[tz] Irish Standard Time vs Irish Summer Time

Clive D.W. Feather clive at davros.org
Tue Dec 12 13:17:00 UTC 2017

Paul Eggert said:
> 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.

Actually, it says "the alternative timezone", with "Daylight Savings Time"
being an *example*.

> But the POSIX TZ requirement is that tm_isdst must be zero during 
> standard time,


> so if IST denotes standard time then current Irish rules should 
> be represented by something like POSIX TZ='IST-1GMT0,M10.5.0/2,M3.5.0/1'.

Looks right.

>>   any dictionary tells us that daylight-saving time
>>   is advanced, and not retarded, over the time used otherwise,

Dictionaries are descriptive, not prescriptive, in English.

> Although that's typical I doubt whether we can take it as an axiom, as POSIX 
> clearly allows DST to be retarded.

Specifically, the same definition of TZ allows any offset for the
alternative timezone; one hour ahead of the standard zone is merely the
default. If it could only be forwards of the standard zone, there would be
wording saying so.

Clive D.W. Feather          | If you lie to the compiler,
Email: clive at davros.org     | it will get its revenge.
Web: http://www.davros.org  |   - Henry Spencer
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