I want to use 24Hr Clock and NOT 12Hr Clock. How do I change this?

I want to use 24Hr Clock and **NOT** 12Hr Clock. How do I change this? If I go to the App on the iOS device or the App on the Mac OSX, I cannot see how to change these setting to show 24hr clock. It's incredibly frustrating. If anyone can help, I'd be very much appreciative. Thanks.