This gave me errors, but I tried a simple subtraction and it worked.

Strangely, they use hundredths of minutes and hours instead of 60, so to subtract 5 hours, multiply 5*100*100 = 50000.

I figured it out with the ISVALIDNUM function, so all is good. 

Now there is an issue. When a data field is 0, it does not read it as numeric.

Example, I have input values of 3.5 and 0 and want to subtract them, but I don't get output

Example, I have input values of 3.5 and 1 and want to subtract them, output is 2.5.