Converting and Subtracting Two Dates I'm trying to get the numeric difference between two dates.  (Ex:  11/30/2018 - 11/05/2018 = 25)

One of the dates (@EndOfMonth) represent the last day of the prior month and is being converted into a 'yyyymmdd' format

BEGIN
DECLARE @EndOfMonth AS VARCHAR(10)
SET @EndOfMonth = (SELECT CONVERT(VARCHAR, DATEADD(m, DATEDIFF(m, 0, DATEADD(m, 1, DATEADD(MM, -1, @maxDwdate))), -1), 112))


The second date that I'm looking to subtract from @EndOfMonth, is a field named "DUEDATE" and it's in a 'mm/dd/yyyy' format.
Assuming that you have a valid reason to convert the date to a string, and that you cannot change that logic: declare yourself a variable to store the intermediate results of the date arithmetic before you convert it to a string, then you can easily get the difference between the two dates:

DECLARE @eom DATE 
      = DATEADD(m, DATEDIFF(m, 0, DATEADD(m, 1, DATEADD(MM, -1, @maxDwdate))), -1);
DECLARE @EndOfMonth AS VARCHAR(10)
    SET @EndOfMonth = (SELECT CONVERT(VARCHAR, @eom, 112))
 SELECT DATEDIFF(DAY, @eom, DUTEDATE) AS DAYS
   FROM t; -- replace with actual table name.