title: Computed properties in Backbone
body: <p>I have a scenario where the data being manipulated on the client is presented and interacted with in a different way than it is represented on the server.</p>
  
  <p>Consider the following <code>event</code> resource returned from the server.</p>
  
  <pre><code>{
    "id": 123,
    "start_at": 1331336004906,
    "end_at": 1331337704906
  }
  </code></pre>
  
  <p>And the following template for editing:</p>
  
  <pre><code>&lt;form&gt;
    &lt;!-- Notice how date and time are separated in the interface --&gt;
    &lt;input type="text" name="start_date" value="{{start_date}}" /&gt;
    &lt;input type="text" name="start_time" value="{{start_time}}" /&gt;
  
    &lt;!-- Instead of asking for an end date/time, we ask for the duration --&gt;
    &lt;input type="text" name="duration" value="{{duration}}" /&gt;
  
    &lt;input type="submit" /&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>How would I go about treating <code>start_date</code>, <code>start_time</code>, and <code>duration</code> as attributes in my Backbone model without sending them to the server? Am I supposed to modify <code>.toJSON()</code>?</p>