Well I ask a few questions here, for one, since when and how did the birth of Christ become a historical point of reference (200,000 BC, etc)? And has there ever been another civilization that's used another system or point of reference?
EDIT: Slight grammar error in title, "been used.
This has been discussed a lot. Take a look at http://www.reddit.com/r/AskHistorians/wiki/calendars and see if that answers your question.