BW customer exit variable to show "NA" based on a key figure value

Hi,

Can you guys please tell me how can I resolve this simple issue?

I have a query ,where two key figures (XYZDATE ,ABCDATE) are there along with other characteristics and key figures.Now formula is built (Replacement path) for both these two key figures to take the date only(from DD/MM/YYYY format) and then another formula(FORM1(ABCDATE)-FORM2(XYZDATE)) is created to calculate the difference between these two.Now the problem I am facing is, there are cases where FORM2(XYZDATE) is populating as # (due to no data is there in the info provider).Now while calculating the difference in BeX analyzer ,it's populating some huge value like 734xxx .

Now my requirement is whenever the ABCDATE will be # ,then the Difference column should populate "NA".From my understanding ,this can be achieved using Customer exit variable.Can you please help me with the logic or code?