As a kid, I thought the correct and only way to write years was “800 BC” (meaning “800 years before Christ”) and “800 AD” (meaning “800 years after death”).
I was wrong on many levels. The BC part is fine, but AD doesn’t stand for “after death” at all – it stands for “anno Domini”, meaning “in the year of the Lord”. And it should be placed before the year, not after: AD 800, not 800 AD. (If you find it hard to remember that BC comes after the year and AD comes before, just say them out loud. It makes sense to say “800 years before Christ”, not “before Christ 800 years”, and “in the year of the Lord 800”, not “800 in the year of the Lord”.)
No sooner had I got my head around those rules than I discovered there was a whole other naming system vying for my attention. Sometimes, especially in academic writing, BC and AD are replaced with BCE and CE, which stand for “before Common Era” and “Common Era”, or sometimes “before Current Era” and “Current Era”. These terms are equivalent to BC and AD – we are currently in the year AD 2017 and the year 2017 CE. (Note that both BCE and CE come after the year, making them slightly less confusing than BC and AD.)
Now that I know the Common Era system exists, I’m all for it. Its big advantage is that it’s secular, i.e. non-religious. Considering that, as of 2010, only 31.5% of the world’s population was Christian, it doesn’t make much sense to measure our lives in relation to Jesus Christ’s. Of course, the Common Era is only secular in a superficial way – even though it doesn’t namecheck Christ, it still uses his birth as its starting point – but I think a token gesture towards inclusivity is better than nothing.
Having said that, I admit you need to consider your audience before switching to the Common Era. If you’re writing for the general public, you risk isolating the many readers who haven’t heard of the system, so it’s safer to stick to BC and AD.
One more note: I imagine some people dismiss the Common Era system as “PC gone mad”, akin to the “War on Christmas”. (Don’t even get me started on that.) If you encounter someone like that, it might help to remind them that this isn’t a newfangled invention. Wikipedia says:
The expression has been traced back to Latin usage to 1615, as vulgaris aerae, and to 1635 in English as “Vulgar Era”. The term “Common Era” can be found in English as early as 1708, and became more widely used in the mid-19th century by Jewish academics.