# What was the rationale in designating midnight as 12AM? **URL:** \ **Category:** Factual Questions\ **Created:** [May 13, 2017, 2:37pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533 "2017-05-13T14:37:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Enola\_Gay](https://avatars.discourse-cdn.com/v4/letter/e/3ab097/32.png) [@Enola\_Gay](https://boards.straightdope.com/u/Enola_Gay)\ **Post date:** [May 13, 2017, 2:37pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/1 "2017-05-13T14:37:27Z")
It has never made sense to me that midnight is 12AM. You go from 8PM to 9PM to 10PM to 11PM to 12 **A** M?? It doesn’t fit the pattern! Same issue with 12PM being noon. 10AM then 11AM then 12PM. It seems like things should start all over at 1. This has always bothered me, but friends and family in my world have no issue with this, nor any rational explanation whatsoever. So Dopers, I turn to you: please tell me why, Why, WHY??? And I am sure this has been asked here before, but my search attempts came up empty. ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [May 13, 2017, 3:01pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/2 "2017-05-13T15:01:18Z")
No one has designated 12am as midnight nor as noon. Both 12am and 12pm are ambiguous and best avoided in favor of the words midnight and noon, or the use of a 24 hour clock (aka, military time). [Wikipedia](https://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noon_and_midnight) explains this in greater detail; am means before midday, while pm means after midday, so neither 12am nor 12pm applies to noon or midnight. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [May 13, 2017, 3:06pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/3 "2017-05-13T15:06:04Z")
> [@Enola\_Gay](#): > > It has never made sense to me that midnight is 12AM. You go from 8PM to 9PM to 10PM to 11PM to 12 **A** M?? It doesn’t fit the pattern! > > Same issue with 12PM being noon. 10AM then 11AM then 12PM. **It seems like things should start all over at 1.** This has always bothered me, but friends and family in my world have no issue with this, nor any rational explanation whatsoever. > > So Dopers, I turn to you: please tell me why, Why, WHY??? > > And I am sure this has been asked here before, but my search attempts came up empty. (emphasis added) I submit that if you think the entire hour between noon and 1PM should be designated AM, then you aren’t properly understanding what _ante meridiem_ and _post meridiem_ mean. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [May 13, 2017, 3:20pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/4 "2017-05-13T15:20:07Z")
For me, it’s consistent and logical if you understand what AM and PM stand for. 12 midnight is neither AM nor PM, but the instant afterward, it’s am. So, to me, it makes sense to designate it as AM. ---
**Author:** ![igor\_frankensteen](https://avatars.discourse-cdn.com/v4/letter/i/839c29/32.png) [@igor\_frankensteen](https://boards.straightdope.com/u/igor_frankensteen)\ **Post date:** [May 13, 2017, 3:27pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/5 "2017-05-13T15:27:22Z")
A more intriguing thing to discuss about this, would be why it is that so much of the world decided to divide their sense of the day in half as they do. Why do so many people see noon as some sort of dividing line? ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [May 13, 2017, 3:35pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/6 "2017-05-13T15:35:33Z")
> [@igor\_frankensteen](#): > > A more intriguing thing to discuss about this, would be why it is that so much of the world decided to divide their sense of the day in half as they do. Why do so many people see noon as some sort of dividing line? Well, it’s at the point where the sun is at its highest in the sky, or at least local noon is. So it seems to make sense to me as a midpoint between sunrise and sunset. Now why it’s “12,” I don’t know. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 13, 2017, 3:36pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/7 "2017-05-13T15:36:24Z")
> [@pulykamell](#): > > For me, it’s consistent and logical if you understand what AM and PM stand for. 12 midnight is neither AM nor PM, but the instant afterward, it’s am. So, to me, it makes sense to designate it as AM. Why is is necessary to designate it as AM or PM? Since there is no universally followed rule for this, if you want to avoid confusion just say 12 noon or 12 midnight. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [May 13, 2017, 3:39pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/8 "2017-05-13T15:39:49Z")
