def to_dictionary(self):
        """Serialize an object into dictionary form.  Useful if you have to
        serialize an array of objects into JSON.  Otherwise, if you call the
        :meth:`to_json` method on each object in the list and then try to
        dump the array, you end up with an array with one string."""

        d = {'start': self.start.isoformat(),
             'end': self.end.isoformat(),
             'tz': self.tz,
             'summary': self.summary.to_dictionary(),
             'series': self.series.to_dictionary()
             }
        return d