Another peeve

Who decided that 12 pm means noon, and 12 am means midnight? Or is it the other way around? I sure can't tell, unless from the context. I mean for crying out loud:

12 pm = 12 hours post meridiem = 12 hours after noon = midnight!
12 am = 12 hours ante meridiem = 12 hours before noon = midnight!

Pisses me off, I tell you.