# 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:** 15\ **Page:** 5
**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 15, 2017, 10:11pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/81 "2017-05-15T22:11:11Z")
> [@SCAdian](#): > > And which airports let them do it? Post #62 goes over this. Even my local large airport (O’Hare) has about a dozen or so flights each night that leave between midnight and 2 a.m. I count 15 for today. ---
**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 15, 2017, 10:14pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/82 "2017-05-15T22:14:23Z")
> [@CookingWithGas](#): > > I don’t following this logic. The reason that 0 is neither positive nor negative is by definition. We can just as reasonably define midnight to be the last point in time for a day before the next day starts. There does not have to be a point defined as a boundary point between two days. > [@CookingWithGas](#): > > Comparing midnight to the number 0 is a false analogy. I… think I agree with you: It looks analogous to me _because_ it’s a matter of definition. Midnight isn’t automatically, inherently, inevitably part of one day, or the next, or neither, but we could choose to designate it in any of these ways. ---
**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 15, 2017, 11:17pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/83 "2017-05-15T23:17:20Z")
> [@Thudlow\_Boink](#): > > Asking which day midnight belongs to is like asking whether zero is positive or negative. I thought midnight always belonged to the next day. On New Year’s Eve, when the clock strikes 12, it is the next year…so wouldn’t that mean that midnight belongs to the next day on any random day? ---
**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 15, 2017, 11:26pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/84 "2017-05-15T23:26:57Z")
> [@Enola\_Gay](#): > > On New Year’s Eve, when the clock strikes 12, it is the next year…so wouldn’t that mean that midnight belongs to the next day on any random day? Not necessarily. Another interpretation is that, if you are hearing the clock strike midnight, that means it’s already past the midnight point and is now the next day/year. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [May 16, 2017, 12:06am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/85 "2017-05-16T00:06:32Z")
> [@CookingWithGas](#): > > You can define 12:00 midnight as the last point in time in a day. There is no reason, mathematical or otherwise, that point cannot be part of that day. It is not required to define a boundary point that belongs to neither day. Ah, I grok what you are saying. It wasn’t clear to me before. My apologies. 🙂 ---
**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 16, 2017, 12:10am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/86 "2017-05-16T00:10:09Z")
> [@Enola\_Gay](#): > > I thought midnight always belonged to the next day. On New Year’s Eve, when the clock strikes 12, it is the next year…so wouldn’t that mean that midnight belongs to the next day on any random day? No. This has been explained over and over again in this thread. ---
**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 16, 2017, 12:16am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/87 "2017-05-16T00:16:56Z")
> [@Colibri](#): > > No. This has been explained over and over again in this thread. Right but it’s still confusing because according to this thread (as I understand it), 12:00 am is today, but 12:01 is tomorrow. Just seems weird & illogical. ETA that not everyone seems to agree on this. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [May 16, 2017, 12:34am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/88 "2017-05-16T00:34:35Z")
24-hour clock (to simplify): 0000 = today 0001 = today … 2300 = today 2359 = today 2400 = today = 0000 tomorrow 0001 = tomorrow The statement that the “instant” of midnight might belong to the day ending rather than the new day was a hypothetical rhetorical suggestion only; do not get confused thinking there is such a standard. ---
**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 16, 2017, 3:31am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/89 "2017-05-16T03:31:30Z")
> [@DPRK](#): > > The statement that the “instant” of midnight might belong to the day ending rather than the new day was a hypothetical rhetorical suggestion only; do not get confused thinking there is such a standard. The instant of midnight \*does \*belong to the day ending rather than the new day. It is _ **by convention** _. There is no reason we couldn’t have another convention that is different, but this is the convention that we have. If you choose to disregard this convention then you might just as well decide there are 100 hours in a day and 100 minutes in an hour, because you think it makes more sense that way. ---
