…for example, if I write 4:00am and I mean to write 4:00pm, I would say “Oops, I used the wrong ______ after 4:00.”
I call it a temporal antecedent. But only in everyday speech.
Or meridiem antecedent.
BTW, in a slight hijack, I contend that 12 p.m. actually is the same as 12 a.m., and they both mean midnight because the Latin a.m. means *ante meridiem * (before noon) and p.m. means *post meridiem * (after noon) Given this classic translation, noon would be 12 m. even though “midnight” starts with “m”.
This was shifted from GQ to GD a couple of years ago in this forum. Just 2 more cents’ worth…