One of the remaining open issues in proposal-canonical-tz how to handle the case when the OS's time zone ID is unrecognized by ECMAScript. Example: an older Electron app running on a newly-patched OS that has its time zone set to a new ID, like America/Ciudad_Juarez that was just added to the IANA Time Zone Database.

There's a PR at https://github.com/tc39/proposal-canonical-tz/pull/25 that proposes a solution: use the same Etc/Unknown ID that ICU uses to handle this case, but also accept as input to Temporal and Intl.DateTimeFormat methods that accept IDs.

I would be grateful for feedback on this PR if you think this is a good (or a bad!) solution, and to hear any concerns that you have. Thanks!