Currently it is impossible to compare data for exact periods of time, for example, compare average of data in June vs July and alert user in case of significant difference. It makes monitoring based on time shifts rather limited and not user friendly. Besides there are performance related issues since Zabbix always use historical data for calculation of trigger expressions.
It would be great if Zabbix supported this functionality. It would enable generation of valuable insights and also significantly improve business value of Zabbix.