js日期级联效果的实现代码

<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))
}