[tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision

Paul Eggert eggert at cs.ucla.edu
Sat Jul 30 17:01:08 UTC 2022

On 7/29/22 00:03, Stephen Colebourne via tz wrote:
> I see no value in adding sub-second precision to time-zone offset data
> beyond the comments. No system is likely to make practical use of it,
> and it is unlikely to be correct in most locations (is the plan to
> change LMT everywhere? if so why?) .

No there was no plan to change LMT everywhere, as the LMT values are 
somewhat arbitrary. The idea was only to write down standard time when 
it was standardized to be some non-integer offset from UT.

Given the other comments in this thread I'm inclined to revert the 
effect of this change, so that vanguard form continues to use integer 
multiples of a second. So I installed the attached further patches, the 
last of which does the reversion.

These patches keep the structured comments about subseconds, though. In 
the future some downstream client may find the subsecond info useful, 
and if so it should be easy to change vanguard form accordingly (it 
should be a one-line change to ziguard.awk). So it might be useful for 
other .zi parsers to parse and discard the subsecond information the 
same way that zic does (i.e., by rounding to even as documented in zic.8).

It's conceivable (though unlikely) that in the future some impractical 
politicians may rebel against the Western concept of "Universal Time" 
and insist on a non-integer UT offset. Something like this happened 
briefly in Beijing in 1949 (which attempted to impose apparent solar 
time, Qing dynasty style!), and there was a milder echo in 2015 when the 
North Korean government rebelled against JST. If a Beijing-style 
revolution occurs somewhere in the future it would break a lot of 
software, ours included; but if the revolution's leaders merely choose a 
non-integer UT offset we'll have an easy fix for tzdata.
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