bug#37130: 27.0.50; Tar mode should use the ISO8601 format to display the timestamps (or support TIME_STYLE or locales)

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@mouse.gnus.org>
> Date: Fri, 23 Aug 2019 05:46:26 +0200
> Cc: 37130@debbugs.gnu.org
> 
> Vincent Lefevre <vincent@vinc17.net> writes:
> 
> > When tar-mode-show-date is set to t (to get the timestamps), the
> > Tar mode displays them in a non-standard format. It should use
> > the ISO8601 format (this is what GNU tar does).
> 
> I think it makes sense to follow GNU tar's lead here, so I've now done
> this on the trunk.

Yuck! what an ugly format!

> > Other formats (e.g. one corresponding to the locales) could be used
> > via an option; in particular, the support of the TIME_STYLE
> > environment variable (see coreutils manual) would be a good idea.
> 
> I think that sounds like overkill for tar-mode, but perhaps there should
> be a general function somewhere for using TIME_STYLE throughout Emacs
> whenever time/dates are displayed.

I think we should just make the format used for the time stamp a user
option, that would be a good-enough solution.  It will also provide a
"fire escape" to get the old behavior for those, like me, who might
dislike the ISO8601 format.  We must provide such
backward-compatibility options anyway when we change behavior in
incompatible ways.

Finally, this change should be in NEWS (including the to-be-added
backward-compatibility shim).

Thanks.