Hell, typing it was the hard part. I typed in "military time" into Wikipedia and got this page:
https://en.wikipedia.org/wiki/Military_time
It had significant holes, so I went to the internal link to this page:
https://en.wikipedia.org/wiki/24-hour_clock
It had a lot of info, but more importantly, in its references, it had "Communication instructions – General, Allied Communications Publication ACP 121(H), Combined Communications-Electronics Board, April 2007" as one of its sources. I clicked the hyperlink, but found the .pdf offline (temporarily? permanent?), so I did a google search for "ACP 121(H)". You have to be smart about searches. Realize they are looking for particular strings of text. If I'd put ACP 121(H) without the quotes, it would look for anything with 121, ACP, or H in it. By grouping them in that manner, it searches for only those in that particular order.
A simple Google search later (https://www.google.com/search?q=%22ACP+121(...lient=firefox-a) and I had the text of the document which showed not only the official military depiction for midnight, but the rationale behind it.
I cut & pasted it into my response. So, 2 searches and 4 clicks yielded the answer. For a former commo guy, that's nothing...
...you should see what I can do with Powerpoint... :-)