Hi Siber,
Try to replace the below two lines
sqlDataSource.SelectParameters.Add("@startdate", Convert.ToDateTime(startdate));
sqlDataSource.SelectParameters.Add("@enddate", Convert.ToDateTime(enddate));
Here you can see I have converted to dateTime type since I believe field in database is in DateTime type.
Note: Before that you need to make sure startdate and enddate has values.
Hope this helps you a bit.
Regards,
RK