bug#25448: RFC 3339 misdescribed in doc of date(1)

[Paul Eggert]

Your bug report raised two points: the numeric time zone -00, and separating 
date from time with space instead of "T".

For -00, you're right that the documentation is messed up. While we're on the 
topic, 'date' should output the numeric time zone '-00' when the time zone is 
indeterminate. There's now a convention for indeterminate time zones in tzdb, 
and 'date' should respect that.

For space instead of T, as it happens I contributed to the drafting of RFC 3339. 
RFC 3339's note that allows a space instead of "T" was specifically put in with 
usages like GNU "date" in mind. It's irrelevant whether this note is specified 
in English or in EBNF: it's part of the spec. That being said, the documentation 
could be worded more carefully.

The examples are intended to explain the GNU 'date' formats well enough that the 
user shouldn't need detailed annotations or citations to chapter-and-verse of 
the relevant standards. In rereading the documentation now, I see an opportunity 
to streamline the wording a bit, to avoid a possible misinterpretation that the 
coreutils documentation describes "the" ISO 6801 format (ISO 8601 does not 
specify just one format) or "the" Internet RFC 3339 format (likewise).

While we're in the neighborhood, Internet RFC 2822 is now obsolete, and several 
URLs in the coreutils manual either no longer work or have been superseded. 
Instead of adding yet another option --rfc-5322 (will we keep adding more and 
more?) I added an option --rfc-email that is intended to stand for the 
latest-and-greatest Internet email date format.

I installed the attached patches to address all the issues that I found, which 
should fix Bug#25448.

0001-build-update-gnulib-submodule-to-latest.patch
Description: Text Data

0002-maint-modernize-URLs.patch
Description: Text Data

0003-date-new-option-spelling-rfc-email.patch
Description: Text Data

0004-date-output-00-for-indeterminate-time-zone.patch
Description: Text Data