there are several ways. You can use this method:
start = @selected_date.beginning_of_day
end = @selected_date.end_of_day
@comments = Comment.where("DATE(created_at) BETWEEN ? AND ?", start, end)
Or this:
@comments = Comment.where(:created_at => @selected_date.beginning_of_day..@selected_date.end_of_day)