效果如下图：
实现过程：
date.html
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
<script type="text/javascript" src="js/date.js" charset="gbk"></script>
</head>
<body onload="init()">
<select id="year" ></select>年
<select id="month" onchange="setDay()"></select>月
<select id="day"></select>日
</body>
</html>
date.js
function init(){
var year= document.getElementById("year");
for(var i=1980;i<=2016;i++){
year.options.add(new Option(i,i));
}
var month=document.getElementById("month");
month.options.add(new Option("-月-",0));
for(var i=1;i<=12;i++){
month.options.add(new Option(i,i));
}
}
function setDay(){
year=document.getElementById("year").value;
month=document.getElementById("month").value;
document.getElementById("day").length=0;
var data=new Array(31,28,31,30,31,30,31,31,30,31,30,31);
var days=data[month-1];
if((year%4==0&&year%100!=0)||(year%400==0)){
if(month==2){
days++;
}
}
for(var i=1;i<=days;i++){
day.options.add(new Option(i,i));
}
}
