No you do not need a customer exit variable.
Input variable is nothing but a characteristic reference variable with manual input as processing type.
as shown in above image, after creating the variable, you can select Value Ranges. Then select the same variable that you created for Calendar Day/Month in the from and to box(in my case it is Var05).
Once you pass it to the adjacent window, you can select it and set offset. Note that you'll get to see two offset windows one after the other.
If you are using Month as input then set offset in first window as -11 and second window as 0.
If you are using Day as input then set offset in first window as -365 and second window as 0.
Hope your confusion is cleared.
Regards,
Gopal