Change in Time zone
Historical data in System 800xA is handled with UTC time format (i.e. with no influences from time zone, nor daylight savings time). Trend clients (e.g. the Trend Display in the workplace) applies the current time zone offset to paint in local time.
On the contrary A&E is handled with local time. I.e. the local time offset is injected on A&E by the PLC or OPC Server (depending on implementation) before being collected by 800xA Event Collector service.
E.g. by changing a client PC's time zone one hour (+1h) ahead the trend data will also move one hour ahead, while your alarms will stay on the date/time they were raised with.