subtracting Dates and saving value in different column of database

<div class="gview"> 
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataSourceID="SqlDataSource1" HeaderStyle-HorizontalAlign="Center"  FooterStyle-HorizontalAlign="Center" CssClass="gview" DataKeyNames="PID" Width="613px">
        <Columns>
            <asp:BoundField DataField="PID" HeaderText="PID" SortExpression="PID" InsertVisible="False" ReadOnly="True" />
            <asp:BoundField DataField="Pname" HeaderText="Pname" SortExpression="Pname" />
            <asp:BoundField DataField="Gender" HeaderText="Gender" SortExpression="Gender" />
            <asp:BoundField DataField="Department" HeaderText="Department" SortExpression="Department" />
            <asp:BoundField DataField="Ward" HeaderText="Ward" SortExpression="Ward" />
            <asp:BoundField DataField="Bed" HeaderText="Bed" SortExpression="Bed" />
            <asp:BoundField DataField="Status" HeaderText="Status" SortExpression="Status" />
            <asp:BoundField DataField="AdmitDate" HeaderText="AdmitDate" SortExpression="AdmitDate" />
            <asp:BoundField DataField="DischargeDate" HeaderText="DischargeDate" SortExpression="DischargeDate" />
            <asp:BoundField DataField="NumOfDays" HeaderText="NumOfDays" SortExpression="NumOfDays" />
        </Columns>
        <FooterStyle HorizontalAlign="Center"></FooterStyle>
        <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    </asp:GridView>

    <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:SMCConnectionString %>" SelectCommand="SELECT [PID], [Pname], [Gender], [Department], [Ward], [Bed], [Status], [AdmitDate], [DischargeDate], [NumOfDays] FROM [Patient]">
    </asp:SqlDataSource>
 </div>

protected void Button6_Click(object sender, EventArgs e)
{

    string str = "update Patient (Department, Ward, Bed, Status, AdmitDate, DischargeDate) set Department= @Department,Ward=@Ward,Bed=@Bed,Status=@Status,AdmitDate=@AdmitDate,DischargeDate=@DischargeDate   where PID = '" + TextBox3.Text + "'";
    cmd = new SqlCommand(str, con);
    con.Open();
    cmd.Parameters.AddWithValue("@Department", DropDownList4.SelectedItem.Text);
    cmd.Parameters.AddWithValue("@Ward", DropDownList3.SelectedItem.Text);
    cmd.Parameters.AddWithValue("@Bed", DropDownList1.SelectedItem.Text);
    cmd.Parameters.AddWithValue("@Status", DropDownList2.SelectedItem.Text);
    cmd.Parameters.AddWithValue("@AdmitDate", TextBox1.Text);
    DateTime.Parse(TextBox1.Text.ToString());
    cmd.Parameters.AddWithValue("@DischargeDate", TextBox2.Text);
    DateTime.Parse(TextBox2.Text.ToString());
    var diff = DateTime.Parse(TextBox2.Text.ToString()).Subtract(DateTime.Parse(TextBox1.Text.ToString()));
    cmd.Parameters.AddWithValue("@NumOfDays", diff);
    cmd.ExecuteNonQuery();
    con.Close();
    show();
    Response.Write("Record Updated");
}

Now when I click the button it generates the following error:

    SqlDbType.Time overflow. Value '10.00:00:00' is out of range. Must be between 00:00:00.0000000 and 23:59:59.9999999.

I did quite a research and found out the idea that i have implemented. But it's no good so far. Any kind of help is greatly appreciated.