Error converting data type nvarchar to date in asp.net

      protected void btnDisplay_Click(object sender, EventArgs e)

        {

            Hashtable ht = new Hashtable();

            ArrayList arrlst = new ArrayList();

            string fromdt = txtFromDate.Text.Substring(txtFromDate.Text.Length - 4) + "/"     + txtFromDate.Text.Substring(3, 2) + "/" + txtFromDate.Text.Substring(0, 2);

            string todt = txtTodate.Text.Substring(txtTodate.Text.Length - 4) + "/" + txtTodate.Text.Substring(3, 2) + "/" + txtTodate.Text.Substring(0, 2);

            arrlst.Add(fromdt);

            arrlst.Add(todt);

            DataSet ds = bal.Search_InventoryDetails(arrlst);

            GrdInventory.DataSource = ds.Tables[0];

            GrdInventory.DataBind();

           

        }