I'm guessing you are using military time.
Note, 23:00 is the same as 11:00 pm, and 12:20 is the same as 12:20 pm
Also note: 1 hour = 60 minutes
1) Change 1 hour inside 23:00 to minutes
23:00 = 22:60
Subtract
22:60 - 12:20 = 10:40
The time between 12:20 and 23:00 is 10 hours and 40 minutes
hope this helps
