Rails - 如何更改created_at时间？

In my controller I have:

在我的控制器中，我有：

@konkurrencer = Rating.new(params[:kon])
@konkurrencer.save
@konkurrencer.konkurrencer.rating_score += params[:kon][:ratings].to_i
@konkurrencer.konkurrencer.ratings += 1
@konkurrencer.created_at = Time.now.strftime("%Y-%m-%d 00:00:00")
@konkurrencer.save
@konku