Gets the time at which each ledger was updated.
        Can be called at any time to get this information.

        :return: an ordered dict of outdated ledgers sorted by last update time (from old to new)
        and then by ledger ID (in case of equal update time)

def get_last_update_time(self):
        '''
        Gets the time at which each ledger was updated.
        Can be called at any time to get this information.

        :return: an ordered dict of outdated ledgers sorted by last update time (from old to new)
        and then by ledger ID (in case of equal update time)
        '''
        last_updated = {ledger_id: freshness_state.last_updated
                        for ledger_id, freshness_state in self._ledger_freshness.items()}
        return OrderedDict(
            sorted(
                last_updated.items(),
                key=lambda item: (item[1], item[0])
            )
        )