For once I’m with huju. The meridian is noon. 12AM is 12 hours before noon, i.e. midnight. And 12PM is 12 hours after noon, i.e. midnight. Noon, by definition, is neither before itself nor after itself!
Joseph says “12PM” is “universally” understood to mean noon. I wonder which universe he refers to, and how he determined this. When I see someone write 12PM I ask them what they meant.