Ember Date Returning "Undefined" from Rails API

I've got an Ember app running a Rails API back-end, and I'm experiencing a weird problem between the two. I want the Message model data stored in the API to send up the stack to Ember. I've seeded the database with a couple messages built to match this migration:

class CreateMessages < ActiveRecord::Migration[5.0]
  def change
    create_table :messages do |t|
      t.text :body
      t.boolean :flagged

      t.timestamps
    end
  end
end

Plus a serializer looking like this:

class MessageSerializer < ActiveModel::Serializer
  attributes :id, :body, :flagged, :created_at, :updated_at      
end

On the Ember side, my model is setup like so:

import DS from 'ember-data';

export default DS.Model.extend({
  body: DS.attr('string'),
  flagged: DS.attr('boolean'),
  createdAt: DS.attr('date'),
  updatedAt: DS.attr('date')
});

Ember shows both the flagged and body params just fine, but it only returns undefined for the createdAt and updatedAt. By checking the Rails console, a quick test for the first Message returns something like this:

message.as_json
 => {"id"=>1, "body"=>"Etsy actually four dollar toast seitan. Schlitz twee kale chips pork belly franzen tote bag.", "flagged"=>true, "created_at"=>Wed, 02 Mar 2016 21:33:55 UTC +00:00, "updated_at"=>Wed, 02 Mar 2016 21:33:55 UTC +00:00} 

I've seen some references on GitHub that do this more or less exactly, and I can't figure out where things have gone off. Any help appreciated!