Calendar shows events exported from Thunderbird in a wrong timezone

Distribution

Mint 21.2

Package version

5.8.4

Graphics hardware in use

No response

Frequency

Always

Bug description

The (Gnome) calendar applet is showing events in a wrong timezone - as if they were specified in UTC and then applies the conversion to local TZ.
For example, events scheduled at 9:30 are shown as occurring at 11:30 due desktop using CEST.

The same applies for the Gnome Calendar app (41.2) view:

Steps to reproduce

• Set up synchronization of calendar in Thunderbird with Gnome Calendar
• Enable "Show calendar events" in the Calendar Applet configuration
• Accept an invitation to an event in Thunderbird

Expected behavior

The times should be the same, no TZ conversion should take place.

Additional information

• Thunderbird 115.3.1 from Cinnamon repo
• Gnome Calendar app 41.2+mint1+vanessa from Cinnamon repo

I found Gnome bugreports such as this one, mentioning this exact problem. Maybe Cinnamon is just missing backports of some of the fixes?

Start by reproducing the Thunderbird synchronization and accepted-invitation flow, then compare the Calendar Applet with GNOME Calendar 41.2 using the linked GNOME issue 444 as context. Trace the existing calendar-event integration; done means events display the same scheduled time without unintended timezone conversion.