[tz] isdst bug Europe/Dublin (tzdb-2019c)

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Thu Dec 12 16:29:03 UTC 2019

On 2019-12-11 11:39, Paul Eggert wrote:
> On 12/11/19 10:12 AM, Michael H Deckers wrote:
>>     the tzdb description in
>>     newctime.3 ... guaranteed (in addition to what C and POSIX require)
>>     that
>>        "Tm_isdst is non-zero if summer time is in effect"
>>     and probably the "if" was even meant as "if and only if".
> I don't think that old wording was ever intended to mean that tm_isdst was to be
> set only in summer, or even only in daylight-saving periods that were used in
> summer and adjacent time periods. Such a meaning would have been contradicted by
> longstanding data, such as Belize's observance of DST from October through
> February in late 1918 through early 1943.
> Instead, it was merely sloppy wording where "summer time" was incorrectly used
> as a synonym for "daylight saving time", the wording that is in the relevant
> standards and that corresponds to the abbreviation "tm_isdst".

It is more likely that the term was correctly used in some other locale, where
summer (and winter) time is often the legal wording in various languages, where
the equivalents of "daylight saving" and "standard", used in North America, are
not used or don't exist; sometimes the equivalents of "civil time" and "advanced
time" are used instead, but nothing helps when standard is advanced relative to
the alternative.

I preferred whichever reference used the terms "standard time" and "alternative
time", as that seems to be a more accurate wording for current (existing) usage.
Of course, summer and winter time are also useful and accurate, but the offset
relative values may not be as expected.

Perhaps tm_isdst should always be set to -1 for inverted zones, where it does
not represent summer or saving time, or even set it for all zones permanently to

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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