If formula for selecting an Hour duration

Hi all,

I have two start times, one at

05:00 and the other at 07:00

The work day finishes at 17:00

In B2 if I select 05:00, I want the formula in B3 to =MAX(B6−"12h","0h")

But if I select B2 to start time at 07:00 I want the formula in B3 to ==MAX(B6−"10h","0h")

Cell C1 is adding 30 mins to Shutdown time for getting back to base,

I am trying to say if someone starts at 05:00 and ends at 17:00 overtime is after 17:00

If someone starts at 07:00 and finishes at 17:00 their overtime is cal after the 17:00

Is there an easier way for this?