Does anyone know when the military started keeping time using “military time”? And is it in general use among military forces around the world?
No, but probably a long damn time ago. It makes so much more sense. Of course so does metric, yet here we are
Hell, I wish civilians used it 
WAG, about thirty seconds after the first time somebody started a military operation 12 hours too late.
What you call ‘military time’ is known is most countries around the world simply as the 24-hour clock, and is not only used by armed forces but in fact is used by most countries as the standard format. Very few countries still use the 12 hour clock as ‘the’ standard – the US and Canada being a couple of exceptions - although it’s common to use the 12 hour clock in spoken language in many countries.
As far as military use goes, the British Army started using it in 1917.
As I said, most countries do, at least in written form.
24-hour time is extensively used in Quebec, although you’re right that usage is negligible in other areas of the country. The one exception I’m aware of is that Air Canada works in 24-hour time.
Go Transit in Toronto issues its schedules in 24-hour time. The TTC and other local transit systems do not.
Another thing we should standardize on. 12-hour time has got to go.
Military time and the metric system are widely used in America outside of the military, if you know where to look.
I’ve said this before but I’m jolly well going to say it again!
Railway timetables in Great Britain are always expressed in the 24-hour clock, but there is one exception - the Merseyrail system in Liverpool. For visitors to Liverpool who are used to seeing trains timed to arrive at 19.46 it is very disconcerting to find one at 7.46 p.m.
Scientists and medical people in the US use the metric system. But for some odd reason some medical people here in NC say “sontemeter” rather than centemeter.
Sounds like they’re trying to say it in French. ‘SON-tih-MEH-truh’
Are they imported doctors? Or from down east? In the North Carolinia piedmont I’d expect them to pronounce it “senuhmeeder”.
“Centimètre” in French would be pronounced rather like [sãtimɛ:tr], depending on the dialect (and I’m not at all sure if this [r] is the correct one). There is no ‘n’ sound, the letter ‘n’ indicates a nasal vowel instead.