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Comment by tomrod | Hacker News Reader

I appreciate your thoughts. You made me realize AM (antemeridian, before noon) and post meridian (after noon, 24 hours mod 12) aren't universal standards!

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One kind of bad localization that happens every now and then is that the time is given in 12-hour clock but the am/pm particles are translated (to some reasonable thing). While a proper localization in those cases would just display 24-hour time without any am/pm equivalents.

Yeah, but because if analog clocks here in Austria we have no problem to convert back and forth. Are there no analog clocks in Norway?

There are, but we just skip the AM/PM part most of time, because it's obvious from context which one you mean. And if it's not you'll just say "... in the morning/afternoon/evening"

AM: morning, PM: afternoon/evening

Yeah, but this is the problem - why noon is "afternoon"?

"12:00:00 + ε" is after noon, but your clock will not show a difference.

Clopen intervals in order to have a well-specified time system.

[0] [12:00 PM, 12:00 AM) for after the noon instant

[1] [12:00 AM, 12:00 PM) for before the noon instant

24-hour time is superior to the mod-12 time system in almost every way but mechanical clock faces and soldier shifts under early systems that used sun dials. However, the equivalent question can be asked by a day is a clopen interval of [00:00 current day, 00:00 next day)

In 24-hour time, those are 12-24 and 0-12, respectively. https://en.wikipedia.org/wiki/ISO_8601

A place open 24 hours/day is open in the period 0-24.

I don't follow your point. Are you correcting my point on clopen intervals and mod bases?

I think clopen is too complicated in that it expresses ranges that are not practically useful, like (10,11], and it complicates expressing something like "Open 0-24", where your notation appears to require a date.

I think you get the essential utility you want in a 24 hour clock by simply letting 24 (or "24:00:00" when fully written out) be a way to represent the end of the day.

It's also already in use, with examples at https://ioi2023.hu/faq/ , http://www.garageferrarimilazzo.com/en/ , and https://jozsefvaros.hu/english/news-in-the-district/2022/12/... .

Noon isn't really before or after noon, but we want to lump it together with one of them, so we picked the one that made 12:00 and 12:01 go together. (Edit: or for that matter, 12:00:00 and 12:00:01, down to arbitrary closeness, as sibling comment points out)

People in 24h cultures always use 12 analog hours in normal situations, but the 24h system is needed for situations where it is easy to make a mistake.

When is dinner?

- It's at seven.

When does the flight depart?

- At 19:00

> People in 24h cultures always use 12 analog hours in normal situations

No, we don't. The only person I know who by default uses 12h is my grandmother. Everybody else always uses 24h.

Do you mind me asking where you are?

All my experience has also been using 12h time in casual situations in 24h countries. I'm curious of a country that doesn't, and whether that's true of the whole region.

Poland. It seems to be more of a generational thing than regional. People over the age of 70 seem to use 12h. Younger ones use 24h.

Ireland, New Zealand and Finland used 24 casually when I was there

Ireland uses 24h clock in writing (including casual writing, text messages, etc.), and it wouldn't be at all unusual for personal digital clocks to be set to 24h clock, but I'd be surprised to hear it in speech.

I've never heard anybody from a 24h country use 24h unless it's regarding situations where there cannot be any ambiguity. You either live in an exceptional place, or interact with exceptional people. World you really say "At fifteen o'clock" instead of "At three o'clock" or "Three in the afternoon"?

Yes.

You obviously haven't been to France.

But it's still not helpful with AM/PM. If I see 12 and it's dark outside then I know it's midnight, otherwise it's noon, and that's what I will say to someone who will ask "what time is it?". I still don't know if it's PM or AM.

Ah right, for that particular time its difficult. Yeah, I have to google that.

Noon and midnight are especially difficult for people used to the 24 hour system. Noon is neither ante nor post meridiem. And midnight is both.

I don't follow - Noon is 12pm, midnight is 12am.

The word "meridiem" means "noon", the middle of the day. Noon itself is neither before nor after noon.

You're right that by convention noon is assigned a pm tag and midnight is assigned an am tag. But midnight, when the calendar date changes, is confusing enough to people that e.g. airplane departures will be scheduled for 11:59 pm rather than 12:00 am, because there's no ambiguity about when 11:59 pm will happen on Wednesday, whereas it isn't clear when midnight will happen on Wednesday -- technically, if it's Wednesday, midnight already happened on Tuesday night.

It is far more common for people to end their personal day after midnight than it is to begin it before midnight, so the convention directly contradicts the way pretty much everyone views "the day".

Which is very confusing. Why does 12pm follow 11am and 12am follow 11pm?

Maybe because 12:00 being consistent with 12:01 was more important than it being consistent with 11?

I personally enjoy using 24-hour clocks on all devices I can.

Speaking of 24 hour clocks: my oven clock shows midnight as 24:00, and then changes to 00:01 at 1 minute past midnight. I've never seen any other clock show that; very strange.

I thought that was normal at least when writing or saying things.

Use of either mechanical displays or on computers time selecter widgets does mean that 24 is not shown anywhere,

Because that's how the numbers are arranged on the clock face/sun dial

I grew up in the UK and still couldn’t tell you this off the top of my head. Most people can’t - when setting up systems with a cutoff date/time I’ve regularly been asked to have them expire at 11:59pm, because it avoids any ambiguity here.

I think that's generally a sensible idea. Anything which required a schedule for humans to read - theatre performances, trains, planes, etc - are better off being scheduled for 23:55, 23:59, 00:01 or 00:05 than exactly at midnight because lots of people don't understand or get confused whether 00:00 on a date means the start or the end of the day.

I do enjoy though where systems have to deal with 'weird' days. I believe in the UK the rail ticketing system works on essentially a 26h day - that is to say if you buy a ticket at on 5th November, it will be valid until 04:00 on 6th November, so in some sense the hours 00:00-04:00 belong to both days.

I remember reading somewhere that some systems use a 26 hour clock in that it's literally valid 00:00 to 25:59, or 28 hr for 00:00 to 27:59. 24:30 10/1 and 00:30 11/1 mean the exact same thing, it's a notational convenience to not have to worry about date boundaries between days.

Trying to find the source again but not having success.

TV stations use this system. A schedule for any given day can be up to 27 hours iirc.

The Thomas Cook European Rail Timetable uses 00:00 for a train departing at midnight, and 24:00 for a train arriving at midnight.

> Most people can’t

Bias. Most people can. I'm in the UK, too.

I have no idea, I can never remember, it makes no logical sense - no, 12 noon is NOT post-meridian, it IS meridian. I usually go 11:59 or 12:01, and then I'm still not sure if it doesn't put the entire hour in some interdimensional hole where this doesn't make sense.

The interval is clopen. Hence the convention that noon accords to PM

Noon is the meridiem that AM and PM reference. At precisely 12:00 it is neither before nor after noon, because it is noon. That makes it hard to intuitively know whether the mid-day 12:00-12:59 should be AM or PM (and vice versa with midnight).

It can obviously be learned, but it's something lots of people have trouble with. I was probably in my teens before I finally got that memorized enough to keep them straight. Lots of things happen at 11:59 PM (due dates being the most prominent example) because it's less potentially ambiguous than 12:00 AM the next day, even though it's functionally the same thing.

And yet we have survived with the convention that 12AM is midnight, and 12PM is noon for centuries.

Noon is also not "12:00" - it is the infinitely small moment at the strike of 12:00. Instantaneously it becomes post-meridiem. 12:00:00.0000000000000000000000000000001 is after noon.

If I could edit the comment after a few hours, I would fix the spelling. Apologies! (Ante meridiem & post meridiem)

Not even in the USA.

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