bmg002 - Tuesday, October 17, 2017 10:51 AM

This may not be the most efficient method, but could you implement something like this:
DECLARE @date DATE = '2017-04-17';

SELECT
    CASE WHEN DATEPART(MONTH, @date) = DATEPART(MONTH, GETDATE())
             OR DATEPART(MONTH, DATEADD(MONTH, 6, @date)) = DATEPART(
                                                                         MONTH ,
                                                                         GETDATE()
                                                                     ) THEN
             'yes'
         ELSE 'no'
    END AS [6monthinc];

so your where caluse would have at the end something like:
AND CASE WHEN DATEPART(MONTH, StartDate) = DATEPART(MONTH, GETDATE())
             OR DATEPART(MONTH, DATEADD(MONTH, 6, StartDate )) = DATEPART(
                                                                         MONTH ,
                                                                         GETDATE()
                                                                     ) THEN
             1
         ELSE 0 END = 1
The only problem I see with your existing code is that if you subtract 3 days from Tuesday, you get Saturday.  

This may not be the most efficient way of doing it but I think it solves your problem, no?

EDIT - the AND section should have used StartDate not @date.