We've had a few BC and AD posts recently (as well as BCE, CE, Holocene dating -- the list goes on). You may be interested in these:

https://www.reddit.com/r/AskHistorians/comments/5izv93/how_confident_are_we_that_the_year_is_actually/

https://www.reddit.com/r/AskHistorians/comments/4wllbq/what_event_if_any_is_responsible_for_the_shift_to/

Or even our section of the FAQ on calendars.