public void Valid_duration(string iso8601, SerializableTimeSpan expected)
        {
            TimeSpan result;

            Iso8601Duration.TryParse(iso8601, out result).ShouldBe(true);

            result.ShouldBe(expected);
        }
 public void Formatting(string expected, SerializableTimeSpan timeSpan)
 {
     Iso8601Duration.Format(timeSpan).ShouldBe(expected);
 }