[tz] Fractional seconds in zic input

Steve Allen sla at ucolick.org
Mon Feb 5 05:59:44 UTC 2018

On Sun 2018-02-04T21:07:54-0800 Paul Eggert hath writ:
> It's not clear to me that the zic man page should specify how much precision
> is intended to be significant.

At the 10th plenary assembly of the CCIR in 1963 Rec.  374 specified
that radio broadcast time signals should be within 100 ms of the
intended correct time.

At the 12th general assembly of the IAU in 1964 Commission 31 (Time)
reported that radio broadcast time signals permitted worldwide
synchronization to within 2 ms.  But the tabulated differences of time
signals published by the BIH show that not all broadcasts were
achieving the 100 ms target.

At the 11th plenary assembly of the CCIR in 1966 Rec. 374-1
still specified 100 ms as the time offset.

At the 12th plenary assembly of the CCIR in 1970 Rec.  460 specified
the use of leap seconds offset from without placing any limit on the

It was not until the 13th plenary assembly of the CCIR in 1974 July
that Rec. 460-1 specified that the signals should not deviate by
more than 1 ms.  The date for this to become effective was 1975-01-01.

So fractional second offsets in tz with a precision any smaller than
1 ms are already beyond the level of technical conformance which was
being achieved by the broadcasts that served as the basis of legal
time for various jurisdictions at 1970-01-01.

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           http://www.ucolick.org/~sla/   Hgt +250 m

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