Date Difference in TWO DATES IN WEBI

Hello friends , I have a requirement to have the number of days between the dates. but when I calculate the formula in WEBI

DaysBetween([Sale Date]; [Invoice Date])

this does not work when i hit ok it says date format is invalid

I checked in universe by default sale date object and invoice date object is of type "character" if i change to date then report does not work at all.

please suggest how do I put the code in WEBI so it works to calculate the difference

2. Also if one date is "Not Assigned then it should take current date or give the output 0

thanks

soniya