DateTime dFrom;
DateTime dTo;
string sDateFrom = "11:56:00";
string sDateTo = "12:12:00";
if (DateTime.TryParse(sDateFrom, out dFrom) && DateTime.TryParse(sDateTo, out dTo))
{
TimeSpan TS = dTo - dFrom;
int hour = TS.Hours;
int mins = TS.Minutes;
int secs = TS.Seconds;
string timeDiff = hour.ToString("00") + ":" + mins.ToString("00") + ":" + secs.ToString("00");
Response.Write(timeDiff);
}