ext时间为空grid就不显示了？

Question

在grid中，时间为空就不显示了。怎么能让有空的也显示呢？
var StudentRecord = Ext.data.Record.create([
    {name: 'id', type: 'int'},
    {name: 'subject', type: 'string'},
   {name: 'signDate', type: 'date',dateFormat : 'time' ,mapping : 'signDate.time'},
    {name: 'state', type: 'string'},
  {name: 'addDate', type: 'date',dateFormat : 'time' ,mapping : 'addDate.time'} 
]);
var store = new Ext.data.Store({
    proxy: new Ext.data.HttpProxy({
 url: 'doContract.do?method=_getList'  
}),
     reader: new Ext.data.JsonReader({
        totalProperty: 'total',
        root: 'data'
    },StudentRecord),
 
    remoteSort: false
});store.load({
   params:{start:0,limit:18},
   callback:function(r,option,success){
   
    if(success==false)
{
     Ext.MessageBox.alert('错误', '你权限不够或服务器异常.' );
  }
          
         } 
     });
 var sm = new Ext.grid.CheckboxSelectionModel();//创建复选框var columns = new Ext.grid.ColumnModel([
   sm,//创建复选框
    {header: 'ID', dataIndex: 'id',sortable: true},
    {header: '合同主题', dataIndex: 'subject',sortable: true},
   {header: '签定日期', dataIndex: 'signDate',sortable: true,renderer :Ext.util.Format.dateRenderer('Y-m-d H:i:s')},
    {header: '合同状态', dataIndex: 'state',sortable: true} ,
    {header: '添加时间', dataIndex: 'addDate',sortable: true,renderer :Ext.util.Format.dateRenderer('Y-m-d H:i:s') } 
   
]);

Answer 1

   {header: '签定日期', dataIndex: 'signDate',sortable: true,renderer :myDateRenderer},加一个自己写的renderer函数function myDateRenderer(val)
{
  if(val)
     return val;
  else 
     return new Date();
}

Answer 2

   {header: '签定日期', dataIndex: 'signDate',sortable: true,renderer :myDateRenderer},加一个自己写的renderer函数function myDateRenderer(val)
{
  if(val)
     return val;
  else 
     return new Date();
}

Answer 3

这样也不行， {name: 'signDate', type: 'date',dateFormat : 'time' ,mapping : 'signDate.time'},
不加 mapping ,取不出来时间。后台格式是这样的：
{"total":2,"data":[{"memo":"","subject":"","tel":"","state":"","deliverDate":null,"property":"2011-04-23","sendUnit":"","code":"222","sendDesignDate":null,"postcode":"","contact":"","conYear":"","id":3,"sendDate":null,"signDate":{"nanos":0,"time":1302592580000,"minutes":16,"seconds":20,"hours":15,"month":3,"timezoneOffset":-480,"year":111,"day":2,"date":12},"address":"","addDate":{"nanos":407000000,"time":1302593382407,"minutes":29,"seconds":42,"hours":15,"month":3,"timezoneOffset":-480,"year":111,"day":2,"date":12},"archive":"","baseDesign":0,"custom":0},{"memo":"","subject":"","tel":"","state":"","deliverDate":null,"property":"2011-04-23","sendUnit":"","code":"23","sendDesignDate":null,"postcode":"","contact":"","conYear":"","id":2,"sendDate":null,"signDate":null,"address":"","addDate":{"nanos":907000000,"time":1302592579907,"minutes":16,"seconds":19,"hours":15,"month":3,"timezoneOffset":-480,"year":111,"day":2,"date":12},"archive":"","baseDesign":0,"custom":0}],"success":true}