Eli Zaretskii <eliz@gnu.org> writes:

>> 
>> It depends on how you start Emacs.  The common case on macOS is that
>> people start Emacs from the GUI, and in that case the OS doesn't
>> configure the locale environment variables.  What the NS port does is it
>> uses the OS API to get the locale and applies the environment variables
>> manually (see ns_init_locale in nsterm.m).
>
> Thanks, I think that explains the issue.
>
>> I see that ns_init_locale only sets LANG, which seems insufficient for
>> this use case.  Perhaps we need to extend ns_init_locale to set other
>> LC_ variables as well (LC_TIME is the key here).
>
> Why not LC_ALL?

Yes, after consulting the POSIX spec, I think setting LC_ALL is the
right thing to do.

I've attached a patch that solves the issue.  If you think it's a good
idea, please install it for me.  Thanks.