bug#28476: 26.0.60; Gnus: broken timezone in citation line

[Mike Kupfer]

I have message-citation-line-format set to

  "On %e %B %Y %R %Z, %N wrote:\n"

When replying to a message whose date field is displayed as

  Date: Sat, 16 Sep 2017 17:18:30 -0700

the citation comes out as

  On 16 September 2017 17:18 +07, Mike Kupfer wrote:

I expect

  On 16 September 2017 17:18 -0700, Mike Kupfer wrote:

which is what Emacs 25.3 gives me.

The message I'm replying to is on an IMAP server, if that makes any
difference.

In GNU Emacs 26.0.60 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2017-09-16 built on alto
Repository revision: 1d599df5e0fbbc52e8592c0aff1d23e978c29b67
Windowing system distributor 'The X.Org Foundation', version 11.0.11604000
System Description:     Debian GNU/Linux 8.9 (jessie)

Recent messages:
Opening nndoc server on /media/kupfer/KBLUE/ToFile/upload-tofile-home...done
Opening nndoc server on /home/kupfer/Desktop/upload-action...done
Contacting host: [redacted]
Contacting host: [redacted]
Checking new news...done
nnimap read 0k from [redacted]
Processing kill file /home/kupfer/News/KILL...done
No more unseen articles
nnimap read 0k from [redacted]
Mark set [12 times]

Configured using:
 'configure --prefix=/usr/new'

Configured features:
XPM JPEG TIFF GIF PNG SOUND NOTIFY GNUTLS LIBXML2 FREETYPE XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11

Important settings:
  value of $LC_TIME: C
  value of $LANG: en_US.utf8
  locale-coding-system: utf-8-unix

Major mode: Message

Minor modes in effect:
  gnus-message-citation-mode: t
  mml-mode: t
  shell-dirtrack-mode: t
  delete-selection-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  auto-fill-function: message-do-auto-fill
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow mh-mime mh-identity mh-tool-bar mh-letter mh-show goto-addr
mh-xface mh-utils mh-comp mh-scan mh-gnus mh-e mh-compat mh-buffers
mh-loaddefs emacsbug cl-print help-fns radix-tree vc-hg nnir sort
gnus-cite mail-extr gnus-async gnus-bcklg gnus-kill gnus-dup gnus-ml
disp-table mm-archive url-http url-gw url-cache url-auth url-handlers
nnrss xml mm-url url url-proxy url-privacy url-expand url-methods
url-history url-cookie url-domsuf url-util url-parse url-vars nndoc
nndraft nnmh utf-7 rfc2104 cl-extra help-mode network-stream nsm
starttls gnus-agent gnus-srvr gnus-score score-mode nnvirtual gnus-msg
gnus-art mm-uu mml2015 mm-view mml-smime smime dig mailcap nntp
gnus-cache gnus-sum gnus-group gnus-undo gnus-start gnus-cloud nnimap
nnmail mail-source tls gnutls utf7 netrc nnoo parse-time gnus-spec
gnus-int gnus-range gnus-win gnus nnheader mdk-mail smtpmail auth-source
cl-seq eieio eieio-core cl-macs eieio-loaddefs sendmail message subr-x
puny seq byte-opt gv bytecomp byte-compile cconv dired dired-loaddefs
format-spec rfc822 mml mml-sec password-cache epa derived epg epg-config
gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode mail-parse
rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr mailabbrev
mail-utils gmm-utils mailheader misearch multi-isearch crm thingatpt
cus-edit cus-start cus-load wid-edit elec-pair warnings server noutline
outline easy-mmode cc-mode cc-fonts cc-guess cc-menus cc-cmds cc-styles
cc-align cc-engine cc-vars cc-defs shell pcomplete comint ansi-color
ring xcscope easymenu advice delsel vc cl-loaddefs cl-lib vc-dispatcher
timeclock mdk-hacks time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote inotify dynamic-setting font-render-setting x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 505215 21681)
 (symbols 48 244263 6)
 (miscs 40 586 680)
 (strings 32 198558 4337)
 (string-bytes 1 6115645)
 (vectors 16 33085)
 (vector-slots 8 887601 27067)
 (floats 8 270 670)
 (intervals 56 1582 28)
 (buffers 992 43)
 (heap 1024 75376 11221))