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I've read in various places the first minute of a day described as 12:00am. Now, whilst I personally prefer to use 24h clock notation and therefore don't have this problem as I can simply describe this minute as 00:00, how is one supposed to express this time using am/pm notation? Is 00:00am common (strange because there is no 00:00pm), or is the least-bad way to say 12:00am (strange IMHO because it seems to imply that the previous hour was 11:00am when it was actually 11:00pm)?

Jez
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    @Carlo I'm not sure what language that is, but in English we almost always use colons (:) to separate hours from minutes. Use of commas (,) or periods (.) is a sure sign of a non-native English speaker. Also, we don't usually have an hour 0. Clocks go from 1 to 12. – nohat May 03 '12 at 22:49
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    General Reference. The day starts at midnight. Midnight belongs to the new day – FumbleFingers May 03 '12 at 23:32
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    @FumbleFingers: but the day also ends at midnight. Midnight is a point in time of zero duration that divides the new day from the old. If I say, 'Let's do it tomorrow at midnight," my intention is that we would stay up late tomorrow night to do it. If I say, "Let's do it tonight at midnight" then I mean let's stay up late tonight." – Jim May 04 '12 at 00:01
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    @Jim: You can also do it at one-thirty tonight, because for most purposes, "tonight" is "until we go to sleep". Just as "tomorrow" starts "after we wake up". – FumbleFingers May 04 '12 at 01:52
  • This is why 24h notation is inherently better. ;) – Sonic42 May 04 '12 at 03:16
  • That's why I am sticking with my 24hour notation! – deutschZuid May 04 '12 at 03:59
  • Your confusion is common, because of what you say, 11pm is the hour before 12am seems strange. But if 00:00 is the same as 00:00am (a pleonasm) which is the same as midnight which is the same as 12:00 midnight, it must be labelled 12:00am. Also, it's what everybody else calls it. – Mitch May 04 '12 at 13:28
  • I did once see a listing "12 m", standing for "meridies" or whatever. But still confusing since some may think "m" stands for "midnight". – GEdgar May 04 '12 at 14:49
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    For a long time, humans did not even have a way to express zero. To this day, most humans don't feel comfortable starting anything with zero. A clock face is numbered with 12 at the top, preceded by 11 and followed by 1. The question of whether midnight should be 12:00 a.m. or 12:00 p.m. is not inherently more confusing than whether midnight should be 00:00 or 24:00. In both cases, there is a convention, you learn it, and you move on. – John Y May 04 '12 at 22:27
  • @Carlo_R That may be the correct convention in your language, but it's certainly not true in English. –  May 05 '12 at 01:58
  • @JohnY "most humans don't feel comfortable starting anything with zero" - this is with the sharp exception of programmers who are used to zero-indexing and have been known to start legal contracts with paragraph zero. :-) – Jez May 05 '12 at 09:36
  • @Jez: Out of the billions of people using clocks, only a small percentage are programmers. – John Y May 05 '12 at 17:12
  • @nohat, What about idiomatic expression zero hour? – Eilia Jun 16 '15 at 05:33
  • @nohat I don't know about you, but expressing times with a full stop or interpuct like 3.40 or 12·30 is fairly common. I use the latter myself. – JDF Jan 23 '18 at 16:08
  • @Eilia "zero hour" derives from military time, which uses a 24-hour clock, which starts at zero. – nohat Jan 23 '18 at 17:54
  • @Deonyi I'm not aware of any English-language usage manuals or style guides that recommend using anything other than colons for unit separators in representing time. Even the international standard for writing times, ISO 8601, specifies either no punctuation or colons. https://en.wikipedia.org/wiki/ISO_8601#Times – nohat Jan 23 '18 at 17:57
  • @nohat http://www.disused-stations.org.uk/l/liverpool_exchange/timetable2.gif and https://forum.wordreference.com/threads/punctuation-for-time.2076421/ disagrees with you. In any case, ISO also leaves out middle dots for decimal points which are used commonly in Australian handwriting and uses them instead for multiplication which is a full stop here (or a cross). – JDF Jan 24 '18 at 11:48

6 Answers6

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The Wikipedia article 12 Hour Clock is worth quoting at length:

Confusion at noon and midnight

It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday. Since strictly speaking "noon" (midday) is neither before or after itself, the terms a.m. and p.m. do not apply. However, since 12:01 p.m. is after noon, it is common to extend this usage for 12:00 p.m. to denote noon. That leaves 12:00 a.m. to be used for midnight at the beginning of the day, continuing to 12.01 a.m. that same day.

