[tz] Fractional seconds in zic input

Paul Eggert eggert at cs.ucla.edu
Tue Feb 6 01:29:05 UTC 2018

On 02/05/2018 11:11 AM, Steve Allen wrote:
> https://www.ucolick.org/~sla/temporary/tzfracsec/
Thanks for doing that work. To answer its questions:

> For the case of sub-second offsets, does the tz project want to split 
> the America/<pickone> zones to indicate that the US and Canada had 
> different time offsets before and after events like this?

No, the intent of tzdb is to record the (idealized) UT offsets for civil 
time. Actual clocks often had errors, and we obviously can't record all 
those errors.

> Should tz prefer the intended time offsets from UT/UTC according to 
> legal decrees only (in the absence of technical details), or the 
> offsets from UT/UTC actually used by people who at best had access to 
> some radio broadcast time signals?
More the former. That is, tzdata attempts to record the intended UT/UTC 
offsets instead of the actual offsets, and similarly for transition times.

> If tz chooses to ignore technical details, then most of the sub-second 
> offsets of local mean time are irrelevant because the contemporary 
> practice of time-keeping had systematic offsets and lack of accuracy 
> which do not justify keeping offsets to even 0.1 s of sub-second 
> precision.

Yes, quite true. Until a very few years ago the wall clocks in Boelter 
Hall at UCLA were set by hand and would stray from the correct time by 
several minutes. Much of the world still runs this way. We're not trying 
to record that.

Instead, for these non-integer UT offsets, we're actually recording the 
longitude of the meridians of the affected civil-time zones. (I suppose 
this should be written down in theory.html somewhere.) My impression is 
that such longitudes could be measured to a precision no worse than 0.1 
s of UT offset (which corresponds to 1.5″ of longitude), even with 
circa-1930 technology. Admittedly I haven't researched this in detail.

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