I am not sure where you are getting the error. Better you debug your code and find out where is the problem.
raf.SDate = Convert.ToDateTime(txtStart.Text).ToString("dd-MM-yyyy"); raf.EDate = Convert.ToDateTime(txtEnd.Text).ToString("dd-MM-yyyy");
If your above code doesn't trough any exception, then try the following:
SELECT ReferAFriend.* FROM ReferAFriend where subscribe = @subscribe and CONVERT(DATETIME,@datetype,103) between CONVERT(DATETIME,@SDate,103) and CONVERT(DATETIME,@EDate,103)