Note to self:

If $this->set('_serialize', [ 'content' ]);  then a request for content type of 'application/json' or 'application/xml' will automagically return the correct data without needing a view file.

But if $this->set('_serialize', false); isn't set then a request for content type of  'application/json' or 'application/xml' will expect a view file to format and return the data and you will see an error

Template file "Timezone/json/ajax_get_time.ctp" is missing.