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_... 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. As I understand it, you're proposing that the current Irish rules be represented by something like POSIX TZ='GMT0IST-1,M3.5.0/1,M10.5.0/2', i.e., that we set tm_isdst=1 during IST in summer and that daylight saving time equals summer time in Ireland. 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'.
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)