GNU bug report logs - #78096
31.0.50; calendar-iso-to-absolute and %W time string format are inconsistent

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#78096: closed (31.0.50; calendar-iso-to-absolute and %W time
 string format are inconsistent)
Date: Sat, 10 May 2025 09:53:01 +0000

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Your message dated Sat, 10 May 2025 12:52:14 +0300
with message-id <86selcaj3l.fsf <at> gnu.org>
and subject line Re: bug#78096: 31.0.50; calendar-iso-to-absolute and %W time string format are inconsistent
has caused the debbugs.gnu.org bug report #78096,
regarding 31.0.50; calendar-iso-to-absolute and %W time string format are inconsistent
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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78096: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=78096
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From: Ihor Radchenko <yantar92 <at> posteo.net>
To: bug-gnu-emacs <at> gnu.org
Subject: 31.0.50; calendar-iso-to-absolute and %W time string format are
 inconsistent
Date: Sun, 27 Apr 2025 15:28:43 +0000

Hi,

This bug was originally reported in
https://orgmode.org/list/174544167676.8.6104466990232889732.690779499 <at> publik.slmail.me

Consider the following:

(calendar-gregorian-from-absolute
  ;; First day of the second week of 2025
  (calendar-iso-to-absolute (list 2 1 2025))) ; => 2025-01-06

Now, let's try to format the same date using `format-time-string'

(format-time-string
  "%Y-%m-%d W%W"
  (encode-time (parse-time-string "2025-01-06 00:00")))
;; => 2025-01-06 W01

As you can see, week numbering is off by 1 between %W and
`calendar-iso-to-absolute', which is unexpected.

I believe that it is a bug.

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.42, cairo version 1.18.2) of 2025-04-22 built on localhost
Repository revision: a9ace435a341918eae3e01f2941f5bbca04d7795
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101014
System Description: Gentoo Linux

-- 
Ihor Radchenko // yantar92,
Org mode maintainer,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>

[Message part 3 (message/rfc822, inline)]

From: Eli Zaretskii <eliz <at> gnu.org>
To: Andreas Schwab <schwab <at> linux-m68k.org>
Cc: yantar92 <at> posteo.net, stephen.berman <at> gmx.net, 78096-done <at> debbugs.gnu.org
Subject: Re: bug#78096: 31.0.50;
 calendar-iso-to-absolute and %W time string format are inconsistent
Date: Sat, 10 May 2025 12:52:14 +0300

> Cc: Stephen Berman <stephen.berman <at> gmx.net>, 78096 <at> debbugs.gnu.org
> From: Andreas Schwab <schwab <at> linux-m68k.org>
> Date: Mon, 28 Apr 2025 18:43:18 +0200
> 
> On Apr 28 2025, Ihor Radchenko wrote:
> 
> > Stephen Berman <stephen.berman <at> gmx.net> writes:
> >
> >> The doc string of `format-time-string' says:
> >>
> >> %U is the week number starting on Sunday, %W starting on Monday,
> >>  %V the week number according to ISO 8601.
> >>
> >> (format-time-string
> >>   "%Y-%m-%d W%V"
> >>   (encode-time (parse-time-string "2025-01-06 00:00")))
> >> => "2025-01-06 W02"
> >
> > That indeed explains it - %W and %V are apparently not the same and
> > `calendar-iso-to-absolute' is closer to %V.
> 
> The difference is that %V is always the same for the whole week, whereas
> %V and %W roll over on 01-01 (and use 0 for a partial first week).
> 
> $ for y in $(seq 2020 2025); do date -d "$y-01-01" +"%a %U %V %W"; done
> Wed 00 01 00
> Fri 00 53 00
> Sat 00 52 00
> Sun 01 52 00
> Mon 00 01 01
> Wed 00 01 00

AFAIU, this bug report was based on a misunderstanding of the
subtleties of time formats.  I've updated the documentation to
clarify the differences between these formats, and I'm therefore
closing this bug.

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