Supplemental question: When did the calendar as we know it become (mostly) accepted by the majority of the world?
There's a section of the FAQ on calendars, featuring a bunch of excellent answers related to your questions. Ones to check out include those by /u/sunagainstgold on the rise to prominence of the BC/AD calendar, by a now deleted account on the calendars used by the Greeks, and by /u/400-Rabbits on the calendars used in Mesoamerica.