H and welcome to the forum
Keep in mind that excel treats time as a fraction/decimal of 1 (day). So 12:00 mid day is actually 0.5, and 06:00am is 0.25.
30 minutes is either 0.0208333333333333 or 30/60/24
plug that into your formula and see how far you get
Another observation - if you are just adding (or subtracting) 2 numbers, you dont need to use the SUM() function.
=IF(C7>0,SUM(C7-B7),"0")
can be shortened to...
=IF(C7>0,C7-B7,0)
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