Problem with dates in GridView while send to mail attachment(as excel)

StringWriter stw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(stw); 
        GridView1.RenderControl(hw);
        MailMessage mail = new MailMessage();
        mail.IsBodyHtml = true;
        mail.To.Add(new MailAddress("abc"));
        mail.Subject = "report";
        System.Text.Encoding Enc = System.Text.Encoding.ASCII;
        byte[] mBArray = Enc.GetBytes(stw.ToString());
        System.IO.MemoryStream mAtt = new System.IO.MemoryStream(mBArray, false);
        mail.Attachments.Add(new Attachment(mAtt, "def.xls"));
        mail.Body = "Please find the attached document for the date" + DateTime.Now.ToString("dd-MMM-yyyy");
        SmtpClient smtp = new SmtpClient();
        mail.From = new MailAddress(efg, hij);
        smtp.Host = "smtp.gmail.com";
        smtp.UseDefaultCredentials = false;
        smtp.Credentials = new System.Net.NetworkCredential(***,***);
        smtp.EnableSsl = true;
        smtp.Send(mail);