The widespread adoption, around 2000 years ago, of the Julian calendar placed most of humanity on the same footing, with Leap Years every four years helping to keep the year in sync with the seasons. By the middle of the second millennium, things had gotten so out of hand that a week-and-a-half needed to be removed from the calendar to set things right. Here's why so many "historical" dates are unreliable.
https://bigthink.com/starts-with-a-bang/humanitys-calendar-failed/