使用时间跨度可以解决问题，因为它有很多属性:

DateTime strt_date = DateTime.Now;
DateTime end_date = Convert.ToDateTime("10/1/2017 23:59:59");
//DateTime add_days = end_date.AddDays(1);
TimeSpan nod = (end_date - strt_date);
Console.WriteLine(strt_date + "" + end_date + "" + "" + nod.TotalHours + "");
Console.ReadKey();

上面的答案是正确的，但是如果你只想把整天作为整数，并且愿意放弃约会的时间部分，那么可以考虑:

(EndDate.Date - StartDate.Date).Days

同样假设StartDate和EndDate是DateTime类型。

得到两个日期之间的差值，然后从下面得到日期:

int total_days = (EndDate - StartDate).TotalDays

你可以试试这个

EndDate.Date.Subtract(DateTime.Now.Date).Days

date格式为"dd/MM/yyyy"

  string[] d1 = txtFromDate.Values.Split('/');
  string[] d2 = txtToDate.Values.Split('/');

  DateTime FrmDt = new DateTime(Convert.ToInt32(d1[2]), Convert.ToInt32(d1[1]), Convert.ToInt32(d1[0]));
  DateTime ToDt = new DateTime(Convert.ToInt32(d2[2]), Convert.ToInt32(d2[1]), Convert.ToInt32(d2[0]));

  TimeSpan TDiff = ToDt.Subtract(FrmDt);
  String DaysDiff = TDiff.TotalDays.ToString();

protected void Calendar1_SelectionChanged(object sender, EventArgs e)
{
    DateTime d = Calendar1.SelectedDate;
    // int a;
    TextBox2.Text = d.ToShortDateString();
    string s = Convert.ToDateTime(TextBox2.Text).ToShortDateString();
    string s1 =  Convert.ToDateTime(Label7.Text).ToShortDateString();
    DateTime dt = Convert.ToDateTime(s).Date;
    DateTime dt1 = Convert.ToDateTime(s1).Date;
    if (dt <= dt1)
    {
        Response.Write("<script>alert(' Not a valid Date to extend warranty')</script>");
    }
    else
    {
        string diff = dt.Subtract(dt1).ToString();
        Response.Write(diff);
        Label18.Text = diff;
        Session["diff"] = Label18.Text;
    }
}