Converting from AM/PM to 24 hour clock:
between 1:00 AM and 12:59 PM, a straight conversion: 10:00 AM --> 1000
between 1:00 PM and 11:59 PM, add 12 hours: 10:59 PM --> 2259
Converting from 24 hour clock to AM/PM:
between 0100 and 1159, straight conversion to AM: 0100 --> 1:00 AM
between 1200 and 1259, straight conversion to PM: 1259 --> 12:59 PM
between 1300 and 2359, subtract 12 hours: 1559 --> 3:59 PM