Zero dark thingy

The Times last week published a crossword that – self-referentially – aroused cross words. The clue (a somewhat feeble one...

<WHOOPS>
I was misled by the angry correspondent's writing, or at least implying by its context, "the Times crossword". It must have been the Quick Crossword, which is a whole 'nother thing. So my "feeble" was undeserved, except to the extent that all such clues are feeble.
</WHOOPS>

....whichever side of The Great am/pm Debate you come down on)   was "12 a.m." and the answer was "midnight".  Aware of the  Great Debate,  their style guide says "Use noon/midnight".

But there is no need for the Debate; it depends on a simple misapprehension. 12 noon is not before or after anything; it is noon on the dot; it is just m.

And the m in "a.m./p.m." does not stand for meridian as many people seem to think...

<DESCRIPTIVIST_CAVEAT>
(except in the sense that all's fair in language use, so that "what people mistakenly think" becomes The Truth.)  But I  know what I know.
</DESCRIPTIVIST_CAVEAT>

... and as I was told at primary school. I lacked the theoretical meta-language to voice my objection (but meridian is an adjective, how can you have BEFORE or AFTER an adjective. Holy Category Error, Batman), but I knew enough to raise an eyebrow. One of the letters to The Times added to the corpus of the poisoned fruits of this widespread misapprehension:

"Surely 12 am follows 11 am, so is noon; any time after 12.00 is post meridian [sic], up to 12 pm."

I'm not sure why Rose Wild (writer of the article in The Times) calls this "logical". By simply swapping a few values, you arrive at the opposite conclusion: "Surely 12 am precedes 01 am, so is midnight".

<CASUISTICAL_EXCURSUS status="dead end">
I suppose it would be possible to argue that – like 'the Orthopedic'  meaning hospital, or 'Local' meaning pub (which itself is an adjective-made-noun, being an abbreviation of  'public house') – meridian is, in the contest of expressions of time, a noun (standing for 'meridian great circle'). The sun has passed that imaginary line. 

That's all very well, but the fact remains that the sun has not passed the line at noon. It is dead on it.
</CASUISTICAL_EXCURSUS>

That m stands for meridiem (the middle of the day). As it happens there is an adjective postmeridian. Most dictionaries carefully define it as of or about something that happens in the afternoon. Onelook will point you to a number of examples. One such (the AHD) is admirably specific:

On the other hand there are some whose obeisance to the God of Usage makes them excessively permissive (in my view, though they would presumably argue that that obeisance reflects the fact that they simply OBEY the trends). But I fear that the obeisance is more a matter of craven idolatry. One such is Collins:

(in which there is only a word space [between after and noon] and one little "in" to emphasize the adjectivity By not emphasizing the "of or relating to" bit, they are holding the door open to illiteracy.

It would be helpful – for those of us who crave such systemic balance – if there were  a Latin word *merinoctis to balance the scales; and although the 'word' does exist in some twilight/fictional/pseudo-gothic/'fanfic' sense (that I don't want to pollute my browsing history by researching any further) it is not mainstream enough to pass muster. If it were, we could have md for noon and mn for midnight., each followed respectively by pmd/pmn times. 

But this is getting rather silly; duty calls. We may as well stick with that style guide, and just say noon/midnight; or use the 24 hour clock. And accept, while we're at it, that crossword compilers cock a snook at style guides.

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