The abbreviation "pm" is for post (after) meridiem (midday), so when speaking of twelve noon or midday is actually incorrect to call it 12pm, because it isn't after noon yet; it is noon. For this reason I prefer to call it 12 noon, or 12 midday, or just noon or midday.
A similar problem occurs at midnight. It is easy to see why 11pm is still referred to as after noon (post meridiem) because it is better described as after the previous noon rather than before the next noon because it is closer to one than the other, even though it is in reality both. Likewise it makes sense that 1am is referred to as before noon because it is closer to the next noon. But midnight is closer to neither the previous nor the next. It is equally am and pm. For this reason I prefer to call it 12 midnight, or just midnight.
Apparently, in an attempt to avoid confusing people, travel times around the world commonly use 12:01pm or 12:01am or 11:59am or 11:59pm instead of messing with the ambiguous 12:00 times.
As for the term "noon". That's a weird one. In the past it meant the ninth hour (nona hora) beginning around dawn, or our 6am, so that the ninth hour would have been our 3pm. So how did that eventually become midday? I don't know. Ancient Roman timekeeping is seriously muddled, and I haven't bothered to untangle it yet. One thing I do like about it though, is that the length of an hour changed according to the season and the location, so that at Rome an hour in summer would be about 75 minutes and in winter about 45 minutes. That makes wonderful good sense to me. Screw this stupid daylight saving time and the constantly shifting rising and setting times of the sun. On the other hand, one of my biggest complaints against daylight saving time is that it makes international meetings via the internet incredibly difficult, and constantly shifting hour lengths would seriously mess with that too.
I expect that sometime in the future we might end up with something like Star Trek's stardate which would resolve all synchrony problems, while completely removing all local relevance. We've already had an attempt at that with UTC, which is basically Greenwich Mean Time (GMT) without silly daylight saving. Incidentally, although UTC is often referred to as Universal Time Code, apparently it stands for the French: temps universel coordonné, which doesn't really make sense as it would be then TUC. It seems actually to stand for Universal Time Coordinated, which is an awful name, clearly chosen by a committee. Being locked to Greenwich in England gets up some people's noses. Admittedly much of the early work recorded in books was conducted at the observatory in Greenwich. But there were a lot of much earlier, very accurate astronomical calculations in India, so Greenwich wasn't the first. Perhaps the invention of the first reliable, portable, mechanical clock by Englishman John Harrison decided things. I don't know.
Once thing is certain: time is a mess. I won't even get started on other aspects of it, such as the 60 seconds in a minute and 60 minutes in an hour, 24 hours in a day, 7 days in a week (making calculations of every second day messy), 28 or 29 or 30 or 31 days in a month (WTF!!!), 52 weeks in a year, and 364 or 365 days in a year. Naming the months mixes everything up still further, with September (sept=7) being the 9th month, October (oct=8) being the 10th month, November (novem=9) being the 11th month and December (dec=10) being the 12th month. (FFS!!) And then to top all this off, adding the recent and completely unnecessary insanity of daylight saving into that wreckage just completely screws everything even further.
(Crossposted from http://miriam-e.dreamwidth.org/331883.html at my Dreamwidth account. Number of comments there so far: )