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bug#21903: date->string duff ISO 8601 negative years

From: Mark H Weaver
Subject: bug#21903: date->string duff ISO 8601 negative years
Date: Sat, 20 Oct 2018 20:40:26 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Mark H Weaver <address@hidden> writes:

> Zefram <address@hidden> writes:
>> 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.
> I agree that this is definitely a bug, but I'm nervous about deviating
> from the SRFI-19 reference implementation, and therefore probably from
> most other implementations of SRFI-19, in this way.

Also see my comments here:


which mention that ISO 8601 apparently requires that the sender and
receiver agree ahead of time whether an extended format will be used, in
which case a sign is *always* required, even when printing years in the
range 0-9999.


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