I've always understood the A.D. reckoning to have spread informally. The pope of Rome, since he is "Pontifex Maximus", and therefore in charge of the calendar of the city of Rome, may have deliberately decided the matter for his city, and his prestige may have influenced others' decision to use the A.D. reckoning, but this is pure guesswork. Was there ever a formal adoption of the A.D. system before circa 1100 ? Maybe, but I can't think of one.

The Gregorian Calendar (which leaves out centurial leaps except the year be divisible by 400) was adopted in 1588 by many Roman Catholic countries, but many Protestant countries continued using the old Julain Calendar for some time. Hence 1700 was a leap year in England and English North America by the calendar then in use, but not in Italy. (This was the almanac-year 1700. The Civil year 1700 didn't start until March 25.)

The Gregorian calendar was adopted in England around 1752. The Encyclopedia Britannica claims that there were riots in Bristol over the adoption of the New Style, but I think these "Bristol calendar riots" are pure myth: they never happened. The calendar may have been an issue in some parliamentary elections around the time of the adoption of the New Style, and in some places there may have been election-related violence--this is what the famous Hogarth picture suggests--but I doubt there were any disturbances deriving purely from the adoption of the calendar.

The Jewish Calendar begins with the year 1, according to every source I have ever consulted.

I must admit that I have written much software that is not Y-10-K (YXK?) compliant. It will have to be modified in the year 9999 to allow for 5-digit year numbers.

T.