**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 16, 2017, 3:49am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/90 "2017-05-16T03:49:02Z")
> [@Enola\_Gay](#): > > Right but it’s still confusing because according to this thread (as I understand it), 12:00 am is today, but 12:01 is tomorrow. Just seems weird & illogical. You still don’t understand it then. According to the AM/PM system\* THERE IS NO 12:00 AM. It doesn’t exist. Nothing. Nada. Nohow. Noway. Midnight belongs neither to today nor tomorrow. It’s between the days. \*As noted, there may be some conventions that designate noon or midnight as being AM or PM, but there is no real consensus on this. ---
**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 16, 2017, 4:11am UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/91 "2017-05-16T04:11:26Z")
> [@CookingWithGas](#): > > The instant of midnight \*does \*belong to the day ending rather than the new day. It is _ **by convention** _. What convention? I think of midnight generally as beginning the day (no doubt influenced by 0:00 of the 24-hour clock), so Tuesday midnight to me means the zero hour between Monday and Tuesday. But it depends on the context. If something is ending at midnight, I assume it belongs to the day before. If something is starting at midnight, the day starting. (Personally, I always says something like “midnight Monday going into Tuesday” for things like this.) ---
**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 16, 2017, 8:09pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/92 "2017-05-16T20:09:51Z")
I will capitulate my position. There are some conventions used where 12:00 PM means noon and 12:00 AM means midnight. However, for the sake of anchoring this discussion to some authority I will refer to the (U.S.) [National Institute of Standards and Technology](https://www.nist.gov/ology.me-and-frequency-division/times-day-faqs). > [@NIST](#): > > Are noon and midnight referred to as 12 a.m. or 12 p.m.? > This is a tricky question because 12 a.m. and 12 p.m. are ambiguous and should not be used. > To illustrate this, consider that “a.m.” and “p.m.” are abbreviations for “ante meridiem” and “post meridiem,” which mean “before noon” and “after noon,” respectively. Since noon is neither before noon nor after noon, a designation of either a.m. or p.m. is incorrect. Also, midnight is both twelve hours before noon and twelve hours after noon. > It is fair to say, however, that the shortest measurable duration after noon should be designated as p.m. For example, it would be applicable for a digital clock changing from 11:59:59 a.m. to 12:00:00 to indicate p.m. as soon as it the 12:00 appears, and not delay the display of the p.m. by a minute, or even a second. The same is true for midnight, but there is an added issue of which day midnight refers to (see below). > Hours of operation for a business or other references to a block of time should also follow this designation rule. > For example, a business might be open on Saturdays from 8 a.m. to noon or weekends from 3:30 p.m. until midnight. > Is midnight the end of a day or the beginning of a day? > When someone refers to “midnight tonight” or “midnight last night” the reference of time is obvious. However, if a date/time is referred to as “at midnight on Friday, October 20th” the intention could be either midnight the beginning of the day or midnight at the end of the day. > To avoid ambiguity, specification of an event as occurring on a particular day at 11:59 p.m. or 12:01 a.m. is a good idea, especially legal documents such as contracts and insurance policies. Another option would be to use 24-hour clock, using the designation of 0000 to refer to midnight at the beginning of a given day (or date) and 2400 to designate the end of a given day (or date). I will continue to maintain that there no particular reason that it has to be this way, and it is by convention. But if the U.S. government says this is the convention, who am I to argue? ---
**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 16, 2017, 8:34pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/93 "2017-05-16T20:34:19Z")
_Which_ convention are you referring to? You quoted something from the NIST that says various things, including that 12am and 12pm are meaningless. ---
**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 16, 2017, 11:02pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/94 "2017-05-16T23:02:09Z")
> [@Dewey\_Finn](#): > > _Which_ convention are you referring to? You quoted something from the NIST that says various things, including that 12am and 12pm are meaningless. Maybe you skipped this part: > [@CookingWithGas with a degree of humility](#): > > I will capitulate my position. After doing a little research I have found that the reason that standards are so great is that there are so many of them. There is much disagreement on this but I have decided to go with the federal agency responsible for setting standards. Which is different than where I started out. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [May 16, 2017, 11:25pm UTC](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533/95 "2017-05-16T23:25:03Z")
> [@CookingWithGas](#): > > … > After doing a little research I have found that the reason that standards are so great is that there are so many of them. … Ref [xkcd: Standards](https://xkcd.com/927/) [Previous page](https://boards.straightdope.com/t/what-was-the-rationale-in-designating-midnight-as-12am/786533.md?page=4)