js获取当前指定的前几天的日期(如当前时间的前七天的日期)

<html>
 <head> 
  <meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> 
  <meta http-equiv="Content-Language" content="zh-cn" /> 
  <title>时间</title> 
 </head> 
 <body>   
  <script>
  function getBeforeDate(n){
    var n = n;
    var d = new Date();
    var year = d.getFullYear();
    var mon=d.getMonth()+1;
    var day=d.getDate();
    if(day <= n){
            if(mon>1) {
               mon=mon-1;
            }
           else {
             year = year-1;
             mon = 12;
             }
           }
          d.setDate(d.getDate()-n);
          year = d.getFullYear();
          mon=d.getMonth()+1;
          day=d.getDate();
     s = year+"-"+(mon<10?('0'+mon):mon)+"-"+(day<10?('0'+day):day);
     return s;
  }
  console.log(getBeforeDate(1));//昨天的日期
  console.log(getBeforeDate(7));//前七天的日期
  </script>
 </body>
</html>
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