Textbox Calendar Validation ISSUE DAte and Expiry Date Calendar Exender.

protected void TextBox2_TextChanged(object sender, EventArgs e)
{
        DateTime date1 = Convert.ToDateTime(TextBox1.Text);
        DateTime date2 = Convert.ToDateTime(TextBox2.Text);
        int result = DateTime.Compare(date1, date2);
        if (result > 0)
        {
            Response.Write("<script type='text/javascript'>");
            Response.Write("alert('not valid');");
            Response.Write("</script>");
        }
}