js如何计算当前日期的前一个月和后一个月？

<div class="query_title_div"><img src="../../images/task/before.png"/><span onclick="monthQuery(-1)">前一月</span></div>
<div class="query_title_div"><img src="../../images/task/search.png"/><span onclick="window.location.href='query_condition.html'" >搜索</span></div>
<div class="query_title_div"><img src="../../images/task/after.png"/><span onclick="monthQuery(1)">后一月</span></div>

//日期快捷搜索 
function monthQuery(num){
   var search_month = search_date.getMonth();
   search_month = search_month + num;
   if(search_month>12){
      search_date.setFullYear(search_date.getFullYear+1);
      search_date.setMonth(1);
   }else if(search_month<1){
      search_date.setFullYear(search_date.getFullYear-1);
      search_date.setMonth(12);
   }else{
      search_date.setMonth(search_month);
   }
    var date = search_date.Format("yyyy-MM-dd");
   params.search_date = date;
   init();
}