Re: bug#54764: encode-time: make DST and TIMEZONE fields of the list argument optional ones

[Max Nikulin <manikulin@gmail.com>]

On 17/04/2022 08:58, Paul Eggert wrote:
> Thanks, I installed that and then installed the attached, which merges
> that with some documentation improvements that I drafted based on this
> thread.

Thank you for further editing of docs. Please, fix a typo.

> diff --git a/doc/lispref/os.texi b/doc/lispref/os.texi
> index 66689f43a9..8366689640 100644
> --- a/doc/lispref/os.texi
> +++ b/doc/lispref/os.texi
> 
> @@ -1687,14 +1660,18 @@ Time Conversion
>   than six arguments the @emph{last} argument is used as @var{zone} and
>   any other extra arguments are ignored, so that @code{(apply
>   #'encode-time (decode-time ...))} works.  In this obsolescent
> -convention, @var{zone} defaults to the current time zone rule
> -(@pxref{Time Zone Rules}), and @var{dst} is treated as if it was
> -@minus{}1.
> +convention, @var{dst} is @minus{}1 and @var{zone} defaults to the
> +current time zone rule (@pxref{Time Zone Rules}).
> +When modernizing an obsolescent caller, ensure that the more-modern
> +list equivalent contains 9 elements with a a @code{dst} element that

                                             ^^^
A typo: double "a".

> +is @minus{}1, not @code{nil}.
> 
> +@lisp
> +;; Try to compute the time four years from now.
> +;; Watch out; this might not work as expected.
> +(let ((time (decode-time)))
> +  (setf (decoded-time-year time)
> +        (+ (decoded-time-year time) 4))
> +  time)
> +@end lisp

> +@noindent
> +Unfortunately, this code might not work as expected if the resulting
> +time is invalid due to daylight saving transitions, time zone changes,
> +or missing leap days or leap seconds.  For example, if executed on
> +February 29, 2096 this code yields a nonexistent date because 2100 is
> +not a leap year.  To avoid some (though not all) of the problem, you
> +can base calculations on the middle of the affected unit, e.g., start
> +at July 1 when adding years.

If I get your idea correctly then "January, 31" + "1 month" should be 
more impressive as impossible date. Year 2096 is too far in future. I am 
unsure concerning expectation. Overflow arithmetic is described above 
and e.g. JavaScript normalizes Date object in a similar fashion. The 
special point is that elisp decoded time requires explicit normalization 
however and 2100 is a good example that updating of any field may 
"break" the date.

> Alternatively, you can use the
> +@file{calendar} and @file{time-date} libraries.

A remark loosely related to your patch. Earlier you mentioned missed 
midnight due to time transition and suggested to use calendrical 
functions in Org. I can not figure out which elisp function can help to 
determine wall time for Aug 1 start of day in Cairo:

Africa/Cairo  Thu Jul 31 21:59:59 2014 UT = Thu Jul 31 23:59:59 2014 EET 
isdst=0 gmtoff=7200
Africa/Cairo  Thu Jul 31 22:00:00 2014 UT = Fri Aug  1 01:00:00 2014 
EEST isdst=1 gmtoff=10800

input: 2014-08-01 Africa/Cairo
(timezone may be implicit as the system one)
expected output: 01:00:00