A.M. or P.M.?

Noon? Midnight? Which is 12 A.M., and which is 12 P.M.? Almost every calendar, clock, and what-not gets it wrong. (Luckily, most contracts side-step the issue entirely by saying '12:01'.)

To begin with, what do "A.M." and "P.M." stand for? The "M" stands for meridian. The "A" and the "P" stand for ante and post respectively, which mean before and after. Meridian means the middle, as in the middle of the day in this instance, as in noon. So, 12 A.M. is the "12" before noon, and 12 P.M. is the "12" after noon. What this means is that neither can actually be noon. Midnight is always either--and both--the 12 [A.M.] before the succeeding noon or the 12 [P.M.] after the preceding noon. Noon is just... noon!

Oh, well. This, however, just does not fit in with common perception, so A.M. is used for midnight (most of the time) and P.M. is used for noon (most of the time)... regardless of their true Latin meanings. Cicero would be proud (not).