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

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

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.

I think that this bug should be reported to the SRFI-19 mailing list.

   https://srfi.schemers.org/srfi-19/

There have been several other bugs reported and fixed in upstream
SRFI-19 over the years, including some as recently as June 2017, so I'm
hopeful that they will take this bug seriously and issue a fix.

Would you like to report it to them?

       Mark