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

Steve Allen sla at ucolick.org
Thu Jul 28 17:40:22 UTC 2022

On Wed 2022-07-27T22:02:41-0700 Paul Eggert via tz hath writ:
> +    Source data in vanguard form now uses subsecond precision,
> +    introduced in release 2018d.  For example, for America/New_York
> +    the UT offset before 1883-11-18 is now -4:56:01.6, and the
> +    transition time that day is now 12:03:58.4.  Although TZif files
> +    have only 1-second resolution so zic rounds these values to
> +    -4:56:02 and 12:03:58 respectively, programs other than zic can
> +    use the extra precision in the few cases where it is available.

I urge that tz never attempt to encode zone offsets with precision
better than 1 millisecond.  During the early 20th century broadcast
signals of time disagreed by more than one second.  Ongoing
improvements in determination of time were summarized by Anna Stoyko
of BIH in 1963


Note that those plots of time differences are *after* removing
persistent systematic differences of 0.03 s or more which resulted
from inconsistencies in the conventional longitudes of observatories.

Millisecond agreement of broadcast time signals was not achieved until
the 1960s when they were based on cesium atomic chronometers rather
than direct astronomical observation.

Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
1156 High Street               Voice: +1 831 459 3046         Lng -122.06015
Santa Cruz, CA 95064           https://www.ucolick.org/~sla/  Hgt +250 m

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