js实现年月日三级联动

当我们注册一个qq的时候，会看到一个三级年月日的联动菜单，下面简单介绍。
<!doctype html>
<html lang="en">
 <head>
  <meta charset="UTF-8">
  <meta name="Generator" content="EditPlus®">
  <meta name="Author" content="">
  <meta name="Keywords" content="">
  <meta name="Description" content="">
  <title>Document</title>
 </head>
 <body>
  <form name=form1>  
<select name=YYYY onchange="YYYYMM(this.value)">  
<option value="">年</option>  
</select>  
<select name=MM onchange="MMDD(this.value)">  
<option value="">月</option>  
</select>  
<select name=DD>  
<option value="">日</option>  
</select>  
</form>  

 </body>
<script language="JavaScript">  

//定义年、月、日方法函数
window.onload = function(){  
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 + "</option>
";  
}  
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 + "</option>
";  
}  
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)  
}  
//月发生变化时日期联动  
function MMDD(str) {  
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)  
}  
//据条件写日期的下拉框  
function writeDay(n) {  
var s = strDD.substring(0, strDD.length - 9);  
for (var i=1; i<(n+1); i++)  
s += "<option value='" + i + "'> " + i + "</option>
";  
document.form1.DD.outerHTML = s +"</select>";  
} 
//判断是否闰平年
function IsPinYear(year){
    return(0 == year%4 && (year%100 !=0 || year%400 == 0))
    
    }  
</script>

</html>