ASP.NET

if ($("#ddlRptType").val() == "Monthly") {

     var url = "reportPage.aspx?rpttype=" + $("#ddlRptType").val() + "&mm=" +  $("#ddlMonths").val() + "&yy=" + $("#ddlYear").val();

     window.open(url);
 }
 else if ($("#ddlRptType").val() == "Daily") {

     var url = "reportPage.aspx?rpttype=" + $("#ddlRptType").val() + "&date=" + $('#dailyDatePickerBox').val();

     window.open(url);
 }

string rpttype = Request.QueryString["rpttype"];

           switch (rpttype)
           {
               case "Monthly":
                   string mm;
                   string yyyy;
                   mm = Request.QueryString["mm"];
                   yyyy = Request.QueryString["yy"];
                   GetMonthlyData(rpttype, mm, yyyy);
                   break;
               case "Daily":
                   string stringdate;
                   stringdate = Request.QueryString["date"];
                   GetDailyData(stringdate, rpttype);
                   break;
           }

stringdate = Request.QueryString["date"];

<div runat="server" id="watermark_cancelled">

And style sheet is:

#watermark_cancelled    
{
    width:720px;    
    position:absolute;    
    z-index: -1;    
    background-image:url('../images/Cancelled_WM1.png');
    background-repeat: no-repeat;
    background-attachment: fixed;
    position:absolute;
    z-index: -1;
    display:block;
}

protected void ExportToExel_Click(object sender, EventArgs e)
        {
            string a = string.Format("{0:MM/dd/yyyy}", dtBeginDate.SelectedDate.Value); //get value of FromDate
            string b = string.Format("{0:MM/dd/yyyy}", dtEndDate.SelectedDate.Value); //get value of ToDate

            //call function to Read SQL database between FromDate & ToDate.
            SqlDataReader dtReader = VNPTTracking.DataAccess.Report.ReportOnlineVehicle(id, a, b);


            // Prepair to export to Excels
            HttpContext context = HttpContext.Current;
            context.Response.Clear(); 

            //Vietnamese, so I have to use this function
            context.Response.ContentEncoding = System.Text.Encoding.Unicode;             context.Response.BinaryWrite(System.Text.Encoding.Unicode.GetPreamble());

            // Write header in row 1 in Excels
            string[] s = { "Biển số", "Tỉnh/TP", "Khách hàng", "Tài khoản", "Mã thiết bị", "Cảm ứng dầu", "Địa chỉ hiện tại", "Thời điểm nhận tin cuối" };
            int n = 0;
            for (int i = 0; i < s.Count(); i++)
            {
                context.Response.Write(s[n] + "\t");  //get value s[n] of s matrix, add "\t" to mark position where write in next column
                n++;
            }
            context.Response.Write(Environment.NewLine); 
            
            
            while (dtReader.Read()) //run the data reader. Each line write a row in Excels
            {
                context.Response.Write(dtReader.GetValue(0) + "\t"); //Biển số
                context.Response.Write(dtReader.GetValue(1) + "\t"); //Tỉnh/TP
                context.Response.Write(dtReader.GetValue(2) + "\t"); //Khách hàng
                context.Response.Write(dtReader.GetValue(3) + "\t"); //Tài khoản
                context.Response.Write(dtReader.GetValue(4) + "\t"); //Mã thiết bị

                if (Convert.ToInt32(dtReader.GetValue(5)) == 1) //Cảm ứng dầu
                {
                    context.Response.Write("Có" + "\t");
                }
                else context.Response.Write("Không" + "\t");

                context.Response.Write(dtReader.GetValue(6) + "\t"); //Địa chỉ hiện tại
                context.Response.Write(dtReader.GetValue(7) + "\t"); //Thời điểm nhận tin cuối
               
                context.Response.Write(Environment.NewLine); 
                            
            }

            // File name
            context.Response.AddHeader("content-disposition", "attachment; filename=Report.xls");
            context.Response.ContentType = "application/ms-excel";
            context.Response.End();
  
        }