获取值 DateTime Javascript “if 语句” on asp.net

问：

我正在尝试调试代码，对于 ExpiredDate 和 EffectiveDate，值为 null。当用户选择日期时如何获得价值？

法典

    function checkDate() {
        var ExpiredDate = document.getElementById("<%=dtExpiredDate.ClientID %>");
        var EffectiveDate = document.getElementById("<%=dtEffectiveDate.ClientID %>");
        var ddEmpStatus = document.getElementById('<%=ddEmpStatus.ClientID %>');
        var ddEmpStatusVal = ddEmpStatus.value;
        var ddEmpType = "<%=ddEmpType.ClientID %>";
        var ddvalue = $("#" + ddEmpType).val();


        if (ddEmpStatusVal == "Expatriate") {
            if (ExpiredDate < EffectiveDate) {
                alert("Expire Date is before Effective Date!");
                return false;
            }
        }
        if (ddEmpStatusVal == "National") {
            if (ddvalue != "Employee" && ddvalue != "Direct Contract") {
                if (ExpiredDate < EffectiveDate) {
                    alert("Expire Date is before Effective Date!");
                    return false;
                }
            }
        }
    }