Rails - How to change created_at time?

In my controller I have:

@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

When I create a new item it the created_at column is:

2012-02-27 16:35:18

I would expect it to be:

2012-02-27 00:00:00