[tz] looking for information about changing time zone

Florian Weimer fweimer at redhat.com
Thu Sep 10 09:32:53 UTC 2020

* Paul Eggert:

> Please see:
> https://data.iana.org/time-zones/tz-link.html#changes
> which says this:
> If your government plans to change its time zone boundaries or
> daylight saving rules, inform tz at iana.org well in advance, as this
> will coordinate updates to many cell phones, computers, and other
> devices around the world. With less than a year's notice there is a
> good chance that some computer-based clocks will operate incorrectly
> after the change, due to delays in propagating updates to software and
> data. The shorter the notice, the more likely clock problems will
> arise; see "On the Timing of Time Zone 
> Changes"<https://codeofmatt.com/on-the-timing-of-time-zone-changes/> for examples.

Note that the notice period of one year is from the time the rule change
becomes official to when the difference becomes observable in practice
(in other words, when the rule change becomes effective).

This is not the same as the time from the start of the entire process to
the time it becomes effective, but this is the way most officials appear
to interpret it.  Most rule change proposals are in legal limbo for a
long period of time, and close to all of them fail to become effective.
This means that distributors of the time zone database can do little to
prepare when the process starts.  Instead, they have to wait until
changes become official.  Only at that point, it is possible to
distribute updates that change time zone rules for future dates.

One thing that might also come as a surprise (particulaly for DST
changes or any change at a DST boundary) is that the database format is
sufficiently flexible to express a rule change at an arbitrary future
date.  It is not restricted to making the change at the next DST
transition.  This means that distributors of the time zone database can
act on any official change immediately, making full use of the
notification period.

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