I mean, do they say “I sleep at 9pm” or more like “I sleep at 2100 hrs” even while they are talking informally? 2100 hrs sounds very formal to me, but yeah, I was just wondering if they used 24 hour format for only official and government proceedings and used 12 hour format for casual stuff.
As an American, the first time I heard this usage by a British person, I assumed it meant half an hour before the hour (7:30, in your example).