There are the following situations about the calculation of time period:

One: If the time and minutes are greater than the former, do subtraction directly.

For example, 09: 20 to 13:25 will do.

13-9=4,25-20=5

Time: 4 hours and 5 minutes.

Second, if the corresponding minute is less than the former, you should borrow 1 hour from the latter, that is, 60 minutes, so that the time of the latter is expressed as:

(A- 1): (B+60), and then calculate by the method of one.

Such as: 09:20 to 13: 10.

Calculation process:

13- 1= 12

10+60=70

12-9=3

70-20=50

So the time period is:

3 hours and 50 minutes.

I don't know if I understand my explanation.