Since the latest Nextcloud update (currently v. 12.0.0), the calendar app defaults to UTC after telling me "you are in an unknown timezone (Etc/GMT-10)".
How do I change it to my own timezone so events are shown on the right times / days ?
It is a known issue related to Nextcloud see: https://github.com/nextcloud/calendar/issues/490
It seems we cannot do much other than wait for developers to fix it...
Actually, the issue is related to default timezone. You can configure it per event (cant change the default yet).
You can set your location like here (click on the globe)
Actually, my issue seems to be a bit different (although the one you describe is an annoyance too) : my (previously created) events all have the right time-zone (set to Brisbane), but the calendar app renders the events in a UTC calendar after telling that I am in an unknown timezone. It seems Nextcloud / the calendar app fails to detect my timezone properly (when previous versions had no issues with that).
... and here is the corresponding conversation: https://github.com/nextcloud/calendar/issues/496
... and a patch that might fix it, if you are keen @pierreozoux : https://github.com/nextcloud/calendar/issues/496#issuecomment-326791894
... and an approved enhancement request that will allow the user to set their timezone themselves: https://github.com/nextcloud/calendar/issues/586
Hopefully that will happen soon.
It seems the calendar now uses the right time zone, and new events created in the web-app do too by default!
Not sure when the fix landed, I only noticed today.
Marking this as fixed.
I had the same issue and it was showing correct timezone automatically. I was pulling my hair off not finding a solution. Thanks to their dev team.