<HTML>
<HEAD>
<title>年月日三联动下拉框</title>
<meta http-equiv='Content-Type' content='text/html; charset=gb2312'>
<meta name='author' content='http://singlepine.cnblogs.com/'>
</HEAD>
<body>
<form name="form1">
<select name="YYYY" onchange="YYYYDD(this.value)">
<option value="" selected>请选择 年</option>
</select>
<select name="MM" onchange="MMDD(this.value)">
<option value="" selected>选择 月</option>
</select>
<select name="DD">
<option value="" selected>选择 日</option>
</select>
</form>
<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.form1.YYYY.options.add(new Option(" "+ i +" 年", i));
//赋月份的下拉框
for (var i = 1; i < 13; i++)
document.form1.MM.options.add(new Option(" " + i + " 月", i));
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();
}
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.form1.MM.options[document.form1.MM.selectedIndex].value;
if (MMvalue == ""){ var e = document.form1.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.form1.YYYY.options[document.form1.YYYY.selectedIndex].value;
if (YYYYvalue == ""){ var e = document.form1.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.form1.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>
</body>
</HTML>
来源：https://www.cnblogs.com/162134810/archive/2011/05/27/2059565.html