As you’ve said yourself, that’s not defined in Discourse. It would be a very bad open source practice to handle it ourselves instead of delegating it to the standard library.
In Discourse we call the Rails Timezone class, which is a wrapper of the TZInfo gem, which reads from the tzdata of the underlying OS. We use Debian for our container image and I believe it sources it from Time Zone Database. So it may be better taken to IANA, in order to fix it everywhere.
EDIT: It appear that this change was already made in the Time Zone Database and it’s coming out on the new release, so our standard updates will catch it automatically.