bug#34608: date +%+4C is unimplemented, contrary to POSIX

[Paul Eggert]

Eric Blake wrote:

> date -d 0001-01-01 +.%+4C.
>
> should produce ".+000.", but currently it produces ".%+4C." because the
> + flag is unimplemented.
>
> See also http://austingroupbugs.net/view.php?id=1184

Surely this bug should be reported against strftime, not against 'date'. POSIX 
does require that strftime behave that way, but it does not place a similar 
requirement on 'date'.