window.returnValue问题

Question

sel_date.aspx :      <script language="javascript">        function GetDate()
        {
            var reVal=window.showModalDialog("day.aspx", '',
                "status:no;center:yes;scroll:no;resizable:no;help:no;dialogWidth:340px;dialogHeight:250px");
            if(reVal!="")
            {
            document.formAct.txt_receive_date.value=reVal;  //返回所选的日期
            };
        };
    </script>------------------------------------------------day.aspx    protected void Calendar1_SelectionChanged(object sender, EventArgs e)
    {
        string sel_date = Calendar1.SelectedDate.ToShortDateString();
        Response.Write("<script>window.returnValue=" + sel_date + ";</script>");
        Response.Write("<script>window.close();</script>");
    } 问题是返回的时候：总是出现选择2008-09-01确是1998等错误的信息