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;
    }
}   

// Difference in days, hours, and minutes.

TimeSpan ts = EndDate - StartDate;

// Difference in days.

int differenceInDays = ts.Days; // This is in int
double differenceInDays= ts.TotalDays; // This is in double

// Difference in Hours.
int differenceInHours = ts.Hours; // This is in int
double differenceInHours= ts.TotalHours; // This is in double

// Difference in Minutes.
int differenceInMinutes = ts.Minutes; // This is in int
double differenceInMinutes= ts.TotalMinutes; // This is in double

你也可以得到秒、毫秒和滴答的差异。

如果有人想要作为双精度的天数(a, b类型的DateTime):

 (a.Date - b.Date).TotalDays

假设StartDate和EndDate是DateTime类型:

(EndDate - StartDate).TotalDays

你可以使用下面的代码:

 int DateDifInSecond = EndDate.Subtract(StartDate).TotalSeconds