Formula needs work

[SCLINDNER]

I have a spreadsheet that keeps track of generator hours so that we know when to change the oil, etc. The hours are typed in every day (day values are stored also) and the formula subtracts the hours from the day prior, then adds this number to another field for a total amount of hours. Here's where the problem comes in. There is a cell that tells what PM (Preventative Maintenance) is due, such as 300 hr, 500 hr, or 1500 hr. When the generator hours exceed the numbers in the formula, the PM due cell still remains. Is there some way to reset this cell without changing the generator start hours again?