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Comment by Jenk | Hacker News Reader

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

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.

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.

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".

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

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

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