Return the string representation of the time range

            :param local: if False [default] use UTC datetime. If True use localtz

def str(self,local):
        """ Return the string representation of the time range

            :param local: if False [default] use UTC datetime. If True use localtz
        """
        s = self.start_time.str(local) \
            + u" to " \
            + self.end_time.str(local)
        return s