title: polymer: using a function in a data binding expression
body: <p>One of the properties of my Polymer element is a timestamp:</p>
  
  <pre><code>Polymer({
    timestamp: null,
    ready: function () {
      this.timestamp = new Date().getTime();
    }
  });
  </code></pre>
  
  <p>In my markup I need to show a formatted timestamp:</p>
  
  <pre><code>&lt;template&gt;
    timestamp={{getFormattedTimestamp()}}
  &lt;/template&gt;
  </code></pre>
  
  <p>where</p>
  
  <pre><code>getFormattedTimestamp: function () {
    return new Date(this.timestamp).toLocaleString('en-US', {timeZoneName: 'short'});
  }
  </code></pre>
  
  <p>But the code above does not update the display when <code>this.timestamp</code> changes because Polymer does not know that it needs to call <code>getFormattedTimestamp</code> when this happens.</p>
  
  <p>I cannot just put the contents of <code>getFormattedTimestamp</code> into the data binding expression because it won't be able to parse it, and I cannot create a computed property either for the same reason.</p>
  
  <p>Is there a way to do it other than creating an auxiliary member variable that holds the formatted timestamp and updating it when the timestamp changes?</p>