I believe that the formula to calculate the date is a little bit wrong.

Here's an example according to what I understood:

DECLARE @Date datetime= GETDATE()

SELECT DATEADD(MM, DATEDIFF(MM, '19010101', @Date), '19000101'), --Using one year difference

DATEADD(MM, DATEDIFF(MM, '19000101', @Date) - 12, '19000101'), --Subtracting one year with same dates

DATEADD(MM, DATEDIFF(MM, 0, @Date) - 12, 0) --Substracting one year using integers instead of dates

Obviously, you'll have to use one of these formulas in your WHERE clause. If you have any more questions, feel free to ask.