2 months comparison in Bex reports

I need to create a reports where the user will enter the parameter for selected month / year.

In the output it should show the result for 2 column .. that is selected month data and (selected month_1) data.

Suppose user enter the 001/2008 means the out put data will have 2 columns 001/2008 data, 012/2007 data.

Which type of selection,variable to be used for the period.

Sure marks for helpful answers.