When and how did our current year counting become common place?

by onestepdown54

How did many of the world's countries agree to the current system?

Who decided when year 1 would be?

voyeur324

/u/algernon_asimov and /u/sunagainstgold have previously written about the origins of the Gregorian calendar. Other calendars remain in use around the world.

EDIT: See also the answer by /u/WelfOnTheShelf in When did people started writing dates as we do now? (specifically the number of the year in its current A.D succession)