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 a native English speaker that used to be able to speak decent German, the one that really screwed with me was 30 minutes past/to the hour. In British English (apparently, an American girlfriend found this confusing when I said it), you can just say “half eight” and everyone knows you mean half past eight. In German, “halb acht” would be 7:30, because it’s assumed to be half to instead of past. Neither is more reasonable than the other, but it definitely took me a while to get over the instinctual understanding of it. I was very late to at least one lunch
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).