GNU bug report logs - #26165
date-week-day screws up prior to AD 1

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Zefram <zefram <at> fysh.org>
Subject: bug#26165: closed (Re: bug#26165: date-week-day screws up prior
 to AD 1)
Date: Sun, 21 Oct 2018 20:42:02 +0000

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Your bug report

#26165: date-week-day screws up prior to AD 1

which was filed against the guile package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 26165 <at> debbugs.gnu.org.

-- 
26165: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=26165
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Mark H Weaver <mhw <at> netris.org>
To: Zefram <zefram <at> fysh.org>
Cc: 26165-done <at> debbugs.gnu.org
Subject: Re: bug#26165: date-week-day screws up prior to AD 1
Date: Sun, 21 Oct 2018 16:41:06 -0400

Zefram <zefram <at> fysh.org> writes:

> Looking at day of the week, via SRFI-19's date-week-day:
>
> scheme@(guile-user)> (use-modules (srfi srfi-19))
> scheme@(guile-user)> (julian-day->date 1721426 0) 
> $1 = #<date nanosecond: 0 second: 0 minute: 0 hour: 12 day: 1 month: 1 year: 1 zone-offset: 0>
> scheme@(guile-user)> (date-week-day (julian-day->date 1721426 0))
> $2 = 1
> scheme@(guile-user)> (date-week-day (julian-day->date 1721425 0))
> $3 = 6
>
> The output for 0001-01-01, Monday, is correct.  The preceding day is
> actually a Sunday, but Saturday was shown.  Looking at the code, this
> bug arises for the same reason as the problem with date-year-day raised
> in bug#26151.

This is fixed by commit a58c7abd72648f77e4ede5f62a2c4e7969bb7f95 on the
stable-2.2 branch.  I'm closing this bug now, but please reopen if
appropriate.

     Thanks!
       Mark

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From: Zefram <zefram <at> fysh.org>
To: bug-guile <at> gnu.org
Subject: date-week-day screws up prior to AD 1
Date: Sat, 18 Mar 2017 23:45:28 +0000

Looking at day of the week, via SRFI-19's date-week-day:

scheme@(guile-user)> (use-modules (srfi srfi-19))
scheme@(guile-user)> (julian-day->date 1721426 0) 
$1 = #<date nanosecond: 0 second: 0 minute: 0 hour: 12 day: 1 month: 1 year: 1 zone-offset: 0>
scheme@(guile-user)> (date-week-day (julian-day->date 1721426 0))
$2 = 1
scheme@(guile-user)> (date-week-day (julian-day->date 1721425 0))
$3 = 6

The output for 0001-01-01, Monday, is correct.  The preceding day is
actually a Sunday, but Saturday was shown.  Looking at the code, this
bug arises for the same reason as the problem with date-year-day raised
in bug#26151.  The date-year value, of the weird zero-skipping year
numbering, is passed to an algorithm that obviously expects astronomical
year numbering.

Looking at the code also reveals a second problem: the algorithm is
written to perform divisions with quotient where it obviously needs
modulo.  This will manifest in erroneous computations for some earlier
years once the above is fixed.

-zefram

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