Hi, I'm not quite sure I follow the original problem, but if you want the SQL equivalent....I've kept the parameter order the way you wanted, though personally I think you want to reverse them.

Here is SQL equivalent.. Not sure if this will solve everything you're encountering, but here goes.

CREATE FUNCTION dbo.DaysToComplete
(@CloseDate Date, @OpenDate Date)
RETURNS INT
AS
   BEGIN
      DECLARE @DayDiff int =  DATEDIFF(D, @OpenDate, @CloseDate)
	  if @DayDiff < 0  
	     set @DayDiff = 0
   return @DayDiff
   end

Here are 2 tests...

select dbo.DaysToComplete(  '5/1/2014', '5/15/2014')    returns zero
select dbo.DaysToComplete(  '5/15/2014', '5/1/2014')   returns 14