On 2017-01-30 10:12 AM, Steve Allen wrote:
On Mon 2017-01-30T14:14:10 +0000, Tony Finch hath writ:
I think what I was trying to get at (as others have already said in this
thread) is that you can use the TAI-UTC delta to translate from UTC to TAI
during a leap second, but you need more information to make the inverse
translation.
As mentioned by Arias in that radio show, there are leaps of one hour
in time that we deal with regularly. The tz project has tzdata that
unambiguously represent when the jump occurs, and the tzcode has a
convention for how to interpret the ambiguous hour.
(One thing tz cannot do is represent the legal specification of the
changes in Spain a century ago when the time of fall back was given
by the decree as 25:00:00 on the date in question. tz encodes
those as 01:00:00 on the next date.)
Indeed, while we debate the exact subtle meanings of UTC and Leap
Seconds the potential inaccuracies of UTC implementations are
overwhelmed by the potential inaccuracies of local time, both in terms
of magnitude (1 second TAI-UTC v.s (at least) one hour DST) and in terms
of clarity of standardization (UTC is pretty well defined, while
specifications of local time is something of the wild west).
It seems to me Tz Database does a great job of describing the meanings
of local time after 1972, thanks to the diligent and obsessive work by
its authors and contributors. As far as I can see, it describes local
time for contemporary timekeeping, albeit with some quirks and
exceptions. It has become a de facto standard.
Describing local time before 1972 is going to be a challenge. When was
the Gettysburg Address delivered? Exactly how was local time understood
at the time and how would it relate to modern timekeeping practice? This
is an interesting, fascinating, and probably valuable exercise, but if
we get too tangled up in those considerations we risk diverting
attention and energy from the more urgent need of arriving at a uniform
treatment of local time in contemporary timekeeping systems.
Time Zone Database has become a de facto standard, and IANA's
involvement has lent it further credibility. But:
A) its only a de facto standard - a due-process standardization process
is required to achieve world-wide adoption
B) its documentation could be improved because it is complex and quirky
C) its data relies on the reports and judgements of contributors and
there is no formal process for registering changes
D) its c implementation code is dated and relies on POSIX-time type
functions and heritage which complicates incorporating Leap Seconds
This is not in anyway a criticism of the years of development in Tz
Database - its really a fantastic piece of work, hats off. I only wish
and hope it will be improved.
-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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