CSS3 year, month, day and date three-level linkage drop-down selection code

<script language="JavaScript">   
function YYYYMMDDstart(){   
	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();  
	for (var i = (y-30); i < (y+30); i++) //以今年为准，前30年，后30年   
		   document.reg_testdate.YYYY.options.add(new Option(" "+ i +" 年", i));   
	
	//赋月份的下拉框   
	for (var i = 1; i < 13; i++)   
		   document.reg_testdate.MM.options.add(new Option(" " + i + " 月", i));   
	
	document.reg_testdate.YYYY.value = y;   
	document.reg_testdate.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Author:meizz   
	document.reg_testdate.DD.value = new Date().getDate();   
}   
if(document.attachEvent)   
	window.attachEvent("onload", YYYYMMDDstart);   
else   
window.addEventListener('load', YYYYMMDDstart, false);   
function YYYYDD(str) //年发生变化时日期发生变化(主要是判断闰平年)   
{   
	var MMvalue = document.reg_testdate.MM.options[document.reg_testdate.MM.selectedIndex].value;   
	if (MMvalue == ""){ var e = document.reg_testdate.DD; optionsClear(e); 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.reg_testdate.YYYY.options[document.reg_testdate.YYYY.selectedIndex].value;   
	if (YYYYvalue == ""){ var e = document.reg_testdate.DD; optionsClear(e); 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 e = document.reg_testdate.DD; optionsClear(e);   
	for (var i=1; i<(n+1); i++)   
	e.options.add(new Option(" "+ i + " 日", i));   
}   
function IsPinYear(year)//判断是否闰平年   
{
	return(0 == year%4 && (year%100 !=0 || year%400 == 0));
}   
function optionsClear(e)   
{   
	e.options.length = 1;   
}   
</script>

一款相当简单的js年月日日期三级联动下拉选择代码，select下拉框日期选择js特效。