为什么JSON.stringify更改时间当我解析json到它

为什么当我转换时间JSON.stringify为什么我的时间变化请帮助我如何处理这个请任何人帮助我，以及如何直接json传输代码隐藏在C＃变量后面为什么JSON.stringify更改时间当我解析json到它

subject_end: Thu Jan 01 1970 09:00:00 
After JSON.stringift 
"subject_start":"1970-01-01T03:00:00.000Z" 
i don't know why this happen 

    $scope.Savesujects = function (item, event) { 
          console.log($scope.moreadditems); 
          var getjsonvar = JSON.stringify($scope.moreadditems); 
          console.log(getjsonvar); 
          var ssa = String(getjsonvar); 
          $.ajax({ 
           type: "POST", 
           url: "classschedule.aspx/savesubhects", 

           data: JSON.stringify({ jsonstring: ssa }), 
           contentType: "application/json; charset=utf-8", 
           dataType: "json", 
           success: function (msg) { 
            alert(msg.d); 

           }, 
           error: function (msg) { 
            alert(msg.d); 
           } 
          }); 
         } 

//CODE BEHINED 

    [WebMethod] 
      public static string savesubhects(string jsonstring) 
      {}