extjs计算两个DateField所间隔的月份（天数）

     var serviceTimeStart = new Ext.form.DateField({
          name: "serviceTimeEnd",//最新交费时间参数为上期终止时间
          id:"startdate",
          fieldLabel: "服务开始时间",
          allowBlank: false,
          width: 180,
          tabIndex: 8,
          format: ‘Y-m-d‘,
          blankText: "不能为空"
          
            });
      var serviceTimeEnd = new Ext.form.DateField({
//          name: "serviceTimeEnd",
           id:"endDate",
          fieldLabel: "服务终止时间",
          width: 180,
          tabIndex: 9,
          format: ‘Y-m-d‘,
         
                     listeners:{ 
                      change:function() 
                      { 
 
                 var e = Ext.util.Format.date(Ext.getCmp(‘endDate‘).getValue(), ‘Y-m-d‘);//格式化日期控件值
                    var s= Ext.util.Format.date(Ext.getCmp(‘startdate‘).getValue(), ‘Y-m-d‘);//格式化日期控件值
 
                       var end = new Date(e);
                    var start = new Date(s);
                    var elapsed = Math.round((end.getTime() - start.getTime())/(86400000*30)); // 计算间隔月数
 
                         Ext.MessageBox.alert(elapsed);
                       }
}
      });