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I used to think PM/AM was correct, but at some point, I switched to using p.m./a.m. for reasons I can't recall. I know that in practical, casual writing, people tend to use whatever form is most convenient to them, but I'm curious about what the official usage—should it exist—actually is.

What is the correct or official form for these initialisms?

  1. Are they capitalized?
    • 4 pm
    • 4pm
  2. Is there a space before them?
    • 4 PM
    • 4PM
  3. If the correct usage is a.m./p.m., how does this affect punctuation at the end of a sentence?
    • Should we meet at 4 p.m.?
    • Let's meet at 4 p.m..

(Let me know if these should be split up into individual questions, but I figured that because they were all related to these initialisms in particular, it might be okay to ask them all in one question.)

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possible duplicate of Do I spell out a time in an essay? – MrHen Apr 26 '11 at 20:27
The formatting questions have been covered before. The question of order is a bit different and could probably use its own question (and I am not sure you actually asked that question.) – MrHen Apr 26 '11 at 20:29
Ah sorry. I did search before posting, but I didn't notice that question. – LucasTizma Apr 27 '11 at 5:18
up vote 17 down vote accepted

Because the initials in question are of Latin origin ("post meridian"/"ante meridian"), it makes sense to keep consistent with other Latin abbreviations, e.g. (oh there's one right there) "i.e.", "e.g." when there are two words.

I would say, use whatever seems natural, but I prefer "10 p.m." and "8:30 a.m.", for example.

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I like this as a reasonable explanation. I hadn't thought of the Latin origin, but that does make more sense now. Thanks. – LucasTizma Apr 27 '11 at 5:19

An alternative from The Guardian Style Guide:


1am, 6.30pm, etc; 10 o'clock last night but 10pm yesterday; half past two, a quarter to three, 10 to 11, etc; for 24-hour clock, 00.47, 23.59; noon, midnight, not 12 noon, 12 midnight;

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I agree with The Guardian. 6.30pm is the simplest and least intrusive form, so it should be used. – FumbleFingers Jul 10 '11 at 21:02
Note that this style guide also uses . instead of : to separate hours and minutes. E.g. 6.30 vs 6:30. – Dennis Apr 23 '14 at 14:58

Instead of AM and PM (in small caps), p.m. and a.m. -- with lowercase and periods -- are the preferred way to indicate time of day.

According to the The Chicago Manual Style used by journalists.

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If you leave out the dots, you should also omit the space, otherwise am looks like an independent word. Personally, I prefer to include both the dots and the space.

In the USA, I've seen them reduced further, with times like 8:00a and 6:00p. Worse, this was on a timetable (aeroplanes), where I'd expect to see the 24-hour clock used. It confused me for a while. I can't imagine how confusing it must have been for travellers from countries where the 24-hour clock is used everywhere.

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protected by RegDwigнt Oct 16 '13 at 7:29

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