[LEAPSECS] The POSIX Time Rationale - in the Working Group's own words

[Brooks Harris]

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


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