendar had drifted out of alignment by almost two weeks, and people began to notice. One important function of the calendar was to track the seasons, and people didn’t want it drifting too far out from the typical norms. So a new calendar was proposed, almost identical but with a different leap year rule. This is the *Gregorian calendar*. In this calendar leap years happen every four years, except when the year was divisible by 100, *except* when the year is divisible by 400 (so 2000 was a leap year but 1900 was not and 2100 won’t be). This gets us much closer to the proper targetfor keeping things on track. There’s still a small margin of error, but it will take thousands of years before the calendar even drifts by a day.
And that’s the answer to your question. Our current calendar was built up slowly over time, rarher than suddenly shifting to what we now know.
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