24 hr time calculations without semi colons

DonkeyOte
Thank you for your quick response and your solution, that seems to do the trick but it has left me with another problem, which I did learn how to solve, but it has got a bit too complicated for me this time. I hope you can help with it. I have posted the sample again.
The calculations now work out nicely thanks, but when I drag the formula down each cell fills with either zeros or the calculation from above, What I want are for the cells to remain blank, but I can't follow my own notes on how to sort it.
Thanks