title: How to watch for polymer element attribute value change from index.html
body: <p>I have a custom element from which I want to read an attribute value from my index .html page. The element looks as follows, simplified:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;polymer-element name="card-list" attributes="JSONsource lastUpdatedOn"&gt;
  &lt;template&gt;
    ...
   &lt;/template&gt;
  &lt;script&gt;
    Polymer('card-list', {
      created: function() {
        this.lastUpdatedOn = 'init';
        setInterval(ajaxReload, 5000);
      }
    });
      
    var ajaxReload = function() {
        console.log('reload'); 
        updateSyncTime();     
    }
    var updateSyncTime = function() {
      var today = new Date();
      var h = today.getHours();
      var m = today.getMinutes();
      // add a zero in front of numbers&lt;10
      h = checkTime(h);
      m = checkTime(m);
      this.lastUpdatedOn = h + ':' + m;
      console.log(this.lastUpdatedOn);
    }
    var checkTime = function(i) {
      if (i &lt; 10) {
          i = '0' + i;
      }## Heading ##
      return i;
    }
    
  &lt;/script&gt;
  &lt;/polymer-element&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>I can access this lastUpdatedOn property to display it in index.html. I've tried like this:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;card-list JSONsource="http://sbewepas118:58573/AllergenStatusService.svc/lineinfo" lastUpdatedOn ="{{ lastUpdate }}" &gt; &lt;/card-list&gt;</code></pre>
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  </div>
  </p>
  
  <p>Somewhere else I have a text in which I include {{ lastUpdate }}. On loading the page, {{ lastUpdate }} gets replaced by 'init', from which I conclude that polymer correctly replaces {{ lastUpdate }}, but it simply has the value I initially set to the lastUpdatedOnProperty. When the property is update after an ajax-update, the indexpage does not detect it.</p>
  
  <p>How can I watch for a change of value in this property from my index.html page, which is not a polymer element?</p>
  
  <p>Thanks,</p>