You can use the Add method to add more than one kind of time interval (days, hours, minutes, seconds, or milliseconds) in a single operation. This method's behavior is identical to that of the addition operator. The DateTime structure also supports specialized addition methods (such as AddDays, AddHours, and AddMinutes) for each time interval.
The Add method takes into account leap years and the number of days in a month when performing date arithmetic.
This method does not change the value of this DateTime. Instead, it returns a new DateTime whose value is the result of this operation. The Kind property of the new DateTime instance is the same as that of the current instance.
// Calculate what day of the week is 36 days from this instant.
System.DateTime today = System.DateTime.Now;
System.TimeSpan duration = new System.TimeSpan(36, 0, 0, 0);
System.DateTime answer = today.Add(duration);