i got this error"Error converting data type nvarchar to datetime. "

<%@ Page Title="" Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="SalesOrder.aspx.cs" Inherits="SalesOrder" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">

    <script type="text/javascript" language="javascript">
        function GetTotal() {
            var val1 = document.getElementById('<%=txtq.ClientID%>').value;
            var val2 = document.getElementById('<%=txtp.ClientID%>').value;
            if (val1 != "" && val2 != "") {
                document.getElementById('<%=txttotal.ClientID%>').value = val1 * val2;
            }

        }

    </script>

    <style type="text/css">
        .style9 {
            height: 34px;
        }
    </style>

</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder3" Runat="Server">
    <asp:Panel ID="Panel1" runat="server">
        <br />
        <asp:HyperLink ID="HyperLink12" runat="server" NavigateUrl="~/Customer.aspx">Customer Details</asp:HyperLink>
        <br />
        <br />
        <br />
        <asp:HyperLink ID="HyperLink13" runat="server" NavigateUrl="~/SalesOrder.aspx">Sales Order</asp:HyperLink>
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
        <br />
    </asp:Panel>
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">
    <h1 style="font-family: Andalus; font-size: x-large; color: #FF00FF;" >Sales Order</h1>
    <table>

<tr>
<td align="right">Customer ID</td>
<td>
    <asp:DropDownList ID="ddlcid" runat="server" DataSourceID="SqlDataSource1"
        DataTextField="Customer_Id" DataValueField="Customer_Id" AppendDataBoundItems="true">
        <asp:ListItem>---select Customer id---</asp:ListItem>
    </asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource1" runat="server"
        ConnectionString="<%$ ConnectionStrings:sqlserverConnectionString %>"
        SelectCommand="SELECT [Customer_Id] FROM [CustomerTable_]">
    </asp:SqlDataSource>
    </td>
</tr>
<tr>
<td align="right">Product Id</td>
<td>
    <asp:DropDownList ID="ddlpid" runat="server" DataSourceID="SqlDataSource2"
        DataTextField="Productcode" DataValueField="Productcode"
        AppendDataBoundItems="true"
        onselectedindexchanged="ddlpid_SelectedIndexChanged" AutoPostBack="True">
        <asp:ListItem>---select Product Id---</asp:ListItem>
    </asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource2" runat="server"
        ConnectionString="<%$ ConnectionStrings:sqlserverConnectionString %>"
        SelectCommand="SELECT [Productcode] FROM [Products]"></asp:SqlDataSource>
    </td>
</tr>
<tr>
<td align="right">Price</td>
<td>
    <asp:TextBox ID="txtp" runat="server"></asp:TextBox>
    </td>
</tr>
<tr>
<td align="right">Quantity</td>
<td>
    <asp:TextBox ID="txtq" runat="server"></asp:TextBox>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
        ErrorMessage="*" ForeColor="Red" ControlToValidate="txtq"></asp:RequiredFieldValidator>
    </td>
</tr>

<tr>
<td align="right" class="style9">Total</td>
<td class="style9">
    <asp:TextBox ID="txttotal" runat="server"></asp:TextBox>
    <asp:Button ID="add" runat="server" Text="add" OnClientClick="return GetTotal();" />
&nbsp;</td>
</tr>
<tr>
<td align="right">Date Of Delivery</td>
<td>

    <asp:TextBox ID="txtdate" runat="server"></asp:TextBox>
  <asp:CalendarExtender ID="calender1" runat="server" TargetControlID="txtdate"
        Format="dd/MM/yyyy"></asp:CalendarExtender>


    </td>
</tr>
<tr>
<td colspan="2" align="center">

    <asp:Button ID="btnsubmit" runat="server" Text="Submit"
        onclick="btnsubmit_Click" />

</td>
</tr>
</table>
    <asp:Label ID="Label1" runat="server" ></asp:Label>

</asp:Content>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

public partial class SalesOrder : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

       // ScriptManager.RegisterClientScriptBlock(this,this.GetType(),"hi","GetTotal()",true);
    }
    

   
    protected void btnsubmit_Click(object sender, EventArgs e)
    {
        if (txtq.Text == txtq.Text)
        {
            SqlConnection con = new SqlConnection("Data Source=CKRBTECH\\SQLEXPRESS;Initial Catalog=sqlserver;Integrated Security=True");

            SqlCommand cmd = new SqlCommand("sp_quantity", con);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@cid", ddlcid.SelectedItem.ToString());
            cmd.Parameters.AddWithValue("@prodid", ddlpid.SelectedItem.Value.ToString());
            cmd.Parameters.AddWithValue("@price", txtp.Text.ToString());
            cmd.Parameters.Add("@qty", txtq.Text.ToString());
            cmd.Parameters.AddWithValue("@total", txttotal.Text.ToString());
            cmd.Parameters.AddWithValue("@date", txtdate.Text.ToString());
            cmd.Parameters.Add("@error", SqlDbType.VarChar, 500);
            cmd.Parameters["@error"].Direction = ParameterDirection.Output;
            con.Open();
            cmd.ExecuteNonQuery();
            string message = (string)cmd.Parameters["@error"].Value;

            Label1.Text=message;
            con.Close();
            //Response.Write("data inserted successfully");
        }
        //else
        //{
          //  Response.Write("doesn");
        //}
       // Response.Redirect("~/Sales.aspx");
       
    }

    protected void ddlpid_SelectedIndexChanged(object sender, EventArgs e)
    {
        showdata(ddlpid.SelectedItem.Value);
    }
    private void showdata(string id)
    {
        DataTable dt = new DataTable();
        SqlConnection con = new SqlConnection("Data Source=CKRBTECH\\SQLEXPRESS;Initial Catalog=sqlserver;Integrated Security=True");
        SqlCommand cmd = new SqlCommand("select Price from Products where Productcode=@prodid", con);
        cmd.Parameters.AddWithValue("@prodid", ddlpid.SelectedItem.Value);
        con.Open();
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        da.Fill(dt);
        if (dt.Rows.Count > 0)
        {
            txtp.Text = dt.Rows[0]["Price"].ToString();
        }


        con.Close();

    }
    private void showdata()
    {
        DataTable dt = new DataTable();
        SqlConnection con = new SqlConnection("Data Source=CKRBTECH\\SQLEXPRESS;Initial Catalog=sqlserver;Integrated Security=True");
      SqlCommand  cmd = new SqlCommand("select quantity from Products where Productcode=@prodcode", con);
      cmd.Parameters.AddWithValue("@prodcode",ddlpid.SelectedItem.Value);
      con.Open();
      SqlDataAdapter da = new SqlDataAdapter(cmd);
      da.Fill(dt);
      if (dt.Rows.Count > 0)
      {
          txtq.Text = dt.Rows[0]["quantity"].ToString();
      }
      con.Close();

    }


    
}