timestamp with time zone

Here's my query:

SELECT foursquare.name, foursquare.city, COUNT(moment_id) AS popularity
FROM foursq_categories
JOIN foursquare USING (foursq_id)
JOIN places USING (foursq_id)
JOIN blocks USING (block_id)
WHERE "primary"
AND (created at time zone timezone)::date = 'yesterday'
AND (country = 'USA' OR country = 'United States')
AND foursq_categories.name @@ to_tsquery('Restaurant')
GROUP BY foursq_id, foursquare.name, foursquare.city ORDER BY popularity
DESC LIMIT 12;

Here's my explain: http://explain.depesz.com/s/xoH

To my surprise, it was not the tsquery that made this slow (which is
awesome, because I was worried about that) but rather the filter: (created
at time zone timezone)::date = 'yesterday'
created has an index (btree if it matters). timezone does not. I'm
wondering if the solution to my problem is to create a joint index between
created and timezone (and if so, if there is a particular way to do that to
make it work the way I want).

Thanks in advance.

-Alessandro