Here in the US, if you’re in the military or work in aviation, you might actually use what most of the rest of the world thinks of as standard, 24-hour time. While it will confuse all your US-living friends and relatives when they ask for the time and you show them your iOS device’s lock screen, that’s their issue, right? You, at least, will have the joy of knowing that you are fully prepared for the type of time tracking that most people around the globe use.
The default iOS clock in the US is a 12-hour clock, with AM and PM appended to the numbers to denote morning or evening. The other, more standard way of representing the time is with the 24-hour clock, in which any time after 12:00 is the logical 13:00, 14:00, and so on. It’s really easy to set on your iPhone or other iOS device.