Take the 2-minute tour ×
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It's 100% free, no registration required.

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.)

share|improve this question
    
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
add comment

4 Answers 4

up vote 10 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.

share|improve this answer
    
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
add comment

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.

share|improve this answer
add comment

An alternative from The Guardian Style Guide:

times

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;

share|improve this answer
    
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 at 14:58
add comment

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.

share|improve this answer
add comment

protected by RegDwigнt Oct 16 '13 at 7:29

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.