Code
var $ = function (id) {
    return "string" == typeof id ? document.getElementById(id) : id;
};

function addEventHandler(oTarget, sEventType, fnHandler) {
    if (oTarget.addEventListener) {
        oTarget.addEventListener(sEventType, fnHandler, false);
    } else if (oTarget.attachEvent) {
        oTarget.attachEvent("on" + sEventType, fnHandler);
    } else {
        oTarget["on" + sEventType] = fnHandler;
    }
};

var Class = {
  create: function() {
    return function() {
      this.initialize.apply(this, arguments);
    }
  }
}

var Extend = function(destination, source) {
    for (var property in source) {
        destination[property] = source[property];
    }
    return destination;
}

var DateSelector = Class.create();
DateSelector.prototype = {
  initialize: function(oYear, oMonth, oDay, options) {
    this.SelYear = $(oYear);//年选择对象
    this.SelMonth = $(oMonth);//月选择对象
    this.SelDay = $(oDay);//日选择对象
    
    this.SetOptions(options);
    
    var dt = new Date(), iMonth = parseInt(this.options.Month), iDay = parseInt(this.options.Day), iMinYear = parseInt(this.options.MinYear), iMaxYear = parseInt(this.options.MaxYear);
    
    this.Year = parseInt(this.options.Year) || dt.getFullYear();
    this.Month = 1 <= iMonth && iMonth <= 12 ? iMonth : dt.getMonth() + 1;
    this.Day = iDay > 0 ? iDay : dt.getDate();
    this.MinYear = iMinYear && iMinYear < this.Year ? iMinYear : this.Year;
    this.MaxYear = iMaxYear && iMaxYear > this.Year ? iMaxYear : this.Year;
    this.onChange = this.options.onChange;
    
    //年设置
    this.SetSelect(this.SelYear, this.MinYear, this.MaxYear - this.MinYear + 1, this.Year - this.MinYear);
    //月设置
    this.SetSelect(this.SelMonth, 1, 12, this.Month - 1);
    //日设置
    this.SetDay();
    
    var oThis = this;
    //日期改变事件
    addEventHandler(this.SelYear, "change", function(){
        oThis.Year = oThis.SelYear.value; oThis.SetDay(); oThis.onChange();
    });
    addEventHandler(this.SelMonth, "change", function(){
        oThis.Month = oThis.SelMonth.value; oThis.SetDay(); oThis.onChange();
    });
    addEventHandler(this.SelDay, "change", function(){
        oThis.Day = oThis.SelDay.value; oThis.onChange();
    });
  },
  //设置默认属性
  SetOptions: function(options) {
    this.options = {//默认值
        Year:        0,//年
        Month:        0,//月
        Day:        0,//日
        MinYear:    0,//最小年份
        MaxYear:    0,//最大年份
        onChange:    function(){}//日期改变时执行
    };
    Extend(this.options, options || {});
  },
  //日设置
  SetDay: function() {
    //取得月份天数
    var daysInMonth = new Date(this.Year, this.Month, 0).getDate();
    if (this.Day > daysInMonth) { this.Day = daysInMonth; };
    this.SetSelect(this.SelDay, 1, daysInMonth, this.Day - 1);
  },
  //select设置
  SetSelect: function(oSelect, iStart, iLength, iIndex) {
    //添加option
    oSelect.options.length = iLength;
    for (var i = 0; i < iLength; i++) { oSelect.options[i].text = oSelect.options[i].value = iStart + i; }
    //设置选中项
    oSelect.selectedIndex = iIndex;
  }
};