title: Extjs date picker. Issue highlighting dates on month change
body: <p>I am highlighting a number of dates in a month when the date picker is shown. This is done by adding a highlightDates function to the Picker component</p>
  
  <pre><code>Ext.override(Ext.form.field.Picker, {
  
      highlightDates: function (picker, dates) {
  
          if (Ext.isEmpty(dates)) return;
          for(var i = 0; i &lt; dates.length; i++) {
              var t = new Date(dates[i].date),
                  dtMillis = t.getTime(),
                  offset = t.getTimezoneOffset() * 60 * 1000,
                  dt = dtMillis + offset,
                  cells = picker.cells.elements;  
  
              for (var c = 0; c &lt; cells.length; c++) {
                  var cell = Ext.fly(cells[c]);
                  if (cell.dom.firstChild.dateValue == dt) {
                      cell.addCls('cell-highlight');
                      break;
                  }
              }
          }
      }
  });
  </code></pre>
  
  <p>The dates object passed into the function above is a variable set from an Ajax call in the controller.</p>
  
  <pre><code>getDates: function (cmp) {
      var me = this;
      Ext.Ajax.request({
          url: 'someurl',
          success: function (response) {
              me.specificDates = Ext.JSON.decode(response.responseText).specificDates;
          }
      });
  }
  </code></pre>
  
  <p>The controller also contains a function which the date picker expand listener will call.</p>
  
  <pre><code>setDatesToHighlight: function(picker){  
      picker.highlightDates(picker.picker,this.specificDates);
  }
  </code></pre>
  
  <p>Finally, this is called when the date picker is made visible using the expand listener</p>
  
  <pre><code>items: [{
      xtype: 'datefield',
      editable: false,
      listeners: {
          expand: 'setDatesToHighlight'
      }
  }]
  </code></pre>
  
  <p>This is all working fine until i change the month in the picker. I cannot seem to trigger the setDatesToHighlight controller function again so the selected months dates get highlighted.</p>
  
  <p>I was thinking i could override the update method on the picker but i am having difficulty obtaining the dates object i need to pass to the highlightDates function.</p>
  
  <p>Thanks for any advice.</p>