[tz] Irish Standard Time vs Irish Summer Time

Paul Eggert eggert at cs.ucla.edu
Mon Dec 11 05:16:25 UTC 2017

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:


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 

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:


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