I recently discovered that since 2006, the ICU file icu4c/source/tools/tzcode/icuzones has had a line "Zone Etc/Unknown 0 - Unknown"[1]. Any POSIX-like system that feeds this line into 'zic' might run afoul of the recently-proposed TZDB guideline[2] that Etc/Unknown is reserved and is not used by TZDB. This is because the line will cause Etc/Unknown to be a valid timezone with UT offset zero and abbreviation "Unknown", so that, for example, tzalloc("Etc/Unknown") will succeed instead of failing. I don't think this is a practical problem, as I don't know of any platform that feeds that icuzones line into zic and thus makes "Etc/Unknown" a valid timezone. However, it might be helpful to add a comment to icuzones about the situation. Proposed ICU patch attached. I am cc'ing this to Markus Scherer (who added the line) and to Yoshito Umaoka (who most recently updated icuzones), to give them a heads-up on the situation. [1]: https://github.com/unicode-org/icu/blob/main/icu4c/source/tools/tzcode/icuzo... [2]: https://lists.iana.org/hyperkitty/list/tz@iana.org/message/5QWH2DCQ6SLY6JFJT...