Calculating Time difference not working

Ok... This is driving me crazy. This should be a simple formula but I can not get it to work.

I am trying to calculate the difference between 2 times such as 19:20 - 23:47. Now this works fine and gives me a result but..... I want to minus 30 minutes from the overall time. I have to do this becuase we always deduct 30 minutes from our overall wait time. Everytime I do this it gives me ########### and tells me that the number is negative. How can I calcutalte the differnce between the times including a deduction of 30 minutes off the over all wait time in the format of hour:minutes

Please help, I have been trying for hours

Thank you Old Chippy. This resource gave me the answer I was looking for