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English Lesson 19: Correct Use of A.M. and P.M. for Time

August 2nd, 2008

am-pm-clock.jpgOver the past few weeks, I have been receiving several search queries about the “correct use of am and pm” or similar variants. Although I already know about standard time conventions, I have always wondered about whether or not there are any established grammatical rules for writing the 12-hour clock designations. After spending at least three hours researching this earlier today, it appears that there is no single hard-and-fast rule for expressing a.m. or p.m. when referring to time. The majority of the grammar-based sites that I encountered listed several variants. Some used capital letters or periods while others did not, but most of these sources agreed that all of these variants were acceptable as long as your usage remains consistent throughout your writing.

Eventually I ran across the Associated Press Stylebook guidelines, which state that for journalistic copy we should use the lowercase letter with period format and write the abbreviations as a.m. and p.m. without spaces between the letters or periods. When expressing an actual time, there is one space between the numerical value and the abbreviation, so for example three o’clock in the afternoon would be written as 3 p.m. Since we bloggers (at least those of us who are serious about our work) often like to think of ourselves as journalists, it’s probably a good idea for me to stick the AP standards on this when writing articles. However, for informal writing it doesn’t seem to make much of a difference.

Meanwhile, there were two other questions that came up frequently in searches regarding proper time expression. Here are the answers to these:

What do a.m. and p.m. stand for?

The abbreviation a.m. is Latin for ante meridiem, which means “before midday” (noon); the one for p.m. is post meridiem and means “after midday.” An interesting historical anecdote to this is that the ancient Romans used the ante meridiem expression literally in the sense that noon (as opposed to midnight, which is common in modern usage) was their reference point for telling time. So a Roman time of 3 a.m. literally meant three hours before noon, or 9 a.m. in modern time.

What about noon and midnight? Are these a.m. or p.m.?

Technically, noon cannot be labeled as a.m. or p.m. because it IS the meridian (midday) and cannot by definition be before or after it. Midnight is somewhat trickier because it can be seen as being 12 hours before or after the meridian, depending on one’s perspective. Most sources, however, state that neither expression can be reliably assigned to this exact moment; it is simply the point at which one day passes into the next according to our standard time conventions.

On digital clocks it is common to see noon displayed as 12:00 PM and midnight as 12:00 AM. Technically this is correct because by the time you see the display, it is at least a fraction of a second past noon (or midnight), meaning that the AM or PM designation would now apply.

Of course, if you wanted to avoid the whole issue of what to do with a.m. and p.m. altogether, you could simply express time in the 24-hour format that has become commonplace in many areas of the world. In the United States, however, most of us still use the old 12-hour clock system with the exception of the military and most computer applications.


10 Responses to “English Lesson 19: Correct Use of A.M. and P.M. for Time”

  1. comment number 1 by: Ireland5

    Thanks for this one! We had a problem one time with a college professor who gave a strict due date for an assignment as Thursday, 12 a.m. - my son was not sure if he meant the beginning of Thursday or Wednesday night.

    This also reminded me - little off topic… I have noticed in some of my readings that A.D. and B.C. have been replaced with CE (common era) and BCE (before common era). Wonder when this started and the reason?

  2. comment number 2 by: Bong (JB)

    I can never get used to Military time. Whenever I encounter it I still reckon time by counting the 12-hour system mentally.

    First time I learned about that CE and BCE.

  3. comment number 3 by: Andreas Bengter

    In Sweden we just use military hours.. but we don’t count them as hundreds but as 14 o clock and 14:30 (or 14 and 30, as we put it in swedish)? something for the Us army and the british?

    Thanks for the tip by the way.. I was working on a flier (with both english and swedish text) and was confused as how to write 12:30 in ur am/pm system; I wrote 0:30 p.m., was that the right way?

    andreas, sweden

  4. comment number 4 by: Karlonia

    That is an interesting note for Sweden; I have seen the 24 hour system used in Europe before, with times usually written as 14:30 as in your example above or as 14h30.

    In the USA, we never use a zero for the hours except in the case of military time, where the first hour after midnight would be indicated by 00:00 - 00:59. For all other uses, we would write 12:30 p.m. for 30 minutes after noon and 12:30 a.m. for 30 minutes after midnight.

  5. comment number 5 by: 285fpb

    Thank you very much for explaining the different and how to use it.

  6. comment number 6 by: Andrea

    I wanted to know which is correct, AM or A.M. Your article used it both ways — A.M. and P.M. in the title and later without the periods — I learned if small letters, a.m., but if capital letters, no periods, AM? Guess you were covering all the bases.

  7. comment number 7 by: Art

    Very helpful! Glad it was a top 10 hit on google!

  8. comment number 8 by: JHerlocker

    As yet another side note, in the US Navy we would use both 00:00 and 24:00. Both the same time (midnight), but one was used at the end of the day and one for the start. So we ended our daily log entry at 24:00 Tuesday and started the next entry at 00:00 Wednesday.

  9. comment number 9 by: DColley

    What I find interesting is that when you click the link for Associated Press Stylebook and put in a.m. and p.m. in the search you get alot of documents that have a.m. and p.m. written different ways. Looking at that it is no wonder this can be a confusing rule to remember - I would guess because there are really no rules just different styles.

  10. comment number 10 by: Anonymous

    am and pm is hard to remember wich is wich for a 2 grader like me so I have no idea or anser gies sory!

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