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.