I was told way back in the 1970s that these were "offensive." What that means is that if you think really, really hard about it, and ignore the fact that both are nothing but conventions, you might wonder whether somebody somewhere is or should be offended.

I've noticed, however, that both "BC" and "AD" have pretty much disappeared from American academic writing over the last twenty years or so. "BCE" and "CE" are now standard.

But still "offensive," if you get what I'm saying.

Perhaps we should date everything from the Big Bang of about 14,000,000,000 years ago. (or "BP" as anthropologists and paleontologists call it: "before the present").

But that would be "offensive" to Creationists, who would date everything from October 23, 4004 BC. (They're OK with AD and BC, though.)

The simplest thing, really, would be to assume that if it happened more than a few months ago, it just isn't important any more.