Overriding Defaults

I have a stored procedure that has @BeginDate and @EndDate as parameters. I created a report with a default for both. They run just fine. After I deployed, I created Linked Reports and wanted to override the defaults. In the defaults, I tried to put in GetDate() for @BeginDate and GetDate()+10 for the @EndDate so this can be passed in the where statement of the stored procedure. I get 'Syntax error converting datetime from character string.'

What I assume is that if I override the default, the stored procedure will process what I put in the @BeginDate and @EndDate parameters.

The where statement looks like:

and (m.BeginDate >= @BeginDate) and (m.EndDate <= @EndDate)

I'm using Reporting Services 2005 and SQL Server 2005.

Thanks, Iris