We have all seen dates such as 20 BC or 45 AD, not only that now with recent changes we have BCE and CE. Ever wondered why we have AD instead of DC or some other random word? Well let me answer the question.
Back in 525 AD the christian church brought in a new calendar using the year of Christ, AD which meant “Anno Domini” and BC for “Before Christ”. Thus the AD and BC comes from the Christian Church but not everyone is Christian and thus the BC and AD were later changed to BCE “Before the Common Era” and CE “Common Era”.
BCE replaced BC and CE replaced AD, not really replaced but it is more common to use CE and BCE as it is universal.