AM → military
For 1:00 AM through 11:59 AM, keep the hour and pad to four digits: 7:05 AM is 0705. Midnight is special: 12:00 AM is 0000, not 1200.
PM → military
For 1:00 PM through 11:59 PM, add 12 to the hour: 1:00 PM is 1300, 5:30 PM is 1730, 11:00 PM is 2300. Noon (12:00 PM) stays 1200.