On 2018-01-26 22:16, Paul Eggert wrote about the tzdb dst bit:
On 01/26/2018 01:39 PM, Michael H Deckers via tz wrote:
Before this change proceeds, it is, in my opinion, necessary to obtain agreement on what the new significant (non-vestigial) role of the dst bit, if any, should be in the future (beyond the single case of Ireland).
I don't think there's much disagreement over what the bit means in the context of the POSIX API. For example, POSIX specifies that this environment setting:
TZ='IST-1GMT0,M10.5.0,M3.5.0/1'
means that standard time is abbreviated "IST" and is 1 hour ahead of UTC, that daylight saving time is abbreviated "GMT" and is at UTC, and that the European Union's current rules are in place for when to change UTC offsets. tzcode and every other POSIX-compatible system support this, and set tm_isdst=1 in winter with a negative DST offset when operating in this locale. As far as I know there's no serious dispute about this, nor is anybody seriously proposing to change this behavior. That is, the tm_isdst API may be vestigial, but it's too late to change what it means.
No, no: POSIX does not say anything about the setting of the tm_isdst member for a given TZ string; nor does it talk which part of the TZ string describes daylight saving time. I am reluctant to accept any assertion of the type it is clear that POSIX means XXX without a specific reference implying it; if I have to write reliable code, I have to know exactly what I can rely on. Michael Deckers.