Sorry, didn't read very carefully...
I think this will do what you want... or most of it.
DECLARE @Date1 DATE = '10-Aug-2015',
@Date2 DATE = '07-Sep-2015'
,@Diff INT
SELECT @Diff = ABS(DATEDIFF(day,@Date1,@Date2));
PRINT @Diff;
the ABS() removes the sign...