날짜 검색 범위 정하기 (jquery)

<script type="text/javascript" src="http://code.jquery.com/jquery.js"></script>
<script type="text/javascript">
$(document).ready(function(){
 
    $("#start_year, #start_month, #end_year, #end_month").change(function() {
         
        var checkNum = 2;//3개월이면 2로 셋팅
         
        //선택된 값을 가져온다.
        var startYear = $("#start_year option:selected").val();
        var startMonth = $("#start_month option:selected").val();
        var endYear = $("#end_year option:selected").val();
        var endMonth = $("#end_month option:selected").val();
        //계산을 위해 명시적으로 형변환
        var startYearNum = Number(startYear);
        var startMonthNum = Number(startMonth);
        var endYearNum = Number(endYear);
        var endMonthNum = Number(endMonth);
         
        //일단 차이를 재서 위에서 정한 월이 넘어가는지 확인  
        var result = ((endYearNum*12)+endMonthNum) - ((startYearNum*12)+startMonthNum);
                 
        if(result>checkNum){
             
            alert("날짜 검색 범위는 "+(checkNum+1)+"개월 입니다.");
             
            if(endMonthNum<=checkNum){
                startYearNum = endYearNum-1;
                startMonthNum = 12-(checkNum-endMonthNum)
            }else{
                startYearNum = endYearNum;
                startMonthNum = endMonthNum-checkNum;
            }
             
            $("#start_year").val(startYearNum).attr("selected", "selected");
            $("#start_month").val(startMonthNum).attr("selected", "selected");
             
        };
         
    });
      
});
</script>
 
<select id="start_year" name="start_year">
    <option value="2015" >2015</option>
    <option value="2014" >2014</option>
    <option value="2013" >2013</option>
</select>
<select id="start_month" name="start_month">
    <option value="1" >1</option>
    <option value="2" >2</option>
    <option value="3" >3</option>
    <option value="4" >4</option>
    <option value="5" >5</option>
    <option value="6" >6</option>
    <option value="7" >7</option>
    <option value="8" >8</option>
    <option value="9" >9</option>
    <option value="10" >10</option>
    <option value="11" >11</option>
    <option value="12" >12</option>
</select> ~
<select id="end_year" name="end_year">
    <option value="2015" >2015</option>
    <option value="2014" >2014</option>
    <option value="2013" >2013</option>
</select>
<select id="end_month" name="end_month">
    <option value="1" >1</option>
    <option value="2" >2</option>
    <option value="3" >3</option>
    <option value="4" >4</option>
    <option value="5" >5</option>
    <option value="6" >6</option>
    <option value="7" >7</option>
    <option value="8" >8</option>
    <option value="9" >9</option>
    <option value="10" >10</option>
    <option value="11" >11</option>
    <option value="12" >12</option>
</select> 
<input type="image" src="/images/button/search.gif" alt="검색" class="img"/>