Wow! Great answer, @tchrist. I've often tried to explain this to people, but from now on I'll just refer them here. One thing you don't mention is the rise of digital clocks and how that affected how we think of AM and PM. Because it's cheaper not to have lights for "NOON" and "MIDNIGHT", digital alarm clocks with a single indicator light for AM/PM have had to choose one or the other. Nowadays people are telling time with "smart watches" and "smart phones" that could easily show the words noon or midnight, so perhaps this 12AM/12PM anomaly will be just a blip in English's evolution.

As a ref: Date/Time representations are standardized in ISO 8601.

I am so happy to live in a country with a language where "seventeen o'clock" is perfectly valid and hence "twelve o'clock" is noon and "zero / twenty-four o'clock" is midnight... ;)

@elzell What country has clocks whose hour hand circles through twenty-four positions once per day instead of twelve positions twice per day? So when your clock is at ninety degrees it's really six o'clock not three o'clock like ours?!? I've never seen a clock like that! I don't know that I could always tell one hour from the next at only fifteen degrees per hour instead of the normal thirty.. Isn't that really hard for visitors to your country?

@hacker9 I put a little bit of that in there now, but I haven't admitted that the 8601 system allows for 00:00:00 of day N+1 and 24:00:00 of day N to name the very same instant, something confusing and blissfully impossible under the old system. I worry that if we talk about standards too much we risk stumbling into the infinite abyss of leap seconds, daylight saving time, and other eldritch monstrosities better left dead and dreaming in their cyclopean tombs. :)

@tchrist, at this point in history there should be no need to take “o’ clock” literally or to pretend that clocks are all analog. In German we use “17 Uhr” without a problem and perceive it as neither strange nor overly formal. In conversation we will still often use a 12 hour based system and refer to this as “5 Uhr”, or as “5 Uhr nachmittags” if we feel that we have to clarify. There is no fixed abbreviation for this, though, because you can always use the 24 hour based system in writing. Indeed, we would not say “9 Uhr nachmittags”, as 21:00 is “abends” (evening/night), not afternoon.

You seem to be using characters that don't show up on Android browsers. They are also critical to understanding the text; e.g. « : Virtually all style guides tell people to stop using the irresolvably ambiguous twelve o’clock and twelve o’clock in favor…» and «…don’t ask why non- / times …»

@JDługosz I'm quite surprised to learn that, considering that I'm using an Android browser and they show up fine for me!

@CarstenS I wonder if there isn’t a mistranslation here. Although you're welcome to say seventeen hundred hours in English, it still sounds rather technical and perhaps even a bit foreign to us, perhaps as though you were in the military. We do use that lingo with computer times, for example. But if you say zero o’clock or seventeen o’clock, it sounds really very strange in English, and you may even be at risk for confusing people.

I understand that the usage in English differs from that in German. I am just saying that the reason is just convention and not that the English expression was fundamentally different (although our “Uhr” does not have a preposition, but I wonder how much the “o’” in “o’clock” is still perceived as one) or our clocks had arms that go around just once in a day. So, yes, “17 o’clock” is not used, but not because it was silly, but just because it isn’t used, even if might indeed be a good idea. The silly thing is more that 17 hundred business, in my opinion :)

@tchrist Such clocks (and watches) have existed: see 24-hour analog dial. Great answer, though (although being pedantic, I must pull you up on "When the bells of Big Ben" ... Big Ben is the main bell, not the clock nor the tower).

Surely military time is read out as though each hour is a hundred hours? When you say "seventeen hundred dollars", you don't mean 17 dollars and zero cents.

@tchrist I have a 24-hour clock widget on my Android phone (that also shows sunrise/sunset); they're quite nice.

@Fax You mean like how 🕗 — meaning eight ᴀᴍ sharp — is pronounced oh-eight-hundred in military time?

@tchrist Yeah right..... exactly like that, almost as if you were reading out the eight hundred minutes which there aren't and you weren't. Cause you were reading out "eight hours" and "a hundreds worth of decimal place holders". A total mess to keep things simple.

@KalleMP What's so hard about 0030, or zero dark thirty ;)?

It's worth noting that the confusion around whether 5:00 is 5:00 AM 24-hr or 5:00 PM 12-hr can be resolved by notating the 24-hr version as 05:00 and most people will at the very least realize that it's in the morning.

@tchrist see android.stackexchange.com/questions/119252/…

"When someone refers to 'midnight tonight' or 'midnight last night' the reference of time is obvious." No, sadly it's not. Some people call "tonight" the night that ended this morning, some the one that starts this evening. There are even people who say "last night" if they mean the night that ended yesterday morning. For some these two terms even change meaning at some point of the day.

@Fabian who says "tonight" to mean the night that ended this morning? I notice your location in your profile is Germany, is it possible these are non-native speakers who are simply mistaken? Oxford and Merriam-Webster both define it as unambiguously being the night that is either ongoing or approaching at the end of the current day, and as a native speaker I've never encountered any other usage.

@ChrisH In German it's "diese Nacht", literally "this night". But no matter which language, if it's currently night, but after midnight, you would probably mean the night you're currently in. And if it's night and before midnight, you also probably mean the night you're currently in. The question then is when the definition changes. At exactly 12:00 noon? But you also sometimes talk about the night without actually knowing the exact current time. So it's complicated as well.

@Fabian I speak German too. But the question's about English, not German. You can't dismiss that by saying "no matter which language", languages are different. Neither "diese Nacht" nor "heute Nacht" is exactly the same as "tonight". If it's currently night then "tonight" means the current night; if it's not currently night then "tonight" means the approaching night. It's absolutely that simple, "tonight" never refers to a night that already ended. The only time there's any room for ambiguity is if it's not clear whether night has turned to morning yet - people might disagree at, say, 5am.

