Need help with an MS Excel Script/Formula

[Dennys]

I'm working on a form for my job (A monthly mileage statement). We record end of day mileage from the bottom up in column A, and the daily use in column B. 31 lines on the form. So Day 1 is Cell A41. (There are header cells)

What I'd like to do is have Cell B40 and up scan down to the next filled A cell and subtract out the Current day's end from the prior day's end to give the actual use. Essentially B40=A40-A41. However it has to be smart enough to skip a null cell (weekends, days out, etc).

Do I need a script or can this be done in a straight formula in a cell?

Thanks
~Dennys