using System;

public class Program

{

    public static void Main()

    {

        var duration = TimeSpan.FromDays(3).Add(TimeSpan.FromHours(9));

        var convertedString = System.Xml.XmlConvert.ToString(duration);

        Console.WriteLine("converted TimeSpan: " + convertedString);

        var iso8601String = "PT72H";

        var convertedTimeSpan = System.Xml.XmlConvert.ToTimeSpan(iso8601String);

        Console.WriteLine("converted iso8601 string: "+ convertedTimeSpan);

    }

}