Big Ben does not have a "face". Big Ben is a bell.

@ChrisH Yeah, there is so much room for dissagreement that is it often best to make things explisit if there is a significant cost to a misunderstanding. Tonight might not even relate to the time it i9s said (5am or 8am) but might change interpretation depending on weather the person has gone to sleep yet (or should have gone to sleep) in for instance doing shift work. Also as Fabian said if one is drunk or in a basement one may have no way to reference which night is in question with tonight. I would say the general reference would be the coming night unless context is clear otherwise.

If it's 01:30 and someone says "It happened at midnight tonight" that would be 1½ hours ago.

Pretty good answer. I would say that teaching kids to start with zero doesn't mean telling them to call the first object zero. What we should do is have kids start with zero (holding up two fists with no fingers) and then count from there. Then you ask them to count zero items. Even in computer science, the first index in an array is labelled zero. The term zeroth is pretty bogus. This is actually perfectly analogous to one of the earliest things we learn: age. You are not 1 until the first year of your life passes. Age is zero based and it's perfectly simple to understand.

I disbelieve that this has much to do with (digital) computers, because the switch from referring to the first hour of the day as "hora prima" to referring to everything inside it as "12:something", where something might be a minute or a minute+second, happened before they existed. But it does have to do with the difference between the former (1-based) and the latter (somewhat bemused) count, and I don't think it affects the rest of the argument whether the catalyst was digital computers or just the practical need for minute precision (which I'd guess was railways).

In any case it could be seen without reference to either digital computers or zero-based counts, that it is in some sense incorrect to refer to the actual (well, OK, the nominal) meridian as being either "am" (meaning, before the meridian) or "pm" (after the meridian). Which of course doesn't imply that people who predate both those things actually saw it that way. When referring to the point of time, clearly it's neither. When referring to the minute commencing at that point of time it is clearly after (in measure theory we'd say that minute is almost-everywhere pm!)

I would say that noon is an infinite small instant at the very moment between 11:59:59.999999~ and 12:00:00.000000~, but clearly the "time guys" at the NIST don't agree. I don't think that the question "what is the duration of noon?" can be easily answered.

@gerrit Tonight is a future or present time, not a past one.

@IllusiveBrian Really? I can't say, "What did you do tonight" or "Did you enjoy the film tonight" at the end of an evening, just before going to bed?

@ChrisH you must be lucky. Tonight is an ambiguous word. Generally, I have to clarify with those around me whether they're talking about "now" or the night period that just elapsed (if it's morning) or talking about the next night period that has not yet begun. Most people don't seem to realize there was a need to disambiguate their usage of the term.

@iheanyi can I ask where you're from? As far as I'm aware I have literally never encountered anybody using "tonight" to refer to a night that is already over. I also looked up in several dictionaries before posting my previous comments just to be sure I wasn't overlooking anything, and didn't find any suggestion in any of them that "tonight" could ever mean "the night that just finished". Now I look again, that sense is on dictionary.com, although it's marked obsolete.

In addition to the conceptual challenge of transitioning to zero based counting, our clocks have also suffered through the turmoil of transitioning from noon as the start of a new day, to midnight as the start of a new day. This happened as a cultural phenomena primarily due to railroad and bus tickets being good for one day, and schedules being published for a midnight-to-midnight period, so rather recently. Some current documents still reflect the former system. I keep my ship's log that way, starting a new entry at noon.

@ChrisH I live in the USA and have lived in various places throughout the country. In every single locale, I've dealt with ambiguous usage of "tonight". Where are you from that this does not occur?

I’m sorry to point this out but Big Ben neither has a face nor bells. Big Ben is the large bell which rings each hour. The face is the the face of the clock in the Elizabeth Tower.

@iheanyi I'm from England. I've posted a question on ELU to see if anybody can shed more light on this - I hope you don't mind me quoting some of your comments to explain where the question comes from.

@ChrisH I'm interested in the responses to that question as well. Thanks for providing a link.

Please could you link to evidence of the way you say the hours used to work? If hours really used to work that way, 12:59pm would've been called "12:59am", and pm would begin at 1pm. Was am used that way?

Your term "traditional one-based numbering system" is misleading, because 1 is not the base; it's just the numerical label for one object, which is no more salient than the label for any other.

@PhilSweet So, before there were railways and buses, up to the early 19th century, a day's morning would be regarded as the same day as the previous afternoon?? Please could you point to some evidence for this? It would mean for example that the religion's holy day (Sabbath or Sunday or whichever) would be an afternoon and the following morning -- which seems odd to me.

@RosieF You have misinderstood. Noon used to be 12am and midnight was 12pm. Nobody ever said that times after noon used to be before noon. That would be something else, and silly. Don't think about minutes or seconds; just think about hours. Otherwise you'll get crazy things like asking whether a quarter before noon is AM or PM.

@tchrist: Perhaps you skimmed over it, but Phil Sweet said, “…our clocks have also suffered through the turmoil of transitioning from noon as the start of a new day, to midnight as the start of a new day.”  This is what Rosie F is asking about.  She never suggested that times after noon were before midday; she is asking about Phil’s statement which suggests that 11 AM Friday would be followed (two hours later) by 1 PM Saturday, and that 11 PM Saturday would be followed (two hours later) by 1 AM Saturday.

I was one day early to a "midnight release" event because I understood midnight to be the very start of that day, but the even was actually at the end of the day. I now see many "midnight release" events with a time of 11:59PM to avoid confusion and it makes me happy when I see that.

@Scott She asked me to link to something that showed the hours used to have noon be AM with the previous eleven hours instead of saying that noon as actually after noon. Lots of stuff used to do that, including the USGPO. And I did link to a reference to that.