On Aug 31, 2023, at 8:59 PM, Paul Eggert via tz <tz@iana.org> wrote:

As mentioned in [1], we suggest that any rule change be announced at least a year before it affects how clocks operate; otherwise, there is a good chance that many clocks will be wrong due to delays in propagating updates.

For example, even if the official Time Zone Database (TZDB) is updated, that doesn't mean that all machines with operating systems and applications that provide the TZDB data to end users will instantly get their copies of the database updated. The TZDB maintainers don't have the resources to provide that service, and the suppliers of those operating systems and applications probably wouldn't want us to do that - they probably want to do their own integration and testing of new TZDB releases and provide them to machines using *their* mechanisms. The TZDB maintainers have no control over how long that process takes, and thus do not have complete control over, for example, when Android or iOS smartphones will be updated; for Android, how long it takes after a TZDB release to get an update onto Android devices is up to Google (and smartphone manufacturers or telecommunications carriers?), and how long it takes after a TZDB release to get an update onto iPhones is up to Apple. The same applies to other suppliers of software that includes the TZDB. So the earlier rule change announcements are provided, the better, as that not only gives the TZDB maintainers time to prepare a release, it gives Google and Apple and Oracle and IBM and so on time to take that release when it comes out, do their own integration and testing, and provide updates.