From: Zefram <zefram@fysh.org>
To: 26165@debbugs.gnu.org
Subject: bug#26165: date-week-day screws up prior to AD 1
Date: Sat, 18 Mar 2017 23:45:28 +0000 [thread overview]
Message-ID: <20170318234528.GL6518@fysh.org> (raw)
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
next reply other threads:[~2017-03-18 23:45 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-18 23:45 Zefram [this message]
2017-03-19 1:01 ` bug#26165: date-week-day screws up prior to AD 1 Zefram
2018-10-21 20:41 ` Mark H Weaver
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