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 https://www.ucolick.org/~sla/leapsecs/BHsHn05p142.html 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@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