Subject: Re: bug#16663: emacs/calc/date

From: Steve Allen <[email-masked]>
Date: Thu, 6 Feb 2014 21:17:40 -0800

On Thu 2014-02-06T22:52:42 -0600, Jay Belanger hath writ:
> For daylight savings time calculation, Calc only differentiates between
> before/after 2007.  I think Calendar might do the same.  I don't know
> about Calendar, but by default Calc bases its DST adjustments based on the time
> zone and whether or not daylight savings time is used at all.  (Each
> person could write their own DST function, but that would be a pain.)
> I don't know whether or not anything too much more sophisticated can be
> done, but I'll look into it after the next Emacs release.  (Perhaps
> something like a user configurable "DST begins this year" and "DST ends
> this year"; as far as I can tell, the beginning and end years depend on
> more than the time zone.)

So this math is not only limited to being valid on earth, but also
limited to US and Canada subsequent to the 1987 change of the Uniform
Time Act?  That was not clear given the dates being investigated.
I leave all timezone questions to the IANA tz community.

> > In this and many other cases of computed proleptic dates the only
> > reasonable interpretation of the time scale is Universal Time (UT, not
> > UTC).
>
> Could you be more precise?  From what I've read, UT isn't precisely
> defined and UTC is one possible interpretation of it.

For dates prior to atomic chronometers UT is defined more precisely
than the sources of time that were available to most users of civil
time.

For dates after atomic chronometers it becomes necessary to decide
whether the time scale relevant to the problem at hand wants to be
based on counting calendar days of earth rotationand subdividing them
(flavors of UT), or based on counting SI seconds and integrating them
(flavors of atomic time like TAI, GPS), or based on the compromise we
now know as UTC where the duration of the second (SI second of cesium
hyperfine transition) is unrelated to the duration of the day (a mean
solar day of earth rotation).

POSIX chose the first of those three yet called it UTC.

Piles of details about all these time scales are here
http://www.ucolick.org/~sla/leapsecs/timescales.html

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