On 01/19/2018 06:58 AM, Paul.Koning@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_... 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.