ns/add-duration"><code>date:add-duration</code></a> function.
      </p><p>
         The string values of the nodes in the node set passed as the argument must be 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>].  If any of the string values of these nodes are not in this format, or if the node set is empty, the funct