Question: how to calculate date time difference in c#?
Answer: DateTime newDate = new DateTime(2000, 5, 1); Here newDate represents year as 2000 and month as May and date as 1 . ... 
System.TimeSpan diff = secondDate.Subtract(firstDate); You can also find the difference between two dates using the following method.
String diff2 = (secondDate - firstDate).TotalDays.ToString();