Ref T11816. My read of RFC 5545 is that applications can do whatever they want here. Although most applications use legal timezonedb values like "America/Los_Angeles", at least one ("Zimbra") has at least one event with a weird value (TZID="(GMT-05.00) Auto-Detected").
Try to puzzle out what these mysterious identifiers might intend. For now, I added a rule to look for "UTC+3", "GMT-2:30", etc.
If we don't have any guesses, just use UTC. If we guess or fall back, raise a warning so the user can see what happened.