On 2016-12-30 12:11 PM, Steve Allen wrote:
On Fri 2016-12-30T10:49:19 -0500, Joseph Gwinn hath writ:
It may prove useful to know why the POSIX Working Group (WG) excluded
leap seconds, in their own words.
A bit more insight comes from the 1986 draft POSIX and the
1988 first version of the standard.
http://www.ucolick.org/~sla/coolpix/posixtime/
Even more insight comes from counting that the 1988 standard has far
more text (with more ruminations and consternation) about time zones
than it has about precision time.
It seems obvious to me that POSIX time can't be changed substantially
since a vast number of systems and applications rely on its behavior.
Maybe it could be refined a little to make it clearer and to recommend
more consistent practice, but it must fundamentally retain its
86400-second-day character. This means it can never accurately reflect
UTC in all cases, in particular at Leap Second introductions. This
limitation has been good enough for use by humans "to coordinate
activity" for a very long time, and will continue to do so, but its
inadequate for accurate UTC "precision" timekeeping.
It seems to me a "POSIX Time 2" specification could be developed that
handled UTC correctly and defined mapping to the legacy POSIX time
(which will necessarily remain ambiguous in those Leap Second cases).
This could be an addition to POSIX and define behavior of high-level
APIs that could yield accurate UTC YMDhms representation.
A big part of that challenge there would be to better define local time.
As Steve points out, local time was a focus of early deliberations and
remains a critical part of timekeeping considerations because humans
care about local time, not UTC or TAI. Tz Database is the only source of
local time information possibly available in the public domain, and has
essentially become a de facto standard. Its recent inclusion at IANA
improves its authority and perhaps the chances of eventually finding
more formal due-process standardization. "POSIX Time 2" could, should,
better define the meaning of local time and provide the means to
represent the necessary and sufficient metadata to describe local time.
-Brooks
--
Steve Allen <s...@ucolick.org> WGS-84 (GPS)
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Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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