We used a zabbix 1.8.2 VMware appliance before. Shortly we decided to migrate to 1.8.3 this way:
"mysqldump --all-databases" on old appliance
import of mysqldump on new appliance

Now the displayed time in the graph slidebar is still off by one hour. According to the changelog of 1.8.3 this bug should be fixed so I blame our upgrade process. Still we need help to solve this.