gistfile1.rb

the plugin:
http://github.com/tamoyal/simple_time_select#readme

in the view: (u don't have to use date_select, but i needed it)

   <%= f.date_select :published_at %>
    <%= f.time_select :published_at, 
      { :default => Time.now.change(:hour => Time.now.hour), 
        :simple_time_select => true, 
        :minute_interval => 30, 
        :time_separator => ""} %>

in the controller:
  def update
    params[:post] = fix_post_attrs(params[:post]) if params[:post]
    @post = Post.find(params[:id])

    respond_to do |format|
      if @post.update_attributes(params[:post])
        flash[:notice] = 'Post was successfully updated.'
        format.html { redirect_to(@post) }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @post.errors, :status => :unprocessable_entity }
      end
    end
  end
  private

  def fix_post_attrs(post_attrs)
    if full_time = post_attrs.delete("published_at(5i)")
      post_attrs.merge!( {'published_at(4i)' => Time.parse(full_time).hour.to_s} )
      post_attrs.merge!( {'published_at(5i)' => Time.parse(full_time).min.to_s} )
    end
    post_attrs
  end