That's because it doesn't make any sense to describe the middle of the day by '12 PM', since 'PM' should mean "after midday". To be sure we have conventions about this, but not knowing those arbitrary conventions is no reason to think that people don't understand the concept. I think better usage is 12 noon and 12 midnight (https://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noo... )—though that still requires further disambiguation at midnight (is "midnight on August 3" between August 2 and 3, or between August 3 and 4? I don't know whether there's a common disambiguation here, as with 12 n versus 12 m).
Sure, although, if we're re-defining how people tell time anyway, we might as well use a 24-hour clock and so avoid the need for AM/PM at all; and, even if we do that, there is still, inevitably, the confusion about on what day 00:00 AM falls. My point was just that not knowing that "12 PM" means noon is a matter of not being aware of an arbitrary convention, rather than of not understanding the concept of AM vs. PM. (Indeed, arguably it is a matter of understanding their meaning all too well!)
Not so much actually: 00:00 is on the day that is starting, 24:00 is on the day that is ending.
That makes a lot of sense!
Except when leapseconds come along and make it 23:59:60
And daylight saving skips an hour, or repeats it.
AM and PM are really anachronisms though, I don't remember the last time I've ever seen "4pm" written down in normal life.
True in the US (and IIRC the Anglophone world more generally), but AFAIK 24-hour is more commonly used outside of military/technical domains in many other countries.
UK TV schedule: https://www.bbc.co.uk/schedules/p00fzl6p
France opening hours: https://www.malathronas.com/wp-content/uploads/DSC_6720.jpg
Thai train timetable: https://teakdoor.com/images/north.gif
I just checked my OS X machine, it has it. My Linux machine has it, though I grant that among the many DEs for Linux, some might not use it. (I use MATE, a fork of GNOME 2, for reference.) And I've Googled a screenshot of Windows, which seems to still use it.
My phone, microwave, and stove, I guess, seem to presume I know whether it is morning or not.
Or maybe 12:00 vs 00:00
12 PM = noon is something I remember learning in kindergarden, and should be common knowledge for most adults in the US.
Well, maybe, although assuming a commonality between others' education and mine is always dangerous; but I've got way more respect for someone who understands a concept but doesn't know about an arbitrary exception to it, than for someone who understands a pile of arbitrary rules and doesn't know how to fit them to any conceptual framework.
I think that explanation fails because, IME, people who know that “PM” means “post meridiem” which means “after midday” are more, not less, likely to also know that 12:00 PM is noon than those to whom PM is just an arbitrary marker.