private String getStartDate() {
        Calendar cal = Calendar.getInstance();
      // 负数当前时间往前，正数是往后
        cal.add(Calendar.DATE, -120);
        Date d = cal.getTime();
     //转换为年月日格式
        SimpleDateFormat sp = new SimpleDateFormat("yyyy-MM-dd");
        String yesterday = sp.format(d);
        return yesterday;
    }