虽然是个简单的效果，还是需要积累一下，记录一下；
源代码如下所示：
<select id="myselect1">
<option value="">— —</option>
<option value="2015级">2015级</option>
<option value="2014级">2014级</option>
<option value="2013级">2013级</option>
<option value="2012级">2012级</option>
</select><br><br>
<select id="myselect2">
<option value="">— —</option>
<option value="2015级">2015级</option>
<option value="2014级">2014级</option>
<option value="2013级">2013级</option>
<option value="2012级">2012级</option>
</select>
var time = new Date();
var month=time.getMonth()+1;
var day=time.getDay();
if(month>5&&day>1){
var objSelect=document.getElementById("myselect2");
objSelect.options.add(new Option("2016级", "2016级"),1);
objSelect.options.remove(5);
}
if(month>7&&day>1){
var objSelect=document.getElementById("myselect1");
objSelect.options.add(new Option("2016级", "2016级"),1);
objSelect.options.remove(5);
}