<body>
<select id="s1">
<option value="2001">2001</option>
<option value="2002">2002</option>
<option value="2003">2003</option>
<option value="2004">2004</option>
<option value="2005">2005</option>
<option value="2006">2006</option>
<option value="2007">2007</option>
<option value="2008">2008</option>
<option value="2009">2009</option>
<option value="20010">20010</option>
<option value="20011">20011</option>
<option value="20012">20012</option>
<option value="20013">20013</option>
<option value="20014">20014</option>
<option value="20015">20015</option>
<option value="20016">20016</option>
<option value="20017">20017</option>
<option value="20018">20018</option>
<option value="20019">20019</option>
<option value="20020">20020</option>
</select>
<select id="s2">
<option value="2001">2001</option>
<option value="2002">2002</option>
<option value="2003">2003</option>
<option value="2004">2004</option>
<option value="2005">2005</option>
<option value="2006">2006</option>
<option value="2007">2007</option>
<option value="2008">2008</option>
<option value="2009">2009</option>
<option value="20010">20010</option>
<option value="20011">20011</option>
<option value="20012">20012</option>
<option value="20013">20013</option>
<option value="20014">20014</option>
<option value="20015">20015</option>
<option value="20016">20016</option>
<option value="20017">20017</option>
<option value="20018">20018</option>
<option value="20019">20019</option>
<option value="20020">20020</option>
</select>
<script>
var s1 = document.getElementById('s1'),
s2 = document.getElementById('s2');
s1.onchange = function () {
s2.value = s1.value;
for (var i = s2.selectedIndex - 1; i >= 0; i--) {
s2.options.remove(i);
}
}
</script>
</body>