Last night I was watching The Pale Blue Eye (2022), a (bad) period piece set at West Point in the year 1830. At the 21m50s mark, Captain Hitchcock (Simon McBurney) snaps at a cadet:
Effective oh-six-hundred tomorrow morning, all men will only attend class, meals, prayer services. Nothing more!
This struck me as an anachronism: I would have expected that "military time" came into use only in the age of radio and other voice-or-print telecommunications, where "8 AM" could get garbled into "8 PM" more easily than "0800" into "2000". I wouldn't have expected military time to be in use at all pre-WWI, let alone pre-Civil-War.
A cursory Google search supports that intuition. In fact there's a meme going around that says very precisely "The U.S. Army didn't officially adopt military time until 1942" — but I have no idea what the meme-maker meant by that. (Was that the date of some sort of proclamation that "from now on thou shalt always talk in hundreds"? For how long before that "official adoption," if any, was military time already in common use by ordinary soldiers and officers?)
So I'm looking for some more rigorous analysis. This is going to be tricky, since we're talking about oral language in the days before film; but maybe a military historian will know: When did the U.S. Army adopt the "oh-six-hundred" manner of speaking?