Subject: date->string duff ISO 8601 negative years
From: Zefram <[email-masked]>
To: [email-masked]
Date: Fri, 13 Nov 2015 14:01:57 +0000
List-Id: <debbugs-submit.debbugs.gnu.org>

The date->string function from (srfi srfi-19), used on ISO 8601 formats
"~1", "~4" and "~5", for years preceding AD 1, has an off-by-one error:

scheme@(guile-user)> (use-modules (srfi srfi-19))
scheme@(guile-user)> (date->string (julian-day->date 0 0) "~4")
$1 = "-4714-11-24T12:00:00Z"

The date in question, the JD epoch, is 24 November 4714 BC (in the
proleptic Gregorian calendar).  In ISO 8601 format, that year is properly
represented as "-4713", not "-4714", because ISO 8601 uses the AD era
exclusively.  4714 BC = AD -4713.

-zefram