Keyfigure type for Total No of days

Hi Experts,

As for my requirment i need to write field level routine for getting total no of days.

I have two char for start date and end date when i am mapping these two char in tranformations to key figure showing error (Source parameter /BIC/xxxxx) is not being used) .

Which type of key figure i need to create for this ?

Thanks - Siva