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>";

            document.form1.YYYY.value = y;
            document.form1.MM.value = new Date().getMonth() + 1;
            var n = MonHead[new Date().getMonth()];
            if (new Date().getMonth() == 1 && IsPinYear(YYYYvalue)) n++;
            writeDay(n); //赋日期下拉框
            document.form1.DD.value = new Date().getDate();
        }

        function YYYYMM(str) //年发生变化时日期发生变化(主要是判断闰平年)
        {
            var MMvalue = document.form1.MM.options[document.form1.MM.selectedIndex].value;
            if (MMvalue == "") { DD.outerHTML = strDD; return; }
            var n = MonHead[MMvalue - 1];
            if (MMvalue == 2 && IsPinYear(str)) n++;
            writeDay(n)
        }

        function MMDD(str) //月发生变化时日期联动
        {
            var YYYYvalue = document.form1.YYYY.options[document.form1.YYYY.selectedIndex].value;
            if (str == "") { DD.outerHTML = strDD; return; }
            var n = MonHead[str - 1];
            if (str == 2 && IsPinYear(YYYYvalue)) n++;
            writeDay(n)
        }

        function writeDay(n) //据条件写日期的下拉框
        {
            var s = strDD.substring(0, strDD.length - 9);
            for (var i = 1; i < (n + 1); i++)
                s += "<option value='" + i + "'> " + i + " 日" + "</option>\r\n";
            document.form1.DD.outerHTML = s + "</select>";
        }

        function IsPinYear(year)//判断是否闰平年
        {
            return (0 == year % 4 && (year % 100 != 0 || year % 400 == 0))
        }