display date in textbox from gridview/database in asp.net

I develop one web form by using ASP.net, I want to select the row and display that row data into web form but the date cannot display in the textbox of type DateTime, someone helps me to sort out this issue

ASP.Net Code

<div class="col-4">
    <asp:Label ID="Label7" runat="server" Text="Birth Date"></asp:Label>
    <asp:TextBox ID="txtBirthDate" runat="server" TextMode="Date"></asp:TextBox>
    </br>
    <asp:Button ID="btnSave" Text="Save" runat="server" OnClick="btnSave_Click" />
</div>
<div class="col-8">
    <asp:GridView ID="studentGrid" runat="server"
        DataKeyNames="Id"
        OnSelectedIndexChanged="StudentGrid_SelectedIndexChanged">
        <Columns>
            <asp:CommandField ButtonType="Button" ShowSelectButton="True" />
        </Columns>
    </asp:GridView>
</div>

c# Code

protected void StudentGrid_SelectedIndexChanged(object sender, EventArgs e)
{
    try
    {
        selId = Convert.ToInt16(studentGrid.SelectedDataKey[0]);

        if (selId > 0)
        {
            using (var ctx = new dbTestEntities())
            {
                var oldObj = ctx.tblStudents.Find(selId);
                if (oldObj != null)
                {
                    txtBirthDate.Text = oldObj.BirthDate.ToString();
                }
            }
        }
    }
    catch (Exception)
    {
        throw;
    }
}