How do we know it's the year 2017 a.d. and not the year 2015 or something similar? I can't imagine they had great global record keeping (way) back in the day.

by LieutenantDave

Also, how do we know the year that anything happened b.c. with any level of accuracy? It's not like people were writing 130 b.c. on their term papers.

jschooltiger

Hi there -- plenty to be said on this topic, but you may be interested in our section on Calendars and Timekeeping in the FAQ.