1. <script> 
2.    function getBeforeDate(n,time){ 
3.        var d = new Date(time); 
4.        var year = d.getFullYear(); 
5.        var mon=d.getMonth()+1; 
6.        var day=d.getDate(); 
7.        if(day <= n){ 
8.            if(mon>1) { 
9.                    mon=mon-1; 
10.             } 
11.            else { 
12.                 year = year-1; 
13.                 mon = 12; 
14.              } 
15.        } 
16.        d.setDate(d.getDate()-n); 
17.         year = d.getFullYear(); 
18.        mon=d.getMonth()+1; 
19.        day=d.getDate(); 
20.        s = year+"-"+(mon<10?('0'+mon):mon)+"-"+(day<10?('0'+day):day); 
21.        return s; 
22.    } 
23. alert(getBeforeDate(3,'2016/11/20')); 
24. /script>