code: function(type, aDates, fnRender) {
          var add = [type,aDates,fnRender];
          this.renderStack.unshift(add); 
          this._renderStack = this.renderStack.concat();
      }
docs: The private method used for adding cell renderers to the local render stack.
  This method is called by other methods that set the renderer type prior to the method call.
  @method _addRenderer
  @private
  @param {String} type  The type string that indicates the type of date renderer being added.
  Values are YAHOO.widget.Calendar.DATE, YAHOO.widget.Calendar.MONTH_DAY, YAHOO.widget.Calendar.WEEKDAY,
  YAHOO.widget.Calendar.RANGE, YAHOO.widget.Calendar.MONTH
  @param {Array}  aDates  An array of dates used to construct the renderer. The format varies based
  on the renderer type
  @param {Function} fnRender The function executed to render cells that match the render rules for this renderer.