code: public static String getLastDayOfMonth() {
          Calendar cal = Calendar.getInstance();
          cal.set(Calendar.DATE, 1);
          cal.add(Calendar.MONTH, 1);
          cal.add(Calendar.DATE, -1);
          return format(DF_DATE, cal.getTime());
      }
docs: 获取本月最后一天
  
  @return 本月最后一天