[tz] Levine's proposal for once-a-decade leap smears

[Paul Eggert]

On 2024-07-29 16:20, Tim Parenti via tz wrote:
> No leap second 2024-12-31, per below.  See attached patch.

Thanks. While we're on the topic of leap seconds, Judah Levine's 
proposal to replace leap seconds was published this month as a preprint 
by Metrologia. Although the paper is supposed to be public domain in the 
US (because it's a product of the federal government), Metrologia 
doesn't make it freely readable, which is not quite cricket; thankfully 
a preprint is publicly available.

Levine's proposal would replace leap seconds with leap smears every 
decade. Until 2100 the smears would adjust clocks by 13 seconds. After 
2100 the "13" could be adjusted upward as needed. The idea is to keep 
UTC to within 100-or-so seconds of UT1, by adjusting via a smear once 
per decade, and to do this adjustment algorithmically (i.e., without 
paying attention to what the Earth actually does) so as to make TAI-UTC 
more predictable. Using a leap smear would keep TAI-UTC continuous over 
time. Levine suggests running UTC clocks half-speed during the smear; 
this means TAI-UTC would be continuous but not continuously differentiable.

My back-of-the-envelope calculations suggest that this scheme would work 
for about half a millennium before accumulated tidal friction would 
necessitate further adjustment or replacement.

If I understand things correctly, Levine would rather get out of the 
leap-second-table publishing business, and so would rather have the leap 
smear implemented directly via code in localtime.c (or whatever), 
instead of being disseminated by a table that needs updating every six 
months. Presumably a new version of the code would need to be 
disseminated before the year 2100 rolls around. If we resisted this hint 
and used a table anyway, the table would have one entry per century and 
would stop working after five entries or so; not sure it'd worth the 
effort to implement it that way.

Proposed documentation patch attached and installed in the development 
sources.
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