Post

PM is the one that happens at midday and AM is the one that happens at night these other comments are correct but some systems don't respect "just M" and this is generally the next best answer

ok are u guys insane though. like i get what you're saying but there's no "just M" in timekeeping. am and pm are just points of reference for clarity in communication about which half of the day they're referring to. noon is pm

if there's no M there would be no reference point for ante or post. meridiem is an infinitesimally small point to contextualize halves. zeno knows what I'm talkin bout and I'd ask him for backup here but unfortunately he's got an unquantifiable number of strikes from bsky

but am/pm timekeeping is not a scientific annotation, it's a simple binary system for people to communicate time, so this "just M" doesnt serve a purpose. also if meridiem is infinitesimally small, which ofc it is, then 99.9999...% of that minute is still after the meridiem since it goes to 12:00:59