Which one? The calendar used by Christians in the 1st century, the Julian calendar, or the Gregorian calendar?
Did you even read the thread?
Post #33:
"The Christian calendar was created by an Eastern European monk named Dionysius Exiguus. He invented the now commonly used Anno Domini (A.D.) era, which counts years based on the birth of Jesus. He came up with this concept in the year 525, or, 525 years after the birth of Jesus." https://abcnews.go.com/blogs/headlines/ ... alculation