However, because practical confusion is still possible, some style guides recommend replacing "12:00 p.m." with "12:00 noon" and "12:00 a.m." with "12:00 midnight".

The American Heritage Dictionary of the English Language (Fourth Edition, 2000) has a usage note on this topic: "Strictly speaking, 12 a.m. denotes midnight, and 12 p.m. denotes noon, but there is sufficient confusion over these uses to make it advisable to use 12 noon and 12 midnight where clarity is required."

Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page, recommend that it is clearest if one refers to "noon" or "12:00 noon" and "midnight" or "12:00 midnight" (rather than to "12:00 p.m." and "12:00 a.m."). Some other style guides suggest "12:00 n" for noon and "12:00 m" for midnight.

The Canadian Press Stylebook (11th Edition, 1999, page 288) says, "write noon or midnight, not 12 noon or 12 midnight." Phrases such as "12 a.m." and "12 p.m." are not mentioned at all.

The use of "12:00 midnight" or "midnight" is still problematic because it does not distinguish between the midnight at the start of a particular day and the midnight at its end. To avoid confusion and error, some U.S. style guides recommend either clarifying "midnight" with other context clues, or not referring to midnight at all. For an example of the latter method, "midnight" is replaced with "11:59 p.m." for the end of a day or "12:01 a.m." for the start of the next day. That has become common in the United States in legal contracts and for airplane, bus, or train schedules, though some schedules use other conventions.

My advice is to use "12:00 noon" instead of "12:00 p.m." and "12:00 midnight" instead of "12:00 a.m." Realize that readers may be confused which day 12:00 midnight belongs to, so make sure context makes it clear.

Community
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Steven Rumbalski
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    Or use 24h notation. :-) – Jez May 03 '12 at 22:50
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    And yet anyone with experience with a digital clock with date function will readily know that 12:00am is the start of a new day since the date increments simultaneously with the change from 11:59pm to 12:00am- there is only one minute per year when a working clock will show Dec 25, 12:00am. – Jim May 03 '12 at 23:11
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    In my opinion, it is more ambiguous to say "12:00 midnight Tuesday" than to say "12:00am Tuesday"- 12:00 midnight Tuesday could be construed to be at the end of Tuesday, while 12:00am always means at the beginning. – Jim May 03 '12 at 23:16
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    12:00 a.m. refers to midnight unambiguously because 12:15 a.m. refers to quarter past midnight. – Kaz May 04 '12 at 07:46
  • In some military contexts is such that there is no 0000; time goes from 2359 straight to 0001. If there a real risk of confusion, it would be equally possible to go from 11.59 a.m to 12.01 pm with nothing between (using long minutes if needed). – Tim Lymington Jun 25 '14 at 13:17
  • Think of it this way: as noted in this answer, 12:01 pm is definitely in the literal 'after noon' period, so p.m. Likewise, 12:00:01 pm is also after noon, and logically also 12:00:00.000000000000001. In fact, by the time even the photons emitted from the clock at exactly noon have reached your eyes, it is already 'after noon'. So, we put p.m. there. – whybird Jun 16 '15 at 03:56
  • And, of course, since AM stands for "ante-meridiem" and PM for "post-meridiem", one could call noon "12:00 m." But that leaves us at a loss for what to call midnight. – Hot Licks Dec 01 '15 at 19:51
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12:00am is correct.
The clock goes from 12:00am (midnight) through 1:00am, 2:00am to 11:59am and then to 12:00pm (noon) through 1:00pm to 11:59pm and repeats.

