日期三级级联

<select name="year" id="year"> 
   <option>选择年</option>         
</select> 
<select name="month" id="month"> 
   <option>选择月</option> 
</select> 
<select name="day" id="day"> 
   <option>选择日</option>
</select>

//取得当前年、月、日！
var nowTime=new Date();
var year=nowTime.getFullYear();
var month=nowTime.getMonth()+1;
var day=nowTime.getDate()
// var day=nowTime.getDay();
var mon=['31','28','31','30','31','30','31','31','30','31','30','31'];
function $(id){
   return document.getElementById(id)
}
 /*
  * 显示日期
  */
function showDate(){
  //写出年
   var num=0;
   for(var i=year;i>=year-30;i--){
	num++;
	$('year').options.add(new Option(i+"年",i));
	if(i==year){	
		$('year').options[num].selected=true;
	}	 
    }
   //写出月
   for(var j=1;j<=12;j++){
	$('month').options.add(new Option(j+"月",j));
	if(j==month){
		$('month').options[month].selected=true;
	}
   }
  //写出日
  changeDay(year,month);
  $('year').attachEvent('onchange',changeYear);
  $('month').attachEvent('onchange',changeMonth);	 
}
//判断是否为闰年
function isLeapYear(year){
     if((year%4==0&&year%100!=0)||(year%400==0)){
	return true;
     }else{
	return false;
     }
}
//根据所选年、月动态改变每月的天数！
function changeDay(y,m){
     var sum=0;
     if(isLeapYear(y)&& m==2){
	sum=parseInt(mon[m-1])+1;
     }else{
	sum=mon[m-1];
     }
     for(var k=1;k<=sum;k++){
	$('day').options.add(new Option(k+"日",k));
	if(k==day){
		$('day').options[day].selected=true;
	}
      }
}
//变化年
function changeYear(){
	$('month').options.selectedIndex = 1;
	$('day').options.selectedIndex = 1;
}
//变化月份
function changeMonth(){
	var Year = $('year').options[$('year').selectedIndex].value;
	var Month = $('month').options[$('month').selectedIndex].value;
	$('day').options.length = 1;
	changeDay(Year,Month); 
	$('day').options.selectedIndex = 1;
}