Working in Time

I am setting up a spreadsheet that calculates plant efficiencies. I would like to find a way to add or subtract in Hours and Minutes that gives me a result in hours and minutes and not a point in time. The spreadsheet works ok most of the time but it gets unpredictable when working around 12:00 hours and 00:00 hours, basically because Excel thinks I am working in real time and it gets confused at the transitions from am to pm and pm to am. Can you give me a couple of clues here please?

Regards

Brian