Datediff start date cannot be greater than the end date

I have been using DAX DATEDIFF statement  to calculate No Days between two dates which are

Target Completion Date and Actual Completion Date

Which works all very well except now I have sometimes the Actual Completion Date is before the Target Completion Date (ie where an Action Officer performed better and completed the task before the "target" date).

However, of course, DATEDIFF doesn't like this and gives error "In DATEDIFF function, the start date cannot be greater than the end date"

Why can't this Dax function just return the negative value in this case?  Is there a way we can enhance this function to return a signed integer?