DateTime st = DateTime.Now;
DateTime ed = DateTime.Now;
TimeSpan ts = ed.Subtract(st);
string output = String.Format("{0:00}:{1:00}:{2:00}", ts.TotalHours, ts.Minutes, ts.Seconds);

//時間與字串之間的轉換
//時間轉字串
string datestring = DateTime.Now.ToString("yyyy/MM/dd HH:mm:ss ffff")
string datestring = DateTime.Now.ToString("s", System.Globalization.DateTimeFormatInfo.InvariantInfo)
"s" 代表Sortable格式 "yyyy/MM/ddTHH:mm:ss"

//字串轉時間
DateTime st = DateTime.Parse(datestring);