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>11){
    search_date.setFullYear(search_date.getFullYear()+1);
    search_date.setMonth(0);
}else if(search_month<0){
    search_date.setFullYear(search_date.getFullYear()-1);
    search_date.setMonth(11);
}else{
    search_date.setMonth(search_month);
}
var date = search_date.Format("yyyy-MM-dd");

}