[tz] [PROPOSED] Improve leapseconds support

[Steve Allen]

On Sat 2018-10-06T13:44:20-0700 Hal Murray hath writ:
> A leap second other than June or December is probably an error in the input
> data.

This is definitely true in this era.  Assuming (bad thing to do) that
rotation of the crust does not undergo another strong deceleration the
two dates per year should be enough for the next 40 years.

In short, it is dangerous for code to accept a leap second on any date
other than June 30 and December 31.

> I would leave the check and add a big comment.  Maybe write the code so that
> it is a simple edit to disable the check.
>
> I think there is provision for an intermediate stage between Jun/Dec and every month.  Apr/Jun/Sep/Dec.

The active rec from ITU-R is version 6 (2002)
http://www.itu.int/rec/R-REC-TF.460/en
"A positive or negative leap-second should be the last second of a UTC
month, but first preference should be given to the end of December and
June, and second preference to the end of March and September."

This is the current evolution of the initial instructions in
CCIR Report 517 (Question 1/7, Resolution 53) (1971)
that was reprinted on page 32 of NBS Monograph 140
https://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph140.pdf
"A positive or negative leap second, when required, should be the last
second of a UTC month, preferably 31 December and/or 30 June."

That was modified in 1973 by the 15th IAU GA
https://www.iau.org/static/resolutions/IAU1973_French.pdf
who were watching the extreme deceleration of earth rotation in
the early 1970s and said
"if the present trend of the rotation of the earth continues, it will
become impossible to maintain UTC within the present limits by the
insertion of leap seconds on two preferred dates only"
But even as they were making that resolution the earth was speeding up
considerably, so their fears were premature.

But all of those were unfunded mandates laying extra responsibility
for international implementation on a perennially underfunded agency
which ceased to exist in 1988.

Note in particular that the perennially underfunded sucessor agency
who inherited this responsibility has not incorporated any changes to
their operational mandate since version 4 in 1986
ftp://hpiers.obspm.fr/iers/bul/bulc/BULLETINC.GUIDE

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