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I happen to have a copy of U. S. Government Printing Office Style Manual of March 1984. (I have no idea how it ended up in a used book store in the Russian city of Vologda where I have picked it up.) In the chapter on abbreviations it says the following:

9.54. References to meridian in statements of time are abbreviated as follows: 10 a.m., 2:30 p.m, 12 m. (noon), 12 p.m. (midnight).

Screenshot of the text quoted above

My question is about “12 m.” and “12 p.m.” I never saw the former before and I think presently “12 p.m.” means noon, not midnight. Was it different in 1984?

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    I've never seen it before either, but it makes sense. As explained here, the 'm' means noon, so 12 midday is neither before (ante) nor after (post) noon. People saying '12 p.m.' is a pet peeve of mine,
    – Kate Bunting
    Commented Nov 29 at 11:47
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    I think that's a bit of disinformation to confuse the Russians.
    – TimR
    Commented Nov 29 at 15:34
  • @KateBunting and midnight does, in fact, come 12 hours after noon.
    – hobbs
    Commented Nov 30 at 1:22
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    @hobbs - Well, of course it does, so '12 p.m.', which is what many people call noon, should logically mean midnight. That's part of my peeve.
    – Kate Bunting
    Commented Nov 30 at 8:48
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    @JensSchauder Absolutely – using the logic that AM and PM mean ‘in the period between midnight and noon’ and ‘in the period between noon and midnight’, respectively. The problem with that logic is that it doesn’t fit with how most people refer to noon and midnight. 12 PM usually means noon, which is obviously not the twelfth hour in the period between noon and midnight; and 12 AM usually means midnight, which is equally obviously not the twelfth hour in the period between noon and midnight.
    – Janus Bahs Jacquet
    Commented Dec 2 at 12:48

3 Answers 3

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Wikipedia quotes The Chicago Manual of Style Online and says that

Although "12 m." was suggested as a way to indicate noon, this is seldom done and also does not resolve the question of how to indicate midnight.

I do not have access to this source, but found a more complete quote:

Except in the twenty-four-hour system (see 9.39), numbers should never be used to express noon or midnight (except, informally, in an expression like twelve o'clock at night). Although noon can be expressed as 12:00 m. (m. = meridies), very few use that form. And the term 12:00 p.m. is ambiguous, if not illogical. (source)

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    CMoS goes against a popular usage here, 12 noon / 12 midnight: << When most people say 12pm, typically they're talking about the middle of the day: 12 noon. When they say 12am, they normally mean 12 midnight. >> [Royal Museums Greenwich]. See also NPL, Wikipedia, Britannica, CD .... // Note that 'midnight' is often used to mean 'around 12 midnight', etc, so the precising is needful on occasion. // But agreed; 12am / 12 pm are better consigned to the mists of time.
    – Edwin Ashworth
    Commented Nov 29 at 12:07
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    The basic principle here is that an hour must be either all "am" or all "pm," so the minute preceding "12:01 pm" must be "12:00 pm."
    – alphabet
    Commented Nov 30 at 6:23
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    @alphabet that's how I remind myself which one is which - the AM/PM never flips during the hour.
    – htmlcoderexe
    Commented Nov 30 at 23:04
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    @EdwinAshworth “When most people say 12pm, typically they're talking about the middle of the day” note that this is precisely what confuses continental europeans about the AM/PM system the most, e.g. we use morning/afternoon as indicators when using 12-hour times, so 12 “afternoon” is midnight, not midday.
    – mirabilos
    Commented Dec 1 at 19:42
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    @mirabilos what continental Europeans are these? I don't know of a European country that does not use this exact system. Even the Greeks, who have a different alphabet, still use two abbreviations which stand for "before noon" and "after noon" (although, confusingly, the Greek πμ—which is the Greek p and Greek m—means before noon, not after it). What countries in Europe use a different system? In what language would "12 in the afternoon" be taken to mean midnight? Are you thinking of "after noon" as opposed to "afternoon" perhaps?
    – terdon
    Commented Dec 1 at 22:50
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...and I think presently “12 p.m.” means noon, not midnight. Was it different in 1984?

Yes, absolutely! Things were different at the U. S. Government Printing Office in 1984:

For some people, including the United States Government Printing Office before 2008, the twelve hours before noon are numbered one o’clock ᴀᴍ up until twelve o’clock ᴀᴍ, and the twelve hours after noon are one o’clock ᴘᴍ up until twelve o’clock ᴘᴍ. Then in 2008, the US GPO switched things around.

Please see the lengthier article linked to above for why “12 a.m. and 12 p.m. are ambiguous and should not be used.”

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    The times they are a-changin.
    – Edwin Ashworth
    Commented Nov 29 at 16:37
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    @EdwinAshworth - is that movie any good?
    – Michael Harvey
    Commented Nov 29 at 19:53
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    @Michael Harvey I've only read the book.
    – Edwin Ashworth
    Commented Nov 29 at 23:16
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    This answer makes it sound like things were different for everyone in 1984. While I don't doubt that the US GPO standards changed in 2008, I don't think the US GPO standards reflect common English (or even US) usage during those time periods. This answer would be improved by some indication that (if?) the US GPO was being idiosyncratic in usage, rather giving the mistaken(?) impression that in 1984 people in general did things differently.
    – R.M.
    Commented Nov 30 at 20:49
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    @R.M. It has never been clear what these mean. That’s why the recommendation is that you should not use them in casual speech. And why sure, a daytime twelve o’clock and a nighttime twelve o’clock are perfectly clear; but translating these to anything more formal pertaining to AM/PM has never been something people did consistently—and in the case of the nighttime one even the date becomes ambiguous.
    – tchrist ♦
    Commented Dec 1 at 16:20
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Of course 12:01 or 1 minute after midday is most definitely pm! I checked the Greenwich Observatory website https://www.rmg.co.uk/stories/topics/noon-12-am-or-12-pm and they say

"When most people say 12pm, typically they're talking about the middle of the day: 12 noon. When they say 12am, they normally mean 12 midnight.

While some people follow this convention, technically it's not quite right – as you'll see from the definition of am and pm below. To avoid any confusion (and to make sure you arrive on time), it might be best to say 12 noon or 12 midnight instead."

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  • I thought the day ran from sunrise to sunset, just as the night runs from sunset to sunrise. There are black-and-white speed limit signs that post twin speed limits, one for day that's black on white and one for night that's white on black. And there are city parks that are only open by day, and which are closed at night.
    – tchrist ♦
    Commented Dec 2 at 18:56

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