Tim Parenti via tz wrote in <cafpi07xpbqdhmkkvwrqgfwqwuqpds5kidxtkygjnggqmflu...@mail.gmail.com>: ... |On Fri, 6 Mar 2026 at 18:27, Mark Davis Ⓤ via tz <[email protected]> wrote: | |> CLDR (and the software that uses it) does not use the TZDB abbreviations |> at all; it only depends on the timezone ID, and the offsets & tm_isdst |> flags produced by the TZDB rules. |> | |On Fri, 6 Mar 2026 at 17:01, Robert Bastian via tz <[email protected]> wrote: | |> All that ICU sees is tm_isdst=0. The link between time zones and their |> "metazone", which determine display names, comes from CLDR. | |A potential constructive suggestion here would be improving programmatic |checks in software using CLDR in conjunction with tzdata, such as ICU. In |particular, it seems wise to shift away from relying solely on the tm_isdst |flag (which per RFC 9636 is subject to the specifics of "law or practice"), |and to instead include calculated offsets in consideration.
A bit hairy to note that just in the 2024 release of the POSIX standard there was Austin Group Defect 1614 is applied, clarifying the requirements for the tm_isdst member of the tm structure. and they made a painstaking effort (including the wonderful kre@ if i recall correctly) to come to The DESCRIPTION section says that mktime( ) converts the specified broken-down time into a time since the Epoch value. The use of the indefinite article here is necessary because, when tm_isdst is negative and the timezone has Daylight Saving Time transitions, there is not a one-to-one correspondence between broken-down times and time since the Epoch values. The description of how the value of tm_isdst affects the behavior of mktime( ) is shaded CX because the requirements in the ISO C standard are unclear. The next revision of the ISO C standard is expected to state the requirements using wording equivalent to the wording in this standard. There is more regarding issue 1614 and tm_isdst there. ... --steffen | |Der Kragenbaer, The moon bear, |der holt sich munter he cheerfully and one by one |einen nach dem anderen runter wa.ks himself off |(By Robert Gernhardt)
