Excel formula - input aviation times to a certain point

I am trying to create a spreadsheet that will allow me to input aviation times to a certain point. Basically my engines need to be serviced at 12, 24 and 50 hour intervals. I want to be able to input times into each respective column and have the column countdown to zero or a negative number in each column and once it has hit that point, to reset to 12, 24 or 50 and start all over again. Any help would be greatly appreciated.

Ryan
in Iraq

Hello Ryan, I can't quite picture exactly what you want to do, can you give a small example...?