My experience in the US Navy left me with the habit of using “zero” rather than “oh”. To my ear, “oh three hundred” sounds a trifle lame and affected - like a TV writer’s idea of how to make someone sound like he’s military.
This also applies to minutes less than ten: 17:04 is “seventeen zero four”, not “seventeen oh four” - though the latter doesn’t grate on the ear half as badly as “oh three hundred”.
It does not apply to minutes greater than or equal to 10: 17:40 would be “seventeen forty”. Though you might well use “one seven four zero” over the radio when there’s lots of static.
It’s the same here in my part of the US, at least. I keep my clocks on 24-hr time because I kind of like it, and got sonewhat used to it living abroad for six years or so. Also—and I realize this is a very edge case, but it’s happened to me—when you lay down exhausted for a nap at 5 pm in the summer and wake up an hour later at 6 pm, there’s no momentary confusion as to whether it 6 pm today or 6 am the next day. Ok, that’s only happened maybe twice to me, but, hey, easier to see 1800 rather than try to make out if the dot for am or pm is lit.
But whenever anybody notices my clocks set to 24 hrs, they ask if I was military. I chuckle and say no, because there is little military about me.
As for what I say, I just translate 1700 hours to 5 pm in conversation. Like I said, it’s not used a lot around here, and saying “It’s seventeen” sounds really odd even to me, and I used the time format. Nobody else would understand if I replied “It’s seventeen.” It’s not that I couldn’t easily get used to that phrasing—it’s absoluteky logical and follows how we express time in my dialect—it’s just that I would need to hear it being used in the wild for my ears to adjust.
My understanding is “Dark” is roughly equivalent to 0500, or maybe 0400, so Dark:30 is ≈ 0530 or 0430; ie. sometime I’d rather be sleeping but up before the sun is.
I don’t think this scenario works for me, but there are times where I just say the minutes. For example, suppose we’re waiting for a bus, and you say “let’s go get coffee real quick”. I might say “It’s 45 now, and the bus comes at 52; we don’t have time”. The hour being understood, since we probably already had a discussion about when to catch the bus just before heading to the bus stop. It’s pretty rare though.
Y’all are overanalyzing it. It just means “middle of the night/wee hours of the morning.” “Dark” doesn’t refer to any specific time. It means exactly what it says: “dark.” Besides, 5:30 it’s starting to get light in much of the world in the summer, as long as you’re not in the tropics. Basically, anytime between like midnight and 4/5 a.m. or so. 3:45 would qualify as “oh dark thirty,” for instance.