Let's say you gave the number 5 in cell A1 and the number 6 in cell B1.
Your formula is in cell C1, =A1-B1 with gives you -1. If you wrap that
formula in an If statement, you can force it to zero:
=IF(A1-B1<0,0,A1-B1)
You are simply saying that if the difference between A1 and B1 is less than
zero, evaluate as zero, otherwise give the positive difference.