It seems no more strange to go from 12:59 to 1:00 than it does to go from 11:59pm to 12:00am

Jim
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  • Disagree partly. While 12:00 midnight might be before noon (and qualify for "am"), how can 12:00 noon be after noon and qualify for "pm"? It's interesting that "midnight" is usually [in the UK at least] at the end of the day (midnight Tuesday is just after 23:59 on Tuesday) but 12:00am would put that time in Wednesday. – Andrew Leach May 03 '12 at 22:58
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    12:00PM is PM because "noon" is just an instant that occurs at the transition from 11:59AM to 12:00PM. The entire 60 second period following 11:59 is represented by 12:00PM, and thus after noon. – nohat May 03 '12 at 23:03
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Anyone who has ever had the kind of digital clock common here in the United States will have observed that the clock, when reset, starts at midnight: it reads 12:00 with the AM indicator set or the PM indicator not set, as the case may be. Then it proceeds for twelve hours to 11:59. After 11:59 it restarts the whole sequence at 12:00 but toggles the status of the AM/PM indicator. This process repeats every 12 hours: reach 11:59, then on the next minute read 12:00 but toggle the status of the AM/PM indicator. Thus the minute that succeeds 11:59PM is 12:00AM.

nohat
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  • Yes, clearly. 12:01 a.m. is clearly past midnight, and it would be completely stupid of 12:00 midnight was p.m., but 12:01 after midnight was a.m. – Kaz May 04 '12 at 07:50
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Legally speaking 12:00 am and 12:00 pm DO NOT exist and should NEVER be used in any contract, statute or anything else that could wind up in court (unless a specific jurisdiction has codified the terms, of which I am unaware). For example look at your car insurance card: most likely it states that the coverage is good from 12:01 am through 11:59 pm on dates in question. Now that doesn't mean there's a two minute gap upon renewal as it's legally understood that coverage runs from the very beginning of the day (12:00:00.0001- am) to the very end of the day (11:59:59.9999+ pm). I understand that some digital clocks and common perception may favor 12:00 am as midnight and 12:00 pm as noon, but that's purely personal preference and enough ambiguity exists to render both terms legally useless. In Latin AM (ante meridian) means before midday and PM (post meridian) means after midday. Since 12:00 noon is neither before nor after midday (it IS midday) it cannot be AM or PM. As for 12:00 midnight it could refer to AM (12 hours before the upcoming noon) or PM (12 hours after the preceding noon). Thus to avoid any confusion you should always use "noon"/"midnight", 11:59/12:01 or rely on the 24-hour clock designation.

Thee Eunice Burns
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    Please provide proof of your legal claims. Seeing as you haven't referenced any jurisdiction, I expect that you will be referencing international laws, or maybe some UN treaties? – curiousdannii Jun 16 '15 at 06:59
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    I speak as an attorney practicing in NYS, specializing in contracts. I haven't done the necessary research, statutory or common law, into this subject and thus ATM I cannot provide requisite "proof" of my claims. However I know from experience that using 12:00 am/pm can lead to problems, sometimes very costly ones. As a contract attorney one of my duties is to eliminate any ambiguity in wording/phrasing that could potentially be detrimental to my client and thus I would never use 12:00 am/pm. I prefer to use the 24 hour clock. – Thee Eunice Burns Jun 16 '15 at 22:58
  • Your preference is not the same as legal fact. – curiousdannii Jun 17 '15 at 00:43
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There is a subtle distinction with digital clocks that has not been mentioned. Most digital clocks display minutes but not seconds. So a valid interpretation of "12:00 AM" on a digital clock is "you are now in the minute that began at 12:00, and it is now ante meridian (AM, that is, before noon). True, the correct suffix, AM, PM, or neither, that should be used to label the instant of midnight is debatable, but since an instant lasts for an infinitesimal length of time, the display is correct for 99.999... percent of the minute.

When one is considering static text (like a contract), rather than a changing display, one can indeed discuss an instant so the correct notation (if any exists) to represent noon or midnight with numerals becomes important.

Gerard Ashton
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Nobody vocalizes twelve colon zero zero a m. So, no, it's not correct to say this.

SrJoven
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