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Say you've written 16:00 PM. Is that correct? Is the PM necessary?

marked as duplicate by Catija, Nathan Tuggy, P. E. Dant, Ben Kovitz, Em. Jun 16 '17 at 23:35

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No, the AM and PM designations are not used when using a 24-hour / military clock.

Hours between 00:00 and 11:59 are implicitly AM and
those between 12:00 and 23:59 are implicitly PM

  • AM/PM might be unnecessary in a 24-hour clock, but I actually remember reading a job description the other day where they were boasting about flexible work hours "between 7:00AM and 21:00PM." So I wouldn't say they're not used exactly. I think the place was in the Netherlands. Maybe it's regional? – cjl750 Jun 16 '17 at 14:25
  • In the Netherlands they may well be confused between a 24-hour clock and AM/PM system. There's no situation where you would need to say 21:00 PM... it's like saying the same thing in two different languages. You'd only do that if you were addressing a mixed group of and want to get the message across in two "languages" at once. :p – Luke Sawczak Jun 16 '17 at 14:30
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    @cjl750 Think of ante meridiem (AM - before noon) and post meridiem (PM - after noon) as another 'digit' to the 12-hour clock. It disambiguates (for example) 1 o'clock before noon (AM) from 1 o'clock after noon (PM). If you're using the 24-hour clock, 19:00 PM would be 19 o'clock after noon. But that brings us to the next day (19 hours starting from the first hour of the period after noon that day = 7AM the next day). By this logic, I think AM and PM should be compulsorily (not optionally) left off 24-hour times. – Lawrence Jun 16 '17 at 15:03
  • @Lawrence I'm not disagreeing, but there about about 50 billion things in English that make no damn sense, and telling a learner "never do this" when it's a common practice in spite of not making sense is not helpful at all. That said, I don't know if it's a common practice to use the seemingly redundant AM/PM on a 24-hour clock, but I'd be interested to hear people from various countries weigh in. Maybe it's just coincidence that I happened to see this usage a couple days ago, or maybe it's actually not that uncommon after all. – cjl750 Jun 16 '17 at 16:09
  • @cjl750 To be clear, it's not common practice at all in any primarily English-speaking country that I know of. (I live in Canada, an interesting case where most of the country is on 12-hour time but some French areas use 24-hour time. They do not write "a.m." and "p.m.".) Another reason it could be considered wrong and not merely redundant is that "a.m." can be understood to mean "counting from midnight" and "p.m." as "counting from noon". 21:00 PM would be the next day. I don't deny, though, that it might be used in countries where it's an attempted (but clumsy) accommodation of 12-hour time. – Luke Sawczak Jun 16 '17 at 17:16
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It's definitely not used. It's about as wrong as saying it's "nought past nine pm in the evening"!

It would actually be misleading, because how would you write a time in the morning? If you write 10:00 AM, then people have no way of knowing you're using a 24 hour clock.

  • Good point. The presence of AM/PM signals the use of the 12-hour clock. – Lawrence Jun 16 '17 at 23:28

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