web.config
<appSettings>
  <add key = "startdate"  value="7"/>
</appSettings>

代码
protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Init_Page();
            BindData();
        }
    }
初始化
private void Init_Page()
    {
        double startdate = 0.0;
        startdate = (-1) * Convert.ToDouble(System.Configuration.ConfigurationManager.AppSettings["startdate"].ToString());
        txt_BeginDate.Value = DateTime.Now.AddDays(startdate).ToShortDateString();
        txt_EndDate.Value = DateTime.Now.ToShortDateString();
    }
private void BindData()
    {
        double startdate = 0.0;
        startdate = (-1) * Convert.ToDouble(System.Configuration.ConfigurationManager.AppSettings["startdate"].ToString());
        string sdate = (txt_BeginDate.Value == "" ? DateTime.Now.AddDays(startdate).ToShortDateString() : txt_BeginDate.Value);
        string edate = (txt_EndDate.Value == "" ? DateTime.Now.ToShortDateString() : txt_EndDate.Value);