"09:00 PM" almost universally (is there any case or format or app that does it differently?) means 21:00 in 24-hour clock format. Not 09:00 (in 24-hour clock).

AM / PM clears up the 0900 vs 2100 ambiguity, but agreed re; American date formats.

For me it clears it up only 92% of the day. I couldn't tell you when 12:xx AM/PM is right now.

PM means post meridiem. That's Latin, but you don't have to remember it. If you simply remember the P means 'post', and the 'M' means the exact middle of the day (12 noon) then you know 12 PM is 12.00 (after noon hence afternoon) and 12 AM is after midnight or 0.00 (the hour after previous day's 23.00 / 11 PM). Then you just need to remember morning is AM, afternoon is PM, evening is PM, and night is AM. That's it. Yeah, I find 24H system easier, but its what my native locale uses, so I am biased... (as is everyone else)

This is in fact so simple to remember that street signs in San Francisco, where AM/PM is native, use 11:59am/pm or 12:01 and never 12:00 to avoid ambiguity. Street cleaning from 12:01am to 3:00am, for example.

Only thing to remember there is: .00 means new hour. Which the number before .00 denotes.

Caters to lowest common denominator.

I had all this explained on elementary school (which was boring and slow as fuck) during English class, and we don't even use 12H system in The Netherlands. It is quite frankly as simple to remember as a logic (tho not necessarily common) grammar rule.

it's really weird tho

12AM 1AM 2AM 3AM 4AM ... 11AM 12PM 1PM 2PM ... 11PM 12AM 1AM

even the way you explain it:

"then you know 12 PM is 12.00 (after noon hence afternoon) "

when I read "12 after noon", I understand "noon + 12 hours" which should give midnight..

Mnemonic: “AM” is “Always Midnight”

What if I remember it as “Always Mid-day”. What you you start to remember it as that from now on now that you’ve read this? There are no winners here.

Mnemonic. ツ

FWIW, there's also 00:00 and 24:00 to distinguish midnight at the beginning of the day versus the end of the day.

The easy way is to remember that the hour following 12 pm is clearly in the afternoon, so 12 pm must be noon.

Yes, I'm not trying to claim that AM/PM is ambiguous, but that me as a person not used to the system (basically everyone except US and a few other countries) have no idea which is which. :)

A native Floridian once told me that the New Year's fireworks display would be starting at “12 PM”. Confused about why they’d be shooting off fireworks in the middle of the day I said, “12 PM!?” She rolled her eyes and made air quotes with her fingers and said, “Well, AM”, as if I was being extremely pedantic. To her, AM meant morning and PM meant night.

Having worked on the UI of an event-related service, I can say the error rate is much lower if you go with the ‘11:59’ workaround. In addition to the confusion about whether 12 PM is 00:00 or 12:00, there’s confusion about which calendar day 12 AM falls on. When does a Friday midnight movie start? 99.9% of people will show up at the end of Friday, but in a literal user interface you’d have to choose 12 AM Saturday. I thought of fireworks girl very frequently when I worked on that UI.

I just use noon and midnight to reduce ambiguity

11:59pm, midnight, 12:01am

11:59am, noon, 12:01pm

"Midnight" doesn't solve the second part of GP's problem:

> In addition to the confusion about whether 12 PM is 00:00 or 12:00, there’s confusion about which calendar day 12 AM falls on. When does a Friday midnight movie start? 99.9% of people will show up at the end of Friday, but in a literal user interface you’d have to choose 12 AM Saturday.

I use 11:59pm or "end-of-day" for most cases.

it is. some ppl say 2359 to avoid saying 12am/pm.

It’s June

Americans use DD/MM/YYYY (regardless of delimiter being e.g. / or - or .). We Dutch use MM/DD/YYYY. The latter results in sorting correct on month if its annual data. Over multiple years it breaks. ISO 8601 has my preference, though in MS Office I need to set to Japanese cause my version lacks the setting.

9.00 PM is very clear to mean 21.00 in 24H format. What isn't clear is if 9.00 is 12H or 24H format. 9.00 AM or 9.00 PM is. However if you may reasonably assume the user uses 24H format, it is clear. It always takes up less space to use 24H format, though 12H format is always clear. Except for TZ (timezone), both all time suffers from that.

[EDIT]Oops I messed up, and that's why I dislike these and prefer ISO8601. Although one nice thing about DD/MM/YYYY is that its little endian (which is easy to remember for laymen as going from small to big). Keeping rest of comment as is.[/EDIT]

This is incorrect, Americans use MM/DD/YYYY, and from my understanding most of the rest of the world (I think the Netherlands too) use DD/MM/YYYY

Lot of Asian countries use YYYY/MM/DD like China, Japan, South Korea etc.

American software engineers use ISO8601 or RFC3339 ... I just wrote 20210108 in my journal this morning (referring to a podcast episode published yesterday).

I'll admit I might be annoying to others ... my wife is specifically irritated when I write a time in military format (but my son has no problem reading it from my phone).

I do that too, but I often replace the month with the 3 letter abbreviation to make it easier for others.

E.g. 2021JAN08.

While I believe that most (?) of Europe uses DD/MM/YYYY, I am pretty sure that most of South East Asia (dunno about India and Nepal) use YYYY/MM/DD.

> I am pretty sure that most of South East Asia (dunno about India and Nepal) use YYYY/MM/DD.

I think you might have meant East Asia. I know Japan primarily uses YYYY/MM/DD.

Singapore, Malaysia, Indonesia, Thailand, Vietnam, Cambodia, Laos, Myanmar, etc (South East Asia) conventionally use DD/MM/YYYY. Interestingly, the Philippines seems to use MM/DD/YYYY.

This looks useful: https://en.wikipedia.org/wiki/Date_format_by_country