title: Rails event start_time+length=end_time
body: <p>I have a model</p>
  
  <pre><code> create_table "events", force: :cascade do |t|
      t.string   "title"
      t.datetime "start"
      t.datetime "end"
  </code></pre>
  
  <p>How to add a "duration value", so that "end"="start+duration"?</p>