undefined method `expect' for DateTime:Class when trying to mock datetime in Minitest ruby

I'm implementing an last_active_at attribute on the User model of an application, which will be updated every time a user sees a page on the website.

When trying to test that the attribute is right in my test suite with a mock on the datetime, I get a NoMethodError: undefined method 'expect' for DateTime:Class.

Here is my test in the file chapter_controller_test.rb :

test "save user last activity timestamp" do
  user = FactoryGirl.create(:user, student_type: User::REMOTE)
  session[:user_id] = user.id
  some_date = DateTime.new(2014, 12, 12, 1, 1, 1)
  DateTime.expect(:now, some_date)
  get :index
  assert_equal(some_date, user.last_active_at)
end

The schema for the user table :

create_table "users", force: true do |t|
  t.string   "name"
  t.datetime "created_at"
  t.datetime "updated_at"
  t.boolean  "admin",              default: false
  t.string   "email"
  t.string   "password_digest"
  t.string   "reset_password_key"
  t.string   "student_type",       default: "remote"
  t.datetime "last_active_at"
end

And the implementation of the activity record in ApplicationController :

before_filter :record_activity

def record_activity
  if current_user && current_user.remote?
    current_user.last_active_at = DateTime.now
    current_user.save
  end
end

I'm using minitest 5.1

Thanks !