[tz] What's "right"?

Steve Allen sla at ucolick.org
Fri Nov 13 03:03:03 UTC 2020

On Thu 2020-11-12T18:13:54-0800 Guy Harris hath writ:
> So what POSIX says in section 3 "Definitions":
>       https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html
> is
>       3.150 Epoch
>       The time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time (UTC).
> and what it says in section A.3 "Definitions" of the Rationale:
>       https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap03.html
> is
>       Epoch
> Historically, the origin of UNIX system time was referred to as
> "00:00:00 GMT, January 1, 1970".  Greenwich Mean Time is actually not
> a term acknowledged by the international standards community;
> therefore, this term, "Epoch", is used to abbreviate the reference to
> the actual standard, Coordinated Universal Time.

Although not specified by any standards nor recommendations, the term
"Coordinated Universal Time" did then exist within the time service
bureaus, and that has a precise meaning, and because Germany had not
yet moved to stop broadcasting old UTC it was a legal time everywhere.

It is also the case that the calendar day based on the rotation of the
earth is not acknowledged by the international standards community,
but at no point have the committees had the balls to create a
recommendation or standard which changes the definition of the
calendar day such that it is unrelated to the rotating earth.

Such an action would violate national laws.
This is the reason we have leap seconds.

It is also the case that the CGPM, overseeing the SI, has never
abrogated the definition of the mean solar second as 1/86400 of the
calendar day.  This is because they never defined it as such, so they
cannot undefine it as such.  In most places other than Germany (where
it was deemed explicitly illegal) the mean solar second still has
legal existence distinct from the SI second.

> Should there be a request for clarification of what they mean by that?

No, mostly because of
where POSIX has the word "approximates" and the phrase "relationship
with the current actual time is implementation-defined."
Also because that goes down the rabbit hole into places where the
writers of the official (and, upon inspection of the source documents,
pedantically incorrect) definitions for the time scales we use dared
not go lest they give national bureaucrats clues about how there are
two qualitatively different meanings for "time".

Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
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