js实现日期级联效果

代码如下:
    <div>
        <select name="YYYY" onchange="YYYYMM(this.value)">
            <option value="">请选择 年</option>
        </select>
        <select name="MM" onchange="MMDD(this.value)">
            <option value="">选择 月</option>
        </select>
        <select name="DD">
            <option value="">选择 日</option>
        </select>
    </div>

    <script language="JavaScript">
        function window.onload() {
            strYYYY = document.form1.YYYY.outerHTML;
            strMM = document.form1.MM.outerHTML;
            strDD = document.form1.DD.outerHTML;
            MonHead = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

            //先给年下拉框赋内容
            var y = new Date().getFullYear();
            var str = strYYYY.substring(0, strYYYY.length - 9);
            for (var i = (y - 30); i < (y + 30); i++) //以今年为准，前30年，后30年
            {
                str += "<option value='" + i + "'> " + i + " 年" + "</option>/r/n";
            }
            document.form1.YYYY.outerHTML = str + "</select>";

            //赋月份的下拉框
            var str = strMM.substring(0, strMM.length - 9);
            for (var i = 1; i < 13; i++) {
                str += "<option value='" + i + "'> " + i + " 月" + "</option>/r/n";
            }
            document.form1.MM.outerHTML = str + "</select>";