Google-sheets – How to modify the formula to take 1 hour off

It looks like you've got some order of operations trouble. I think this will give you what you want:

=((B2-A2)*24)+((D2-C2)*24)

Of course, you also need to ensure that the cell uses a decimal number format. And that your afternoon values need to be actual afternoon values. You need to use 24-hour format or am/pm format.

In any event, using your values above, this formula, and appropriate formatting, this is what I get:

 8:20 | 12:00 | 12:30 | 17:02 | 8.20

Try:

=mod(C2-B2,1)

and drag down as far as needed. Make sure the format the cells as duration. For the 'total time' a simple sum should suffice.