Well, I currently see "Day" on the OP (as it should do, now that it's "Day" by the same time standards) whereas previously it was "Night". But I only got "Night" after following the link and getting the client-side javascript serving me the image, so I'm not entirely sure what was and what wasn't cached.
This might mean that it works (at least for someone who has visited the 'faux image' javascript page at least once. The reliance upon client-side scripting might make it unstable, though.
It's still returning (when I go there manually)...
HTTP/1.1 200 OK
Server: nginx
Date: Mon, 09 Nov 2015 15:18:15 GMT
Content-Type: text/html
[etc, including <script>]
...where the ideal option "works everywhere" option would be something server-side that returns "Content-Type: image/png" and contain the binary for a PNG starting with the magic string "‰PNG".
However, maybe browsers, these days, are far more forgiving. Standards have changed a
lot since I knew, for sure, what was actually going on... (I blame Netscape!
)
I'm not comfortable voting (because I have doubts about whether I'm seeing around the 'cache', despite the image change, and you want a solution that works even for those who haven't followed a link to 'cache' it... and I'm not willing to wipe mine just for this one test) but if you want to take this as a valid "time-dependent" image result, please do... With the prior caveats.