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

[Mark H Weaver <mhw@netris.org>]

Mark H Weaver <mhw@netris.org> writes:

> Mark H Weaver <mhw@netris.org> writes:
>
>> Zefram <zefram@fysh.org> 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:
>
>   https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21904#17
>
> 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.

Since writing this, I've discovered that the SRFI-19 reference
implementation's formatting of negative years is very badly broken.  For
example, when the year is -2, it prints "00-2".  Guile's behavior was
similarly broken for a short time, after I applied upstream fixes.

Since the current behavior of SRFI-19 and Guile is so clearly broken in
the case of negative years, I'm no longer concerned with maintaining
compatibility with SRFI-19 here.  I also feel more urgency to apply a
fix.

So, I went ahead and implemented your recommended behavior, in commit
a58c7abd72648f77e4ede5f62a2c4e7969bb7f95 on the stable-2.2 branch.

I'm closing this bug now, but please reopen if appropriate.

     Thanks,
       Mark