How to pass SSRS params into a URL
(I don't have an SSRS server handy at the moment to test/confirm this, so I'm going a bit off-the-cuff here, and might not be 100% spot-on)

I've never had luck with using the ReportManager URLs when I want to pass in specific parameters. Try using the ReportServer URLs instead. 

BOL gives this example format for passing dates using the ReportServer URLs:
http://myserver/ReportServer/Pages/ReportViewer.aspx?%2fProduct_and_Sales_Report_AdventureWorks&amp;SellStartDate=7/1/2005