Excel Minus Time Calculations

Does anybody know how to get Excel to show a negative minutes and seconds value? My work involves timing TV scripts – having done an initial estimate of a scene length, I enter it formatted as minutes and seconds. In the next column when the scene is recorded, I enter the actual time the scene lasted. In the third column is a formula to show the difference between the two, which when it’s a negative figure appears unhelpfully as ‘#####’. Is there any way round this?