Question: how to calculate time difference between two times manually?
Answer: Convert both times to 24 hour format, adding 12 to any pm hours. 8:55am becomes 8:55 hours (start time) ... 
If the start minutes are greater than the end minutes… ... 
Subtract end time minutes from start time minutes… ... 
Subtract the hours… ... 
Put(not add) the hours and minutes together – 6:45 (6 hours and 45 minutes)