Suppoese two textboxes as dtpReqDate and tpIssueDate.
DateTime(int year,int month,int day)
so break the string according to the above parameteres.
DateTime dtEndDate = new DateTime(int.Parse(dtpReqDate.Text.Substring(6, 4)), int.Parse(dtpReqDate.Text.Substring(0, 2)), int.Parse(dtpReqDate.Text.Substring(3, 2)));
DateTime dtStartDate = new DateTime(int.Parse(dtpIssueDate.Text.Substring(6, 4)), int.Parse(dtpIssueDate.Text.Substring(0, 2)), int.Parse(dtpIssueDate.Text.Substring(3, 2)));
TimeSpan devDate = dtStartDate.Subtract(dtEndDate);