假设StartDate和EndDate是DateTime类型:

(EndDate - StartDate).TotalDays

DateTime xmas = new DateTime(2009, 12, 25);
double daysUntilChristmas = xmas.Subtract(DateTime.Today).TotalDays;

假设StartDate和EndDate是DateTime类型:

(EndDate - StartDate).TotalDays

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。

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;
    }
}   

我认为这会达到你的目的:

DateTime d1 = DateTime.Now;
DateTime d2 = DateTime.Now.AddDays(-1);

TimeSpan t = d1 - d2;
double NrOfDays = t.TotalDays;