[tz] [PROPOSED] Improve leapseconds support

Paul Eggert eggert at cs.ucla.edu
Sat Oct 6 23:42:23 UTC 2018

Hal Murray writes:

> A leap second other than June or December is probably an error in the input 
> data.

Only if the input is intended to be the current official list of leap seconds. 
The recent code change was intended to make the code match the official rules 
better, which lets us better test software on outlandish-but-valid input. It's 
like the long-existing tzdb code that supports negative leap seconds, something 
that would also be an input error in data intended to be the official list.

Steve Allen wrote:
> 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.

My eyeballing of the "extrapolated long-term trend" of Figure 18 of Stephenson 
et al. 2016 says that 2 leap seconds per year should be enough until 2300 or so.

This prediction is based entirely on historical data, though, and global warming 
is throwing another monkey wrench into such predictions. Although there's not a 
consensus on how global warming should affect length of day, you might be amused 
to read Mazzarella & Scafetta 2018, which reports that since 1850 the length of 
day has been negatively correlated with the global sea surface temperature, 
which suggests that global warming should mean that leap seconds will be rarer. 
If they're right, we may have to wait even longer than Stephenson et al. suggest 
before worrying about leap seconds other than June or December.

Stephenson FR, Morrison LV, Hohenkerk CY. Measurement of the Earth's rotation: 
720 BC to AD 2015. Proc Royal Soc A. 2016 Dec 7;472:20160404. 

Mazzarella A, Scafetta N. The Little Ice Age was 1.0–1.5 °C cooler than current 
warm period according to LOD and NAO. Clim Dyn. 2018. 

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