<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);
 }