ion returns an empty string (<code>''</code>).
      </p><p>
         The result is a string in the format defined for <code>xs:duration</code> in [<a href="http://www.w3.org/TR/xmlschema-2/#duration" class="offsite">3.2.6 duration</a>] of [<a href="http://www.w3.org/TR/xmlschema-2/" class="offsite">XML Schema Part 2: Datatypes</a>].
      </p><p class="note">
         The durations can be summed by summing the numbers given for each of the components in the durations.
      </p></div><h2>Implementations</h