You may want to try converting your strings to dates when you pass them to the parameters (assuming the paramters are not typed as strings). For example
CRT.StoredProcParam(0) = "@firstdate = '" & CDate(strStartDate) & "';true"
CRT.StoredProcParam(1) = "@lastdate '" & CDate(strEndDate) & " ';true"
It may resolve the issue since it the error message seems to reflect a data type problem.
Hope this helps
Regards,
John W.