When I create an appointment in my calendar that is due to take place in a different time zone, it appears in my calendar with incorrect times.
I don’t know how to fix your current issue, but Multiple time zones “is planned for eM Client 11”as per @Michal_Burger post below which then once implemented should “I presume also fix that issue” for setting an event time in a different time zone.
If that’s not correct someone will update this thread.
You might want to add your comments to the Multiple Time zone Sleekplan for adding events.