Javascript to Default date in date/value prompt to last weeks date

[actcognosuser]

Hi All,
            I have a value  prompt which needs to display previous week.I have defaulted the date in the query.So the drop down always shows previous week.This previous week date parameter is reference by multiple queries.

When i run the report I need the prompt to execute with previous weeks date ,without the user having to select a date.i.e the finish button should be enabled as soon as the user clicks run.