> [@Colibri](#): > > Why is is necessary to designate it as AM or PM? > > Since there is no universally followed rule for this, if you want to avoid confusion just say 12 noon or 12 midnight. That is exactly what I do. But then it just shifts the OP’s question to 12:01. Following their logic, one minute after midnight would be 12:01 p.m., wouldn’t it? ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [May 13, 2017, 3:42pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/9 "2017-05-13T15:42:42Z")
Let’s say you have a nanosecond precision clock. Consider the following three times: 11:59:59.999999999 PM 12:00:00.000000000 ?? 12:00:00.000000001 AM There’s a big rollover heading into midnight, not at all heading out. The switch from PM to AM fits right in with being part of the rollover. Seems quite simple and logical to me. But that doesn’t mean that there aren’t people who have problems with that. What a surprise. Remember those old mechanical digital clocks with the numbers on flaps that flipped over? The AM/PM designation would be on the hour flaps. Imagine the weirdness of designing such a clock if midnight was 12PM. At midnight it would flip to the 12PM flap. Then at 12:01 it would have to flip the hour flap again to read 12AM. (Or you’d have to have separate flaps for AM/PM that would flip at 12:01.) That would be odd enough that people would _really_ question the “logic” of it. And if the clock maker was in the “neither” crowd then there’d still be the same issue. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 13, 2017, 3:44pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/10 "2017-05-13T15:44:53Z")
> [@pulykamell](#): > > That is exactly what I do. But then it just shifts the OP’s question to 12:01. Following their logic, one minute after midnight would be 12:01 p.m., wouldn’t it? I think this is part of the logic for why noon is 12 PM. One minute past noon is 12:01 PM, by definition of PM. So if noon were 12:00 **A** M and one minute later it was 12:01 **P** M, things would be truly confusing. It’s easier to remember that 12:00 noon to 12:59 is PM and 12:00 midnight to 12:59 is AM. ---
**Author:** ![Enola\_Gay](https://avatars.discourse-cdn.com/v4/letter/e/3ab097/32.png) [@Enola\_Gay](https://boards.straightdope.com/u/Enola_Gay)\ **Post date:** [May 13, 2017, 3:52pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/11 "2017-05-13T15:52:16Z")
> [@Colibri](#): > > Why is is necessary to designate it as AM or PM? > > Since there is no universally followed rule for this, if you want to avoid confusion just say 12 noon or 12 midnight. Can you tell this to the airlines? This is where I always get into trouble–they have flights departing at 12:16AM or PM. If they used noon & midnight, this nonsense wouldn’t bother me so much. ---
**Author:** ![GESancMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gesancman/32/2923_2.png) [@GESancMan](https://boards.straightdope.com/u/GESancMan)\ **Post date:** [May 13, 2017, 3:56pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/12 "2017-05-13T15:56:47Z")
I understand the logic of all of the replies, but the way I’m understanding the OP’s question is: why was the number 12 designated as the divider? Why isn’t everything shifted “down” by an hour, so that the changeover from AM to PM happens at 1:00, where this is regarded as midday? Such that the last hour of “morning” would be 12:00:00 to 12:59:59, in the same way that the 11:00 hour is the last hour of morning now? Then noon is at 1:00:00, and so on? ---
**Author:** ![Channing\_Idaho\_Banks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/channing_idaho_banks/32/250_2.png) [@Channing\_Idaho\_Banks](https://boards.straightdope.com/u/Channing_Idaho_Banks)\ **Post date:** [May 13, 2017, 4:08pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/13 "2017-05-13T16:08:34Z")
> [@GESancMan](#): > > I understand the logic of all of the replies, but the way I’m understanding the OP’s question is: why was the number 12 designated as the divider? Why isn’t everything shifted “down” by an hour, so that the changeover from AM to PM happens at 1:00, where this is regarded as midday? Such that the last hour of “morning” would be 12:00:00 to 12:59:59, in the same way that the 11:00 hour is the last hour of morning now? Then noon is at 1:00:00, and so on? The same reason the calendar starts at year one. There is no year zero just like there is no hour zero. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [May 13, 2017, 4:14pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/14 "2017-05-13T16:14:23Z")
