[tz] Irish Standard Time vs Irish Summer Time

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Dec 11 06:40:36 UTC 2017

On 2017-12-10 22:16, Paul Eggert wrote:
> Michael H Deckers via tz wrote:
>>       The definition
>>       of the tm_isdst flag does not even mention standard time
> True, but other parts of POSIX make it clear that when tm_isdst is zero,
> standard time is intended. See:
> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
> and look at the TZ environment variable: POSIX says that its first few letters
> correspond to standard time and that later letters correspond to daylight saving
> time.

They actually say that the alternative time zone is in effect, such as for DST.
So tm_isdst actually means the alternative non-standard time zone "tm_isalt" and
not necessarily DST per se.

>> any dictionary tells us that daylight-saving time is advanced, and not
>> retarded, over the time used otherwise
> Although that's typical I doubt whether we can take it as an axiom, as POSIX
> clearly allows DST to be retarded. Also, multiple sources talk about having
> standard time in summer and daylight-saving time in winter. See:
> https://en.wikipedia.org/wiki/Winter_time_(clock_lag)

That could be usefully cross-referenced to the relevant zones or rules.
I can see headaches coming for testers and implementers who did not allow for
those cases!

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

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