月份的判定，由于涉及到过多了判定条件，如果用if else会大大降低性能，建议用switch 语法

代码如下：

完整的源代码：

    this.init();
}
GySetDate.prototype = {
    init:function(){
        var _that = this;
        // 初始化日期
        this.setYears({\'start\':this.startYear,\'end\':this.endYear});
        this.setMonths({\'start\':this.startMonth});
        this.setDays({\'year\':this.startYear,\'month\':this.startMonth,\'start\':this.startDay});
        // 年选择
        this.eYear.onchange = function(){
            var year = parseInt(this.value);
            switch(true){
                case (year == _that.startYear):{
                    _that.setMonths({\'start\':_that.startMonth});
                    _that.setDays({\'year\':_that.startYear,\'month\':_that.startMonth,\'start\':_that.startDay});
                };break;
                case (year == _that.endYear):{
                    _that.setMonths({\'start\':1,\'end\':_that.endMonth});
                    if(_that.endMonth>1){
                        _that.setDays({\'year\':_that.endYear,\'month\':1,\'start\':1});   
                    }else{
                        _that.setDays({\'year\':_that.endYear,\'month\':1,\'start\':1,\'end\':_that.endDay});
                    }
                };break;
                default:{
                    _that.setMonths({\'start\':1});
                    _that.setDays({\'start\':1,\'year\':year,\'month\':1});
                }
            }
           
        }
        // 月选择
        this.eMonth.onchange = function(){
            var year = parseInt(_that.eYear.options[_that.eYear.selectedIndex].value),
                month = parseInt(this.value);
            switch(true){
                case (year==_that.endYear&&month==_that.endMonth):{
                    _that.setDays({\'start\':1,\'year\':year,\'month\':month,\'end\':_that.endDay});
                };break;
                case (year==_that.startYear&&month==_that.startMonth):{
                    _that.setDays({\'year\':_that.startYear,\'month\':_that.startMonth,\'start\':_that.startDay});
                };break;
                default:{
                    _that.setDays({\'start\':1,\'year\':year,\'month\':month});
                }
            }   
           
        }
    },
    /*设置年，月，日
    ----------------------------------
    参数值都为Number类型
    */
    // 参数 {\'start\':xx,\'end\':xxx}
    setYears:function(opt){
        this.eYear.innerHTML = \'\';
         for(var n=opt.start;n<=opt.end;n++){
            this.eYear.add(new Option(n,n));
        }
    },
    // 参数 {\'start\':xx,\'end\':xxx}
    // 参数 \'end\' 为可选，忽略，则开始到12月
    setMonths:function(opt){
        this.eMonth.innerHTML = \'\';
        var months = opt.end || 12;
        for(var n=opt.start;n<=months;n++){
            if(n<10) n = \'0\'+n;
           this.eMonth.add(new Option(n,n));
        }
    },
    // 参数 {\'start\':xx,\'year\':xxx,\'month\':xx,\'star\':xx,\'end\':xxx}
    // 参数 \'end\' 为可选，忽略，则开始到本月底（根据月份判断的）
    setDays:function(opt){
         this.eDay.innerHTML = \'\';
         var days = opt.end || this.getDays(opt.year,opt.month);
         for(var n=opt.start;n<=days;n++){
            if(n<10) n = \'0\'+n;
            this.eDay.add(new Option(n,n));
         }
    },
    /* 根据 年，月，返回正确的天数，如 2016-2，返回是29天（润年）
    --------------------------------------------------------------
    参数值都为Number类型
    */
    getDays:function(year,month){
         // var aDay = [31,28|29,31,30,31,30,31,31,30,31,30,31];
        // 二月份的天数数据处理
        var FedDays = year%4==0?29:28,
            returnDays = \'\';
        var month = month<10?month = \'0\'+month:month.toString();
        switch(month){
            case \'01\':
            case \'03\':
            case \'05\':
            case \'07\':
            case \'08\':
            case \'10\':
            case \'12\': returnDays = 31;break;
            case \'04\':
            case \'06\':
            case \'09\':
            case \'11\': returnDays = 30;break;
            case \'02\': returnDays = FedDays;break;
        }
        return returnDays;
    },
    /*工具辅助函数
    ----------------------------------
    */
    getId:function(id){
        return document.getElementById(id);
    }
}