ต้องการแปลง DateFormat จากแบบไทย กลับไปเป็นแบบสากลครับ

<p contenteditable = "true" onBlur = "saveToDatabase(this,'DataIn','1')" style = "display: inline;" >08/07/2561</p>

    <script >
    function saveToDatabase(editableObj, column, GuestID) {
        var date_edit = editableObj.innerHTML;
        var date_format = date_edit.split('/');
        var new_format = date_format[2] - 543 + '-' + date_format[1] + '-' + date_format[0];
        console.log(new_format);
        $.ajax({
            url: "SaveChange.php",
            type: "POST",
            data: 'column=' + column + '&editval=' + new_format + '&GuestID=' + GuestID,
            success: function (data) {
                // alert('OK');
            }
        });
    }
    </script>