rooms availability overview wrongly handles timezone

Steps to reproduce

1. Have a room

   • For example by creating it through:

     php occ calendar-resource:building:create \
       --address="Testweg 23, 12345 Berlin, Germany" \
       "Berlin office"
     
     php occ calendar-resource:story:create 1 "Second floor"
     
     php occ calendar-resource:room:create \
       --wheelchair-accessible=1 \
       --capacity=25 \
       --room-number=201 \
       --contact-person-user-id="demouser" \
       1 \
       "berlin-office-room-201" \
       "Meeting room 201" \
       "room.berlin.office.201@example.invalid" \
       "Meeting room"
     

2. Under "Calendar settings > General" select a timezone different from your browser/OS
   • e.g. browser/OS has "Berlin" and "Tokyo" in settings
3. Start creating an event
4. Search for a room
5. Check the rooms availability

Expected behavior

Selected time (11AM - 1PM, shown in blue in the screenshot) matches with the time of the event (4AM - 6AM).

Actual behavior

Calendar app version

6.5.2

CalDAV-clients used

No response

Browser

No response

Client operating system

No response

Server operating system

No response

Web server

None

Database engine version

None

PHP engine version

None

Nextcloud version

No response

Updated from an older installed version or fresh install

None

List of activated apps

Nextcloud configuration

Web server error log

Log file

Browser log

Additional info

Note that we have at least four timezones when looking at an event:

1. The timezone provided by the browser.
2. The timezone set in personal Nextcloud settings.
3. The timezone set in Calendar settings.
4. The timezone set on the event.

(4.) is set to (3.) by default but can be changed by the user.
In the room availability overview always use the timezone set on the event that is currently edited (4.).

This probably needs only fixing in the frontend. It is probably an issue with either (or many of):

• the timezone set in the timeline UI component
• the timezone conversion after receiving free-busy data