We mainly use 12 hours here in the United States. It's uncommon to see a 24 hour format being used, unless you're using a Linux machine or a few game consoles like the GameCube you will only see a 12 hour clock being used. I often have to look up what the 24 hour format is, we call it "Military time" since it's the format used mainly by military officials. Though since I've been using Lubuntu more as my main operating system, it's been helping me get used to the format better.