We're trying to set the clock on our tablets to have 24-hour (military time) format, but that option is missing. There is a guide that details how to set a number of date/time-related settings, but nothing for formatting.
If you use Samsung or Zebra device, you can config time format using "OEM settings"
When turn OEM settings on, date/time settings appers, and you can set time format to 24 hours.
If you're using devices from a manufacturer other than Samsung or Zebra, depending on the manufacturer, you may be able to configure it using the OEMConfig app.