[tz] Irish Standard Time vs Irish Summer Time

[Paul Eggert]

On 01/19/2018 06:58 AM, Paul.Koning at dell.com wrote:
> Does POSIX say anything?

POSIX uses different phrases in different places to talk about tm_isdst 
and related notions. In its discussion of the TZ environment variable:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03

it talks about "the alternative (dst -such as Daylight Savings Time) 
timezone". In its discussion of time.h:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html

it says "The value of tm_isdst shall be positive if Daylight Savings 
Time is in effect, 0 if Daylight Savings Time is not in effect, and 
negative if the information is not available."; this is language taken 
directly from the ISO C standard, section 7.27.1.

The language of the C standard is older, and the POSIX language for TZ 
was added later, I suppose by a committee that concluded that calling it 
"the alternative timezone" would allow more freedom to implementations 
for situations where calling it "daylight saving" was inappropriate.

Robert Elz's summary of what tm_isdst was actually used for matches my 
impression as well.