> [@pulykamell](#): > > That is exactly what I do. But then it just shifts the OP’s question to 12:01. Following their logic, one minute after midnight would be 12:01 p.m., wouldn’t it? > [@Enola\_Gay](#): > > Can you tell this to the airlines? This is where I always get into trouble–they have flights departing at 12:16AM or PM. If they used noon & midnight, this nonsense wouldn’t bother me so much. There is no ambiguity if you understand what AM and PM mean. 12:16 AM is unequivocally 16 minutes after midnight, and 12:16 PM is 16 minutes after noon. No one considers 12:16 AM to be 16 minutes after noon. AM covers all the time between midnight and noon. PM covers all the time between noon and midnight. It’s really very simple, once you realize that the “12” points are zero markers and not part of either AM or PM. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [May 13, 2017, 4:15pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/15 "2017-05-13T16:15:07Z")
> [@Channing\_Idaho\_Banks](#): > > The same reason the calendar starts at year one. There is no year zero just like there is no hour zero. But there is an hour one. ---
**Author:** ![GESancMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gesancman/32/2923_2.png) [@GESancMan](https://boards.straightdope.com/u/GESancMan)\ **Post date:** [May 13, 2017, 4:17pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/16 "2017-05-13T16:17:43Z")
> [@Channing\_Idaho\_Banks](#): > > The same reason the calendar starts at year one. There is no year zero just like there is no hour zero. You’re missing the point. I didn’t say anything about an hour zero. A rollover of 12 -\> 1 would make more sense than a rollover of 11-\> 12, with 1 then coming later. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [May 13, 2017, 4:19pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/17 "2017-05-13T16:19:33Z")
> [@Colibri](#): > > There is no ambiguity if you understand what AM and PM mean. That’s pretty clear from my first reply to this question (in which I also use the 12 midnight convention, instead of the 12 a.m. one). But people who are confused on this may also be confused as to 12:16 AM, since that means the 11 PM hour is followed by the 12 AM hour. It seems to me the OP is wondering/objecting about this, as well, but maybe the OP is, indeed, just asking the question at the end of my first post: why does midnight and midday start at 12 instead of 1? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 13, 2017, 4:20pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/18 "2017-05-13T16:20:41Z")
The question has been answered; just wanted to mention a related but even more dangerous ambiguity. The old-school convention, at least for sailors and astronomers, was to start/end the day at noon (easily determined by observing the sun) rather than midnight. The trouble was that some would reckon the end of a particular date at the same instant others considered it beginning. IIRC at least one ship was wrecked because the navigator misread his almanac and screwed this up. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [May 13, 2017, 4:22pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/19 "2017-05-13T16:22:45Z")
> [@Colibri](#): > > There is no ambiguity if you understand what AM and PM mean. 12:16 AM is unequivocally 16 minutes after midnight, and 12:16 PM is 16 minutes after noon. No one considers 12:16 AM to be 16 minutes after noon. > > AM covers all the time between midnight and noon. PM covers all the time between noon and midnight. It’s really very simple, once you realize that the “12” points are zero markers and not part of either AM or PM. Well… a.m. means _ante meridiem_, which translates to “before noon [midday]”. 12:16 a.m. is not 16 minutes after anything by meaning of “a.m.”; it’s the 16th minute after 12:00 in the half of the day before mid-day (noon). ---
**Author:** ![Acsenray](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/acsenray/32/4519_2.png) [@Acsenray](https://boards.straightdope.com/u/Acsenray)\ **Post date:** [May 13, 2017, 4:23pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/20 "2017-05-13T16:23:26Z")
> [@Dewey\_Finn](#): > > No one has designated 12am as midnight nor as noon. Both 12am and 12pm are ambiguous and best avoided in favor of the words midnight and noon, or the use of a 24 hour clock (aka, military time). [Wikipedia](https://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noon_and_midnight) explains this in greater detail; am means before midday, while pm means after midday, so neither 12am nor 12pm applies to noon or midnight. This is wrong. Regardless of the technical definition of meridian, as a practical matter, there is no ambiguity. 12 am is midnight and 12 pm is noon. Whether that should be the case is a nice theoretical argument, but it’s silly to pretend that in practice there’s any ambiguity. [Next page](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533.md?page=2)