Can't seem to get it sorted...

The API feed, I mean.
I'm requesting: http://api.giantbomb.com/promos/?api_key=1234567&format=json
At the time of writing this feed returns 9 items (the last 3 of which are currently on the Giant Bomb home page). The results contain a field named "date_added" on which I'm trying to sort, like so:
http://api.giantbomb.com/promos/?api_key=1234567&format=json&sort=date_added
However, the order of the search results are unaffected. 
Am I missing something?