$params['sort'] = array('updated_at');
However I found performing a descending sort much less intuitive. Eventually I figured out it should be:
$params['sort'] = array('updated_at:desc');
Formatting of data should only occur in the final steps of output. Until that point, and as a rule, internally data should remain in a base format that can easily be converted into many others – without being converted into another more basic format first.
For example Unix timestamp for dates or a floating point number for money. Both can readily be converted into more expressive formats without writing code to first parse or disassemble the initial format.
However in a situation where the flow is very specific and unlikely to ever be used to generate a different output it is permissible, even desirable, to generate data in the format it will be finally outputted.