# 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:** 2
**Author:** ![GinoC](https://avatars.discourse-cdn.com/v4/letter/g/f4b2a3/32.png) [@GinoC](https://boards.straightdope.com/u/GinoC)\ **Post date:** [May 13, 2017, 4:27pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/21 "2017-05-13T16:27:36Z")
In military operational writing the problem is avoided by never using 0000 or 2400. Only 0001 or 2359 are permitted. ---
**Author:** ![Ann\_Hedonia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ann_hedonia/32/319_2.png) [@Ann\_Hedonia](https://boards.straightdope.com/u/Ann_Hedonia)\ **Post date:** [May 13, 2017, 4:28pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/22 "2017-05-13T16:28:32Z")
> [@DPRK](#): > > 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. I used to work for a company that did setups for large corporate events. It was not uncommon for us to get access to the facility starting at midnight. Our crews would then work all night to set up a large breakfast or luncheon event. Our crew call sheets ALWAYS gave the call time as (Day of Week, 12:01AM). It kept people from showing up on the wrong day. ---
**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, 4:28pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/23 "2017-05-13T16:28:41Z")
> [@GESancMan](#): > > 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. Thank god for you **GESancMan**. I finally feel understood 🙂 ---
**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, 4:28pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/24 "2017-05-13T16:28:42Z")
Here’s a more interesting question. To me, Tuesday night lasts until dawn in Wednesday morning. Wednesday morning doesn’t start until dawn. ---
**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, 4:28pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/25 "2017-05-13T16:28:44Z")
> [@pulykamell](#): > > 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? Because you are just looking at the hour rather than at the minutes. 12:01 PM is one minute post meridian. 1:00 PM is one hour post meridian. If you are using an ante/post meridian system, then it makes no sense to start it at 1 hour past the meridian rather than one minute (or one second). ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [May 13, 2017, 4:28pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/26 "2017-05-13T16:28:58Z")
If you’re using the 24 hour clock, then there is an hour 0. Anyway, isn’t noon really “12 m”? ---
**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, 4:32pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/27 "2017-05-13T16:32:57Z")
> [@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? Yeah it would seem to make more sense if 6am/pm were the dividing lines. Start off the day in the fields/farm at 6am, return at 6pm. Better yet, I wish we’d just switch to military time, but despite what I was told in grade school, that’s a pipe dream, just like converting to the metric system ☹ ---
**Author:** ![Siam\_Sam](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Siam\_Sam](https://boards.straightdope.com/u/Siam_Sam)\ **Post date:** [May 13, 2017, 4:37pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/28 "2017-05-13T16:37:34Z")
In Thai hotels, it’s common for English signs at the front desks to say, “Check-out time, 12:00 AM.” They mean noon, of course. At the English-language newspapers I worked at there, we were instructed never to use 12am or 12pm but rather midnight and noon. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [May 13, 2017, 4:37pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/29 "2017-05-13T16:37:50Z")
> [@Monty](#): > > Anyway, isn’t noon really “12 m”? Yes, yes it is. ---
**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, 4:38pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/30 "2017-05-13T16:38:29Z")
> [@Colibri](#): > > Because you are just looking at the hour rather than at the minutes. 12:01 PM is one minute post meridian. 1:00 PM is one hour post meridian. If you are using an ante/post meridian system, then it makes no sense to start it at 1 hour past the meridian rather than one minute (or one second). One can also think of it as “the first hour” post meridian, which to me makes as much, if not more sense, than saying 12:05 is five minutes past meridian, rather than 12 hours 5 minutes post meridian or something like that. Look, I understand what a.m. and p.m. mean. It’s a fair enough question to ask why one minute post meridian is 12:01? Doesn’t that seem odd to you at all? ---
**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, 4:44pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/31 "2017-05-13T16:44:22Z")
To people arguing there is obviously no ambiguity, the Wikipedia table linked to above gives plenty of examples with midday designated 12 a.m., on one hand (e.g, by the US Government Printing Office in 2000), and with it designated 12 p.m. on the other (e.g., by the same US GPO in 2008!) So there is plenty of ambiguity. Not to mention the confusion evinced in this thread! BTW any reasonable calendar obviously has a year zero: for instance 0 a.d. = 1 b.c, -1 a.d. = 2 b.c., etc. (Note this Christian calendar actually started up after 500 A.D. anyway.) To keep the number positive, astronomers use Julian days. Which start at noon. ---
