i’m sure someone has answered this question, but i guess since i have a guest membership, i can’t seem to access it.
anyway, it’s always seemed strange to me that 12:30am comes before 11:30am. why doesn’t the day start at 1am?
In military time it’s 00:30 hours, not 12:30am.
This probably doesn’t really answer your question, but it’s all I’ve got…
I think that’s probably at least half the answer…
the day doesn’t start at 1 o clock, it starts at zero o’clock, or one second after zero o’clock. But there’s no zero on the clock face… it’s marked as twelve. That’s where things started to get confusing. 
Cecil claims it’s arbitrary. Although the day doesn’t start at midnight, midnight is the dividing line between one day and the next. So 12:00 A.M. is actually semi-ambiguous. Calling it 12:00 midnight would be more precise, and the day begins at whatever discrete time measurement after midnight.
As to why the day is arbitrarily set to start at (12:00mid + dt) A.M., I don’t know.
Our clocks start at 0:00.
Your ‘am’ and ‘pm’ is pretty confusing to me. 0:00 is midnight here. 12:00 is noon. 2:00 is two at night, 14:00 is two in the afternoon.
You say “your” as if “we” invented it. The division of the day into the period from the highest point of the sun (“noon”) to its antipode and vice versa, as well as the subsequent division of each into twelve segments, is pretty ancient. IIRC, the twelve comes from Babylonian (correct me if I’m wrong) numerology. When clocks were made, it’s a whole lot easier to just make twelve divisions on the face than 24. Only with the rise of digital time displays has it become convenient to keep 24-hour time.
I’m not at all sure about that requirement for digital displays to refer to 24-hour time.
This may not be entirely relevant, but I’ve seen a french schoolhouse clock face marked with two different numbers at each hour… 0 and 12 at the top, 3 and 15 at the extreme right, etcetera.
You still need to keep straight in your head whether it’s appropriate to use the large numbers or the small numbers, just as people who use ‘am/pm’ need to keep straight in their heads which of those are relevant. But it seemed quite a workable system to me. (Admittedly, when I saw such clocks was AFTER digital time displays were invented, so it could have been retro-engineering.)
This site: http://homepage.mac.com/pete.boardman/24hourclock/history.html (Which admittedly sells “24 hour analog clocks”) claims that the 24-hour, double-numbering system goes back to the very earliest clocks, even tracing its origin to roman sundials, and claims that the 12-hour system was introduced later. :] It also talks about a shift from ‘0 is dawn’ to ‘0 is midnight’ that happened fairly early on.
Because our timekeeping protocol was designed by people that didn’t have a numeral “zero”. If we were designing a system from scratch today, we’d have a 24-hour day running from 0:00:00 through 23:59:59, as they do in the military. Unfortunately, by the time Europeans adopted Hindu-Arabic numerals and became comfortable with using zero, the one-through-twelve, a.m. and p.m. system was entrenched too firmly in everyday life to be changed.
Originally posted by Mathochist
But we did. We’re ancient Hindu-Arabic-Roman-Babylonians in disguise.
I admit I tried to gloss over this point for compactness. There have been any number of other timekeeping systems over the years, but who the hell remembers what “eight bells” is? (braces for them all to be readers of SDMB)
Ultimately, there was a division of the day in half and a division of each half into twelve. This twelve is what the OP is asking about, whether it “makes sense” or not. 24-hour time is, whenever it arose, an attempt to unify that first division and put both collections of 12 into one collection of 24.
Well, all of us who have served in the Navy, the Marines, or the Merchant Marine are going to remember.
8 bells is 4:00, 8:00, and 12:00. Each watch (four hour “shift” on a ship) is marked by a series of bells on the half-hour, with 1 bell indicating 4:30, 8:30, or 12:30, 2 bells indicating 5:00, 9:00, or 1:00, etc., until one arrives at the change of the watch at 8 bells. (Unless you are on the John Sherwin while Pat (Pasquale) was the wheelsman, then some variable ringing of the bells simply meant that it was some half-hour or hour increment on the Third Mate’s (8:00 - 12:00) watch. Pat could never quite get the hang of ringing individual bell pulls.)
That’s funny, because this Dutch table clock has a face numbered I II II IV . . . XII. So, it appears that you have recently changed your way of telling time. (Of course, the change may have preceded the birth of a really young person. Orwell poked some fun at the 24 hour clock in his 1948 novel.)
Actually, I think Freddy is closer to the heart of the matter… 24 hours or twelve hours twice doesn’t really make a difference. The question is, do you start the day, whenever it is, at zero hours, (which requires enough mathematical sophistication to understand the value of a zero generally,) or do you refer to ‘quarter after twelve’ or ‘half past twenty-four’… tying the first hour of each period into the HIGHEST number of hours you’re referring to? (I have to admit, I think it’s quite likely that the early twenty-four hour clocks put twelve and twenty-four together, because as Freddy pointed out, they weren’t using zeroes back then.)
Just $0.02 worth as usual.
heh. You’re right. I even have an old clock with those numbers.
I ought to have said: Our digital clocks start at 0:00.
Also; the official time indication uses the 24 hours.
Well at that point one may as well recognize that hours are counted modulo 12 (or 24), where 12 (or 24) is zero.
And of course there are the “dog” watches from 16:00 to 18:00 and from 18:00 to 20:00, which are changed after 2 hours, to ensure that no one has to stand the same watches day after day. Someone with a more recent recollection may correct me, but I don’t believe the bells are “re-set” - they continue on to 8 bells at 20:00.
I ignored the dog watches specifically because they do not alter the four hour cycle of bells in any way. They are certainly important to the sailors who don’t want to find themselves standing the dawn watch every day for weeks at a time, but they are not really relevant to the time keeping.
what i was really interested in was once someone (?) decided to spilt the clock into 12 numbers, why not make it 1…12, rather than 12, 1, 2, 3 … 11. so the day would start at 1:01am and 11:30am would come before 12:30am
I would guess (and I have not yet found evidence) that even without a concept of zero, people had a concept of completion and they chose to end each half-day on the greatest hour, 12. Originally, they did not count minutes (having no mechanical method to guarantee anything like accuracy to 1/60th of an hour), so they placed their hours in such a way that when they reached the highest number, they had finished with that part of the day.
Recall that prior to digital time reckoning (which obviously did not require digital clocks), the standard method of noting the minutes was to say so many minutes after or before the hour. (Cue Chicago singing Twenty-five or -six to four.) We still say “quarter to 11” (or “quarter till 11” depending on dialect) and use similar phrases, although the increase in digital time reckoning has reduced the frequency of that type of expression even in my lifetime.
For the same reason we don’t say you’re 1 year old at the moment of birth. One o’clock means one full hour past midnight or noon, just as one year old means one full year past birth. It would seem more sensible if we talked about zero o’clock. Military time does refer to zero hundred hours (with 1:00 being one hundred hours), and apparently Europeans refer to 0:00 and even have it on their clocks as chrisk mentioned, but for the most part in the U.S. we simply refer to twelve o’clock with the understanding that it is equivalent to zero. You need to embrace that understanding.