Change your calculation like below
datediff('month',[date],today())<=[How Many Months?]+1
It will give the correct O/P
Hope this helps
Plz mark this answer as correct or helpful to close the thread
Why are you confusing here. If you want to select last 12 months or any months in your db then create simple parameter with all and
create calculation last()<[last N Parameter].
Drag to filter then it will filter what you typed in parameter.
If you need to get selected month from some date then go with your calc and add that date parameter and remove today().