The 24 hr clock has a different number for each hour of the day.
The 24 hr clock runs from 00:00 to 23:59
To change from 24 hr time to normal 12 hr, we subtract 12 from the
hours it it is after 12:59pm.
To change from 12 hr time to 24 hr time, we add 12 to the hours it is
If it is before and a single digit we have to add a '0' to the beginning of
the number. (12:00 to 12:59 is 00:00 to 00:59)
Notice we have dropped
the 'am' and 'pm'