Thanks for the bug report.

I see two issues here. First, the change in message-citation-line-format's 
treatment of %Z was deliberate. The new treatment (unlike the old one) is 
consistent with format-time-string and this consistency simplifies the 
documentation and code. If backward-compatibility is a significant hassle we can 
re-complicate Emacs, but I'm hoping that users can change %Z to %z in their 
settings and move on. In the meantime I installed the first attached patch into 
the emacs-26 branch so that the documentation matches the new behavior. (Sorry, 
I missed that part of the documentation before.)

Second, (format-time-string "%Z" nil -25200) returns "+07" where it should 
return "-07" since -25200 denotes 7 hours west of UTC. This is due to a blunder 
in my commit fdb1ba144ca61185e6457f092f38f59dd9bbe6a0 dated 2016-04-12 09:19:11 
-0700: the %Z code gets the sign wrong on purely-numeric time zones west of 
Greenwich. To fix this bug I installed the second attached patch into the 
emacs-26 branch. This fix should cause your example to come out as:

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

which at least gets the sign right.