@abc.abstractmethod<EOL><INDENT>def get_duration(self, units):<DEDENT>
return<EOL>
Gets the duration of the event.

        :param units: the units of the duration
        :type units: ``osid.calendaring.DateTimeResolution``
        :return: the duration
        :rtype: ``osid.calendaring.Duration``
        :raise: ``NullArgument`` -- ``units`` is ``null``

        *compliance: mandatory -- This method must be implemented.*