code: public static String nextDay(int day) {
          Calendar cal = Calendar.getInstance();
          cal.setTime(now());
          cal.add(Calendar.DAY_OF_YEAR, day);
          return format(DF_DATE, cal.getTime());
      }
docs: 取得距离今天 day 日的日期
  
  @param day
  天数
  @return 日期字符串