Sure, sorry I didn't clarify clearly. This is only one parameter, which has 20 drop-downs of 20 date range. And we can choose any of it to render report. First one is based on the date renders report, and following are previous 20 cycles(months)

For example,

1. Today is 4/2/2012, then for this parameter, there are 20 drop-downs and we can choose any of them to render report. first drop-down should be 3/21/2012-4/20/2012, then 2/21/2012-3/20/2012, 1/21/2012-2/20/2012, 12/21/2011-1/20/2012, 11/21/2011-12/20/2011, 10/21/2011-11/20/2011......and so on

2. If report renders at 2/12/2012, first drop-down is 1/21/2012-2/20/2012 because date 2/12/2012 falls into this range, then 12/21/2011-1/20/2012, 11/21/2011-12/20/2011, 10/21/2011-11/20/2011, 9/21/2011-10/20/2011.....and so on