From TimeAndDate.com:

The abbreviations am and pm derive from Latin:

AM = Ante meridiem: Before noon PM = Post meridiem: After noon

But I like to call it “After Midnight”, and “Past Midday”.

@odd I use the same expansions as you do.

@parag Really? Maybe most people make this up the same way? 🤔

@odd @parag … and this is why it makes no sense to speak of 12 am or 12 pm. They’re 12 noon and 12 midnight 😜

@artkavanagh Right! I never use AM/PM in daily life, as we use a 24h clock here, and in daily talk most people know if it’s in the daytime or night, and if not, you use “9 this evening”, or “3 tonight”, and so on. ⏰ @parag

@odd Curious why it can't be Post Midnight and After Midday :P

@amit Post midnight is in the night, but PM is associated with the period from 12:00-24:00, so up until midnight?

@odd “after midnight” and “post midnight” are the same thing. Or do you have some other definition of “after” and “post”? ;)

@Archimage We were talking about “AM” and “PM”, and if PM is an acronym for “Post Midnight”, it gets wrong.

@odd Nice!

@odd @artkavanagh @miraz AM and PM is one of those things I never really get my head around. I usually think of the album 3.00 AM Serenades by the Swedish indie pop artist Marit Bergman to get it right. Would be weird if the girls referred to three in the afternoon.

@ndreas I think about 3am eternal by KLF.