怎样动态生成下拉列表?

Question

<script>
    now = new Date()    function writeYears(obj, date)
    {
            var years = date.getYear()+10
            for (var i=date.getYear()-10; i<years; i++)
            {
                var opt = document.createElement("option")
                opt.text = opt.value = i
                obj.add(opt)
            }
    }
    
    function writeMonths(obj)
    {
            for (var i=1; i<13; i++)
            {
                var opt = document.createElement("option")
                opt.text = opt.value = Math.floor(i/10) ? i : "0"+i
                obj.add(opt)
            }
    }
    
    function writeDate(obj, year, month)
    {
        var date = new Date(""+year+"/"+(parseInt(month)+1)+"/"+0).getDate()
        var selectedIndex = (date-1)<obj.selectedIndex ? (date-1) : obj.selectedIndex
        var len = obj.options.length
        if (date>len)
            for (var i=len+1; i<=date; i++)
            {
                var opt = document.createElement("option")
                opt.text = opt.value = Math.floor(i/10) ? i : "0"+i
                obj.add(opt)
            }
        else
            for (var i=date; i<len; i++)
            {
                obj.remove(obj.length-1)
            }
        obj.selectedIndex = selectedIndex
    }    function changeDate()
    {
        var year    = eval("document.forms[0].year")
        var month   = eval("document.forms[0].month")
        var date    = eval("document.forms[0].date")        writeDate(date, year.options(year.selectedIndex).value, month.options(month.selectedIndex).value)
    }
</script><form>
              <select name="year" onchange="changeDate()">
              </select>
              年 
              <select name="month" onchange="changeDate()">
              </select>
              月 
              <select name="date">
              </select>
              日 </nobr></td>
            <script language="javascript" type="text/javascript">
            writeYears(document.forms[0].year, now)
                document.forms[0].year.selectedIndex = 0
                
                writeMonths(document.forms[0].month)
                document.forms[0].month.selectedIndex = now.getMonth()
                
                writeDate(document.forms[0].date, now.getYear(), now.getMonth()+1)
                document.forms[0].date.selectedIndex = now.getDate()-1
    </script>
</form>