Description

Importing an .ics file into Proton Calendar fails when a VEVENT references a custom timezone (e.g. "Customized Time Zone").

This appears to happen when the TZID parameter is used directly in event fields.

Example

Example snippet from the ICS file:

BEGIN:VEVENT
DTSTART;TZID=Customized Time Zone:20260316T083000
DTEND;TZID=Customized Time Zone:20260316T093000
SUMMARY:Example Event
END:VEVENT
Expected behavior

Proton Calendar should either:

resolve the custom TZID via the VTIMEZONE definition in the ICS file, or

fall back to the provided UTC offset.

Actual behavior

The import fails because the custom timezone cannot be processed.

Context

ICS files exported from calendar clients such as Outlook often use custom timezone identifiers like "Customized Time Zone".

Supporting these would improve compatibility when importing external calendars.