I think that this will solve what you are asking, let me know if I misunderstood what you are looking for.

DECLARE

@numMonthsINT

,@targetDateDATETIME

,@firstDayDATETIME

,@lastDayDATETIME

SELECT @numMonths = 2

SELECT @targetDate = DATEADD(MM, @numMonths * -1,...