[tz] JavaScript IANA date/time library, now with support for leap seconds, TAI, TDT

[Steve Allen]

On Mon 2021-04-26T17:17:13+0100 Tony Finch via tz hath writ:
> My aim back then was to sketch what might be a reasonable way to bridge
> between pre-atomic time and UTC, for code that can only support an integer
> offset between UTC and TAI. If your code supports rubber seconds then it
> should probably use the USNO table that records the rate and phase offsets
> between 1962 and 1972. (I don't know another source of that table, nor
> where you can get a copy while maia.usno.navy.mil is down.)

Keep in mind that the USNO numbers only apply to time signals
broadcast in the US, and before 1968 they ignore the many microseconds
of offset between NBS and USNO.  Germany did not start to broadcast
the old UTC until the late 1960s, until 1974-01-01 broadcast both old
UTC and SAT, and stopped providing old UTC as of 1970-04-01 because it
had been deemed illegal.  Soviet and Chinese time signals never used
the old UTC.

Sussing out the particulars for individual cases of national time
signals needs finding remnants of the high-acid paper that each
national time service used for bulletins issued to its users, and/or
diving through the BIH Bulletin Horaire and Circular D to pull out the
received timing of each official broadcast.

It is much simpler to ignore the 50 to 200 ms jumps that were put into
old UTC and say that before 1972 all available sources of civil time
provided UT (nevermind the particular flavor, nobody set their clock
that accurately) and after 1972 UTC with leap seconds.

For civil time I do not see any advantage in a model with greater
complexity than just plain UT as the basis time scale before
1972-01-01.  This is adequate for any IANA tz library.

For precise time no general model applies.  Each precise timestamp
must be meticulously referenced to the particulars of the clocks that
were used to obtain it.  No available clocks were using TAI before
1972-01-01, and this does not belong in any IANA tz library.

Constructing a model that supposes it is possible to do better is akin
to writing The Silmarillion.

Look at the BIH plots of the accuracy of clocks

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

millisecond agreement between different national authorities was not
achieved until 1960 during the UK/US experiment that became old UTC.

>   * TAI is a retrospective timescale; it isn't defined in the future.

TAI also cannot be defined prior to mid 1955, and it was not
generally availble for use by any system until 1972-01-01 when
broadcast time signals began to be based on TAI.

The BIPM will never tolerate TAI becoming part of an international
agreement.  The best that can be hoped for in a new international
agreement on time is one (or more) new TAI-like uniform scale which
will be offset by some integer number of seconds from TAI.

The reason we use UTC is because it is what is in the current
international agreement.  Everybody involved in the technical aspects
eschewed the use of UTC for their systems from before 1972-01-01.

Look at the ATSC agreement for a technically-competent time scale.
They chose GPS time because it is practically available.  ATSC was not
bound by international bureaucratic requirements, so they could make
that choice.  Because it is ultimately controlled by USNO GPS system
time will never become part of an international agreement.

On Mon 2021-04-26T12:42:19-0400 John Sauter via tz hath writ:
> In looking at historical estimates of the rotation of the Earth based
> on astronomical observations of occulations and eclipses, I found that
> the values of UT1 are reliabe only since 1825.  (Nevertheless, my table
> of proleptic leap seconds goes back to the year -2000.)

Morrison, L. V.; Stephenson, F. R.; Hohenkerk, C. Y.; Zawilski, M.
DOI: 10.1098/rspa.2020.0776
Bibliographic Code:  2021RSPSA.47700776M
Proceedings of the Royal Society A, Volume 477, Issue 2246, id.20200776
all tabulated at
http://astro.ukho.gov.uk/nao/lvm/

That is the place to look, and it will continue to be so for
the lifetime of the authors.

But what is it mean to ask reliable?
That is very much the question given that prior to "reliable"
everybody's clock was set to local apparent time.

What exactly is the question that is trying to be answered by
an algorithm like this?

Ultimately it is necessary to model two kinds of time:
1) earth rotation time (which before the 20th century differed by a
second or more depending on who was setting the clock) because that is
a subdivision of internationally agreed calendar days
2) uniform time (which becomes possible to interpret as atomic time
beginning in 1955).

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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