getBeforeDate(n) {//传入天数

 var n = n;

 var d = new Date();

 var year = d.getFullYear();

 var mon = d.getMonth() + 1;

 var day = d.getDate();

   if (Number(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();

   var s = year + "-" + (mon < 10 ? ('0' + mon) : mon) +"-"+(day < 10 ? ('0' + day):day);

   return s;

 }