Calculate the last day of the current month, the date of the penultimate day

the requirement is to display a warm reminder within the penultimate 5 days of each month. The code is as follows: calculate the last day of the month, the current time, and the penultimate 5th day, and compare them with the timestamp of three times. but there is always a problem: can you help me to see what"s wrong with my time calculation?

<!--  -->
                <div class="inquire_search message_warm" id="message_warm">
                    <ul>
                        <li>
                            <img class="inquire_img" onclick="selectDeviceList()"  src="${pageContext.request.contextPath}/resources/new/${sessionScope.skin }/image/message_hot.png"/>
                            <span class="inquire_remind"><spring:message code="warmTip.PaySIMcard"/></span>
                        </li>
                        <li>
                            <img class="inquire_img" onclick="selectDeviceList()"  src="${pageContext.request.contextPath}/resources/new/${sessionScope.skin }/image/message_hot.png"/>
                            <span class="inquire_remind"><spring:message code="warmTip.PaySIMcard"/></span>
                        </li>
                    </ul>                
                </div>        

/*   */
   $(function(){
       // 
       var mydate = new Date();
       var timestamp = Date.parse(new Date());

       // 
       var currentMonth=mydate.getMonth();
       var nextMonth=PPcurrentMonth;
       var nextMonthFirstDay=new Date(mydate.getFullYear(),nextMonth,1);
       var oneDay=1000*60*60*24;
       var lastMonthDay = nextMonthFirstDay-oneDay;

       // 5
       var fiveLastDay =  (nextMonthFirstDay-oneDay)-oneDay*5;
       
       // 5
       if(fiveLastDay<timestamp){
           if(timestamp<lastMonthDay){
               $("-sharpmessage_warm").show();
               setInterval("AutoScroll("-sharpmessage_warm")", 3000);
           }
       }else{
           $("-sharpmessage_warm").hide();
       }
    });

/*   */
   function timestampToTime(time) {
    //  yyyy-MM-dd hh:mm:ss
       var date = new Date(time);
       Y = date.getFullYear() + "-";
       M = (date.getMonth()+1 < 10 ? "0"+(date.getMonth()+1) : date.getMonth()+1) + "-";
       D = date.getDate() + " ";
       h = date.getHours() + ":";
       m = date.getMinutes() + ":";
       s = date.getSeconds(); 
       
          return Y+M+D+h+m+s;
   }