[tz] suggestion: split "backward" into "backward" and "deprecated"

Steve Allen sla at ucolick.org
Tue May 5 21:05:08 UTC 2020

On Tue 2020-05-05T13:14:44-0600 Brian Inglis hath writ:
> On 2020-05-04 19:58, Paul Gilmartin via tz wrote:
> > On 2020-05-04, at 18:26:22, Andrew Gierth wrote:
> >> (To the best of my knowledge there are no reference clocks available to
> >> which one could synchronize in order to use a mean solar time, so your
> >> argument seems to me to support this choice.)
> >>
> > Do you mean mean solar time at some arbitrary precise longitude
> > or mean solar time at the Prime Meridian?  If the latter, is
> > interpolated UT1 a close enough approximation?:
> >     https://www.nist.gov/pml/time-and-frequency-division/time-services/ut1-ntp-time-dissemination
> No, as the precision of UT1 based on the Earth Rotation Angle is ms a day; UT1R
> has 62 smoothing terms; UT2 smoothes out seasonal variations; some variation of
> UT would be required to provide mean solar time at a consistent rate.
> NIST are still applying radio clock accuracy standards with ~ms precision, where
> the current norm is ~us from GPS, certainly over local LAN and even over decent
> remote WAN links.

The UK radio broadcast time signals stopped providing GMT in 1953.
Starting then they transmitted Provisional Uniform Time, PUT,
which was their own forerunner version of UT2.

The US radio broadcast time signals made similar changes around then.
The USNO version of uniform time scale had names like N3c.
So since the 1950s there has been no real-time source of mean solar
time, only quartz and cesium clock time adjusted to follow that.

> The older standard is adequate for MS products which require only
> watch-and-eyeball accuracy, but more systems are needing more accurate time.

Different sources of radio broadcast time signals did not agree to
within 1 ms until 1960 when the US and UK began to coordinate their
transmissions using cesium.

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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