[tz] historic sub-second time offsets

Paul Eggert eggert at cs.ucla.edu
Wed Nov 5 19:15:47 UTC 2014

On 11/05/2014 07:47 AM, Steve Allen wrote:
> 1960 is after the availability of cesium atomic chronometers and
> during the era when Heure Definitive was UT2.

Was heure définitive really UT2?  My French is pretty weak, but page 253 
of your source has a table entitled "Temps Universel 2 - Heure 
Définitive", which makes it appear to be listing the difference between 
UT2 and heure définitive.  Also, I just now looked at a 
seemingly-authoritative paper on this (see citation below), and its 
pages 177-8 seem to say that heure définitive was closer to what is now 
called UT1.

It's funny: just today I was wading through the HTML5 spec, which says:

    "Times in dates before the formation of UTC in the mid twentieth 
century must be expressed and interpreted in terms of UT1." 

I'm not brave enough to put anything that specific into the tz 
documentation.  I suspect that nobody putting an accurate circa-1957 
time stamp into an HTML5 document has ever consulted a UT2−UT1 
difference table in order to conform to the HTML5 standard.  And I'm a 
bit puzzled as to why the HTML5 committee was so specific about which UT 
variant to use before UTC was introduced.

Should we write to the HTML5 committee and say that they got their 
pre-UTC timescales slightly wrong and they shouldn't have tried to be 
that precise anyway?  Or would that be too brave in the opposite 
direction?  (When was UT1 introduced anyway? :-)

My source:

Guinot B. History of the Bureau International de l'Heure. Polar Motion: 
Historical and Scientific Problems, ASP Conference Series, Vol. 208, 
also IAU Colloquium #178. 2000. pp 175-84. 

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