function Yearcompare()
{
var year1=$("#dropdownlistboxes1").val();
var year2=$("#dropdownlistboxes2").val();
if(year1 < year2){
alert("Start year must < than end year");
}
}
Call this Function on your dropdownlistboxes1 on change event.
<select id="dropdownlistboxes1" önchange="Yearcompare()" name="xxx">
</select>