How to use the croniter.croniter._datetime_to_timestamp function in croniter

rule['aggregate_alert_time'][aggregation_key_value] = alert_time
                agg_id = pending_alert['_id']
                rule['current_aggregate_id'] = {aggregation_key_value: agg_id}
                elastalert_logger.info(
                    'Adding alert for %s to aggregation(id: %s, aggregation_key: %s), next alert at %s' % (
                        rule['name'],
                        agg_id,
                        aggregation_key_value,
                        alert_time
                    )
                )
            else:
                # First match, set alert_time
                alert_time = ''
                if isinstance(rule['aggregation'], dict) and rule['aggregation'].get('schedule'):
                    croniter._datetime_to_timestamp = cronite_datetime_to_timestamp  # For Python 2.6 compatibility
                    try:
                        iter = croniter(rule['aggregation']['schedule'], ts_now())
                        alert_time = unix_to_dt(iter.get_next())
                    except Exception as e:
                        self.handle_error("Error parsing aggregate send time Cron format %s" % (e), rule['aggregation']['schedule'])
                else:
                    if rule.get('aggregate_by_match_time', False):
                        match_time = ts_to_dt(lookup_es_key(match, rule['timestamp_field']))
                        alert_time = match_time + rule['aggregation']
                    else:
                        alert_time = ts_now() + rule['aggregation']

                rule['aggregate_alert_time'][aggregation_key_value] = alert_time
                agg_id = None
                elastalert_logger.info(
                    'New aggregation for %s, aggregation_key: %s. next alert at %s.' % (rule['name'], aggregation_key_value, alert_time)