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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)?

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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 3 '12 at 22:49
General Reference. The day starts at midnight. Midnight belongs to the new day –  FumbleFingers May 3 '12 at 23:32
@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 4 '12 at 0:01
@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 4 '12 at 1:52
This is why 24h notation is inherently better. ;) –  Sonic42 May 4 '12 at 3:16

4 Answers 4

up vote 11 down vote accepted

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 a.m." and "12:00 midnight" for "12:00 p.m." Realize that readers may be confused which day 12:00 midnight belongs to, so make sure context makes it clear.

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Or use 24h notation. :-) –  Jez May 3 '12 at 22:50
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 3 '12 at 23:11
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 3 '12 at 23:16
12:00 a.m. refers to midnight unambiguously because 12:15 a.m. refers to quarter past midnight. –  Kaz May 4 '12 at 7:46
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). –  TimLymington Jun 25 '14 at 13:17

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

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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 3 '12 at 22:58
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 3 '12 at 23:03

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.

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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 4 '12 at 7:50

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

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Methinks thou readest too literally. -1 –  medica Oct 16 '14 at 11:38
@medica I'm willing to take the downvote. But nobody answered the question as stated, so :) –  SrJoven Oct 16 '14 at 11:43

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