**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, 4:48pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/32 "2017-05-13T16:48:42Z")
> [@pulykamell](#): > > Look, I understand what a.m. and p.m. mean. It’s a fair enough question to ask why one minute post meridian is 12:01? Doesn’t that seem odd to you at all? As I mentioned, noon used to be 0h astronomical time. But the civil day begins at midnight, so noon is 12h. Therefore one minute past noon must be 12:01. (If you are up during the wee hours of the night, is it late, or just really early? Past midnight you are closer to noon, so it must be a.m.) ---
**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, 4:59pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/33 "2017-05-13T16:59:22Z")
ETA 12 and 0 are congruent modulo 12… So 12:01 p.m. = 0:01 p.m. I see your point about wanting a minute after noon to be 0:01 p.m., but it seems this convention was never used except by the aforementioned astronomers who were using a 24-hour clock anyway. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [May 13, 2017, 5:55pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/34 "2017-05-13T17:55:35Z")
> [@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? > [@Channing\_Idaho\_Banks](#): > > The same reason the calendar starts at year one. There is no year zero just like there is no hour zero. > [@Enola\_Gay](#): > > Thank god for you **GESancMan**. I finally feel understood 🙂 I see the OP’s point that it seems to make little sense that the label 12 is the _beginning_ of the morning or the _beginning_ of the afternoon. But it would make even less sense to have the morning of afternoon start at 1:00. When you’re ten minutes into the morning, the time should logically be zero hours and 10 minutes, not 1 hour and ten minutes. So if you just think of the time that we label 12:XX as indicating zero hours and XX minutes (as **DPRK** says just above), it does make sense. ---
**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, 6:13pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/35 "2017-05-13T18:13:20Z")
> [@GinoC](#): > > In military operational writing the problem is avoided by never using 0000 or 2400. Only 0001 or 2359 are permitted. Well, 2400 makes about as much sense as saying it’s 13 o’clock (civilian time). And if you have enough functioning brain cells to understand what 0001 is then understanding clearly what 0000 is should hardly be an issue. ---
**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, 6:53pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/36 "2017-05-13T18:53:32Z")
> [@Riemann](#): > > I see the OP’s point that it seems to make little sense that the label 12 is the _beginning_ of the morning or the _beginning_ of the afternoon. But it would make even less sense to have the morning of afternoon start at 1:00. When you’re ten minutes into the morning, the time should logically be zero hours and 10 minutes, not 1 hour and ten minutes. So if you just think of the time that we label 12:XX as indicating zero hours and XX minutes (as **DPRK** says just above), it does make sense. When I become Queen, I’m changing 12 to 00. I was always a whiz at those ‘what comes next in the sequence’ tests, so the fact that 12AM & PM deviate from the pattern just grinds my gears. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 14, 2017, 2:12am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/37 "2017-05-14T02:12:15Z")
While we’re at it with the hypothetical nanosecond clock: When it’s daytime and the clock reads 12:00:00.000000001, that’s after noon, right? But now think: Most clocks are not nanosecond clocks. At that exact moment (which we agree is after noon), what would an ordinary, 1-second clock be reading? ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [May 14, 2017, 2:18am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/38 "2017-05-14T02:18:07Z")
> [@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? It really isn’t 'much of the world". Actually, it’s only the British & former British colonies that still use AM/PM times. Nearly everyone else uses 24-hour clocks now. Of course, that includes the UK & USA, rather important countries in the economy. ---
**Author:** ![Lord\_Feldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lord_feldon/32/341_2.png) [@Lord\_Feldon](https://boards.straightdope.com/u/Lord_Feldon)\ **Post date:** [May 14, 2017, 3:53am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/39 "2017-05-14T03:53:43Z")
> [@Acsenray](#): > > Here’s a more interesting question. To me, Tuesday night lasts until dawn in Wednesday morning. Wednesday morning doesn’t start until dawn. They overlap for me. 4:00 AM is Tuesday night if I’m still up from Tuesday, but Wednesday morning if I have to get up early. ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [May 14, 2017, 4:34am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/40 "2017-05-14T04:34:50Z")
> [@Ann\_Hedonia](#): > > Our crew call sheets ALWAYS gave the call time as (Day of Week, 12:01AM). It kept people from showing up on the wrong day. The starting date/time and ending date/time of my auto insurance policy (and I presume ALL auto insurance policies) are shown similarly. [Previous page](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533.md?page=1) [Next page](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533.md?page=3)