on returns the year of a date as a number.  If no argument is given, then the current local date/time, as returned by <code>date:date-time</code> is used as a default argument.
      </p><p>
         The date/time string specified as the first argument must be a right-truncated string in the format defined as the lexical representation of <code>xs:dateTime</code> in one of the formats defined in [<a href="http://www.w3.org/TR/xmlschema-2/" class="offsite">XML Schema Part 2: Datatypes</a>].  The permitted fo