判断两段时间之间的时间差

DateTime t1 = datenow;
                DateTime t2 = Convert.ToDateTime(ipcon.AddTime);
                int timesp = Convert.ToInt32(((TimeSpan)(t1 - t2)).TotalMinutes);
                if (timesp < 15)
                {
                    Page.RegisterStartupScript("ServiceManHistoryButtonClick", "<script>window.alert('你已经投过票了，谢谢参与！')</script>");
                }