[tz] What's "right"?
sla at ucolick.org
Mon Nov 16 16:34:59 UTC 2020
On Fri 2020-11-13T22:38:56-0500 Kevin Kenny hath writ:
> Most applications that are neither astronomical nor navigational find it
> convenient to treat the nominal time since epoch as
> "86400 times the number of days since 1 January 1970 + the number of
> elapsed seconds in the current day." The reason for ignoring leap seconds
> is that a great many applications deal with *civil* dates and times in the
> future, and it is impossible to predict more than a year or two in advance
> when a leap second might occur.
Not "Most". All. Even in astronomy and navigation.
Every technical person involved in the process knew this in 1970 when
the decision was made to implement leap seconds. Every technical
system continued to use an underlying continous count of seconds, SI
seconds of TAI for radio broadcasts and automated navigation systems,
and mean solar seconds of UT for almanacs of astronomical phenomena.
If the underlying count of seconds in POSIX were based on TAI then
leap seconds do not cause more trouble for planning future civil
events than is already caused by bureaucrats changing time zones.
Technically it was only feasible to run radio broadcast, collision
avoidance, or any laboratory systems using SI seconds.
Legally it was necessary for calendar days to remain based on watching
the sun in the sky. Each delegate to the international science
and regulatory meetings could not introduce nor support a
recommendation which would have been illegal in their country.
The proceedings of the 1970 IAU referenced earlier include Gerhard
Becker remarking that action was needed in order to make atomic-based
time legal in all countries. He had already been saying, and would
continue to say, the same at other meetings for several years, but
he never got any traction. I suspect that is because the urgent need
to create leap seconds was caused by the legislation Becker introduced
in Germany which made old UTC illegal. I think nobody else wanted to
risk pointing out to other governments how atomic time differed from
watching the sun in the sky lest some other government make atomic
time explicitly illegal and thus poison the possibility for all radio
time broadcasts to agree worldwide. Agreement of all radio broadcast
time signals was the reason that the 1912 time conference was called,
the reason that BIH had come into existence, and a primary goal of the
international time committees ever since then.
Of all the persons visible in that 1970 IAU meeting, and at all the
other meetings of other agencies, only H.M. Smith expressed happiness
about UTC with leap seconds, and that was in the context of having
found something on which all governments could agree. Every other
memoir by folks who were part of the process contains tones of regret.
Regret about the situation that we find ourselves in because POSIX
chose a simple solution based on what was legal and available. But
also the only feasible option because, unfortunately, after the
sourness of the 1970 decision all of the technical principals backed
away from the issue. Nobody created a means of distributing the
history of leap second information more robust than having the BIH
send letters to the heads of national time service bureaus.
Steve Allen <sla at ucolick.org> WGS-84 (GPS)
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