Usually the date value is passed in something like below.

http://server/ReportServer?/dir/Report Name&rs:Command=Render&StartDt=2012-11-01&EndDt=2012-11-10

or

http://server/ReportServer?/dir/Report Name&rs:Command=Render&StartDt=2012/11/01&EndDt=2012/11/10

Since you are using a parameter just pass in the parmetername.