|You don't really need a formula since you're only doing it one time.|
It's probably easiest to convert to 24-hour format:
So 8:10 PM becomes 20:10
20:10 - 4:28
Take an hour (60 minutes) from the 20 and add it to the 10 minutes to make it easier to do:
19:70 - 4:28
gives you 15:42, that is, 15 hours and 42 minutes.
Real men don't use AntiVirus; they just reformat