You must learn How to play with Offset Variables in Bex as you are new to Bex.
1) Create a interval variable by using "Current Month Standard Variable" and your range variable should look like below and restrict your KF with it.
Current Month Variable(which will always current system date/month)-23 -- Current Month Variable(which will always current system date/month)
2) Create a Variable for "End Date" as User entry based. So user enters some date will cross check with above whole KF and returns you values accordingly.
I mean you must hide your Step 1 KF and use the same KF in a New Selection restricting with Step 2 variable as well. Hence you are restricting your KF with 24 Months range as well as with "End Date" logic. If you are not able to understand my steps, you can show this to your colleagues.