JS由日期算出年龄,该怎么处理

JS由日期算出年龄
如：已经知道 "2002-10-12" 通过时间戳怎么算出年龄啊。。谢谢~~

[解决办法]

HTML code

    <form>        <label for="birthday">生日：</label>        <input id="birthday" name="birthday" type="text" value="2002-10-12" />        <input id="button" type="button" value="提交" />        <div>            <span>年龄：</span>            <span id="result"></span>        </div>    </form>    <script>        document.getElementById("button").onclick = function(){            var now, birthday, age;            now = new Date();            birthday = new Date(document.getElementById("birthday").value);            age = new Date(now.getTime() - birthday.getTime()).getFullYear();            document.getElementById("result").innerHTML = age - 1970;        }    </script>