You could wrap you subtraction in an IF condition to test for a black 2nd data.
If Date2 is empty return empty, if Date2 is not empty return Date1-Date2.
If either is blank:
isEmpty (Date1) OR isEmpty (Date2);""; Date1-Date2
thanks guys... that helped a lot!
The key word is is ALL. Do not evaluated if ALL the fields are blank means exactly that. If one field is empty and the other not, then clearly you do not have a case where All the fields are blank and the caldulation still evaluates--treating the empty field as a value of 0.
Thanks Phil... I realized that after. I wasn't sure if there was just a check-box somewhere I missed or if I'd have to use an IF statement. My first project has come together quite nicely - thanks!