in date picker parameter how can i set default for last 30 days or 7 days?

hi

i'm using SSRS 2008 and i have an MDX report thatneed to display the data for last 30 days (until yesterday) and 7 days (until yesterday)

for example: today is 3/4/2013 if i choose the report for last 30 daysi want to see data from 4/3/2013-2/4/2013

and if i choose the report for 7 days i will see data from 27/3/20130 to 2/4/2013

i also want the user to be able to change the date with the date picker.what is the best way to do this?

i thought about maybe using a drop down list or something to choose between the 30,7 days and the dates in the date picker to be changed accordingly