bug#22302: 25.1.50; time-stamp ignores time-stamp-time-zone

[Harald Hanche-Olsen]

Run emacs -Q and insert this text:
Time-stamp: <>

Then do:
M-: (require 'time-stamp) RET
M-x set-variable RET time-stamp-time-zone RET "UTC" RET
M-x time-stamp

Expected result: A UTC time stamp is inserted.
Actual result: A time stamp in the local time zone is inserted.

My analysis:
time-stamp uses time-stamp-string to compute the string to insert.
And that calls:
      (format-time-string (time-stamp-string-preprocess ts-format)
                          nil time-stamp-time-zone)
However, time-stamp-string-preprocess does all the work,
leaving nothing for format-time-string to do.
And it does so without consulting time-stamp-time-zone.

Admittedly, the doc string for time-stamp (no longer?) mentions 
time-stamp-time-zone.
However, the variable does exist, and its doc string indicates that it will be 
used by time-stamp.

– Harald

In GNU Emacs 25.1.50.1 (x86_64-apple-darwin15.2.0, NS appkit-1404.34 Version 
10.11.2 (Build 15C50))
 of 2015-12-17
Repository revision: 23b5c22703eeee7b4fe6608ce12ffe3b87794933
Windowing system distributor 'Apple', version 10.3.1404
Configured using:
 'configure --with-ns'

Configured features:
RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS

Important settings:
  value of $LC_CTYPE: en_US.UTF-8
  locale-coding-system: utf-8-unix