user exit for a variable to calculate the last 3-month period

Dear all,

I am working on BEx query and try to create a variable with user exit so it doesnu2019t require user to enter anything to run. I would like to create a variable that automatically calculate for the last 3 month period.

E.g. today is March 19, 2009. The variable should show the range from 12/2008 to 2/2009. I am not good at ABAP and I wonder whether anyone can give me some examples so I can start to work on. Thanks.