为什么JSON.stringify在我解析json时改变时间

Question

为什么当我转换时间JSON.stringify为什么我的时间改变请帮助我如何处理这个请任何人帮助我，以及如何指导json转移代码C＃变量背后

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)
            {}