This is possibly a really stupid question, but why do people even bother to write AD after a year? If it doesn’t specify BC, one assumes AD.
Most people only use AD when it isn’t clear if it is AD or BC. It all depends on context.
Sometimes it’s done as a politcal statement to include a reference to God on a plaque left on the moon.
I don’t. I write AD the proper way - before the year 
It can be confusing if you’re talking about some event around 2000 years ago. If I said that Publius Quinctilius Varus was born in 46 BC and died in 9, for example, people might not be sure if he was 37 or 55 years old when he died.
B.C.E. (Before Common Era) and C.E. (Common Era) are coming into use more and more. They take the religion out of dating.
While true, the same question applies to the BCE/CE dating convention.
I thought this thread was going to be about the new season of America’s Best Dance Crew.
Nope. It’s about America’s Dest Bance Crew.
I guessed it was about Gene DeFilippo, the Athletic Director for Boston College.
No one doesn’t, at least in contexts where one has to refer to a lot of dates on both sides of the epoch. Another place that specifying AD (or CE) is necessary is when you need to refer to years in many different dating systems, or when you’re writing something for posterity and can’t assume that your future audience is going to know what dating system you’re using. For example, Jewish monuments are often dated with the Anno Mundi chronology, and where the AD/CE epoch is more prevalent the date is specifically marked as “AM”, in part to avoid confusion with the former.