[tz] Single source of truth for timezone data

Mark Davis ☕️ mark at macchiato.com
Thu Dec 8 19:59:34 UTC 2022

> As I understand it, if TZDB releases version X, Android and Apple
currently wait for ICU/CLDR to release a new version that takes X's
changes into account.

The only changes that CLDR worries about are ones that would have an effect
on the choice of 'labels' in formatting dates and times. You can see some
of those in

For example, if a version X of the TZDB has no new zones, and changes to
the rules for a zone don't require a change to a metazone (e.g. which zones
it encompasses, or a new metazone), then nothing needs to happen.

Even when a change is needed, in many cases the normal fallback among the
different types of 'labels' means that people will see something reasonable
until an implementation upgrades to the latest CLDR.

Moreover, CLDR has become over time more insulated from unnecessary
instabilities in the TZDB, such as changes in spellings for identifiers,
changes in the zone1970.tab file (the backward-compatibility aids from the
TZDB are extremely helpful for that!), etc.

However, just as with the TZDB, there are cases where countries change
their rules on crazy-short notice, and where CLDR needs to respond quickly.
We try to do dot releases for those cases, but often translations for zones
or metazones will lag since we have over 100 languages to manage. Offline,
people can contact me at mark at unicode.org if they have suggestions for


[Yoshito is the expert, so he can correct this if some details are off.]

On Wed, Dec 7, 2022 at 10:08 AM Paul Eggert via tz <tz at iana.org> wrote:

> On 2022-12-07 03:37, Almaz Mingaleev via tz wrote:
> > Does that mean that libc APIs will behave differently from whatever
> > is using ICU files?
> No, as I understand it the idea is to allow automatic rebuilding of the
> timezone-related part of the CLDR/ICU data, whenever a new TZDB version
> is released, without having to wait for updates from CLDR/ICU.
> As I understand it, if TZDB releases version X, Android and Apple
> currently wait for ICU/CLDR to release a new version that takes X's
> changes into account. The idea is that we could improve on this by
> developing a procedure that, given a new TZDB release X and an older
> CLDR/ICU release W, would produce a slightly-modified copy W' of the
> CLDR/IDU data that takes X's changes into account, without having to
> wait for ICU/CLDR to produce a new version. Of course when ICU/CLDR get
> around to producing a new version X themselves, you could discard W'.
> The main obstacles I see to this are:
> * Copyright issues. Unicode, Inc. does not allow people to redistribute
> modified versions of their data files, even if the modifications are
> clearly needed.
> * Institutional inertia. ICU/CLDR is big and is unaccustomed to moving
> fast, and timezone info is not high priority for ICU/CLDR.
> Although we haven't so far seen any sign of interest here from the
> Unicode, Inc. side, perhaps they'll get around to it if some big
> customers ask them in the right way.
> By the way, what bad things would happen if Android and Apple *didn't*
> wait? In other projects I've dealt with, if an upstream program P adds a
> new timezone abbreviation "XYT", downstream distros like Ubuntu can
> immediately ship the new P without waiting for i18n updates. P will
> still work fine in all locales, except that the new "XYT" abbreviation
> (should it be needed) will appear as-is until translators catch up.
> Could Android and Apple do something similar with timezone-related strings?
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