bug#64972: bibtex-autokey-get-year does not recognize biblatex date ranges

[Roland Winkler]

On Sat, Aug 05 2023, Eli Zaretskii wrote:
>>       date         = {2020-10/2020-12},
>> 
>> Results in user-error "Year or date field ‘2020-10/2020-12’ invalid".
>> 
>> The "date" field, however, is valid `biblatex' syntax for a date
>> range, according to the section "2.3.8 Date and Time Specifications"
>> of `biblatex's manual.
>
> Roland, any comments?

biblatex permits dates and date ranges according to ISO 8601.

The problem in bibtex-autokey-get-year arises because iso8601-valid-p
believes that the string "2020-10/2020-12" is not valid according to
ISO 8601, though iso8601-parse-interval parses that string correctly.

So it seems to me that this problem requires, first of all, some changes
in iso8601.el.  Who is familiar with this code and can possibly make
suggestions?

Roland