To remove the #NUM from the resilt of taking difference between two sharepoint column values.
Here is a scenario :
There are two dates , date1 and date 2
date 2 - date1 will give result if date 2 is after date1, if not it will give the result #NUM
This can be resolve by using this formula :
If the difference between the two dates is negative ( ie, Date1 - Date2 = -ve value ) then it will return -(Date2-Date1)
for eg : 2008/02/02 - 2008/02/01 will give 1 as value, but 2008/02/01 - 2008/02/02 will give #NUM as result normally, but the above calculated will give -1 as result.