In http://www.mediawiki.org/w/api.php?action=help&modules=main#main.2Fdatatypes
we read "All times are in UTC, any included timezone is ignored."

I say non UTC timezones should instead raise errors!

Why?

Because one day, when you finally do implement parsing timezones,
the system will be upwardly compatible.

Each day you let users enter timezones that are ignored, one day when
you finally do parse them correctly, you'll have all the more users
scratching their heads as to why are results suddenly different.

(Sure you can blame the users for not reading the instructions. But it
is more likely they have already added a skew to correct for what
turns out to be an ignored time zone.)