int last_day_of_month(std::tm cur) const {
           if (cur.tm_mon + 1 > 11) {
              cur.tm_mon = 0;
              cur.tm_year++;
            } else {
              cur.tm_mon++;
            }
            cur.tm_mday = 1;
            cur.tm_hour = 0;
            cur.tm_min = 0;
            add(cur, std::chrono::hours(-24));
            return cur.tm_mday;
        }

        Clock::time_point cron_to_prev(const Clock::time_point from) const {
          int loop_times = 0;
          auto now = Clock::to_time_t(from);
          std::tm prev(*std::localtime(&now));
          std::tm old = prev;
          prev.tm_sec = 0;
          add(prev, std::chrono::minutes(-1)); // last min
          bool change_mon = false;
          bool change_day = false;
          bool change_week = false;
          bool change_hour = false;
          while (true) {
            if (loop_times++ > MAX_LOOP_TIMES) {
              printf("!!! seems dead loop in cron_to_prev !!!\n");
              return Clock::time_point(Clock::duration(0));
            }
            if (minute != -1 && prev.tm_min != minute) {
              add(prev, std::chrono::minutes(-1));
              continue;
            }
            if (hour != -1 && prev.tm_hour != hour) {
              add(prev, std::chrono::hours(-1));
              continue;
            }
            if (day != -1 && prev.tm_mday != day) {
              add(prev, std::chrono::hours(-24));
              continue;
            }
            if (day_of_week != -1 && prev.tm_wday != day_of_week) {
              add(prev, std::chrono::hours(-24));
              continue;
            }
            if (month != -1 && prev.tm_mon != month) {
              if (prev.tm_mon == 0) {
                prev.tm_mon = 11;
                prev.tm_year--;
              } else {
                prev.tm_mon--;
              }
              add(prev, std::chrono::seconds(0)); // bug fix
              continue;
            }
           
            break;
          }

          if (old.tm_hour != prev.tm_hour) {
            change_hour = true;
          }
          if (old.tm_mday != prev.tm_mday) {
            change_day = true;
          }
          if (old.tm_wday != prev.tm_wday) {
            change_week = true;
          }
          if (old.tm_mon != prev.tm_mon) {
            change_mon = true;
          }
          if (change_hour || change_day || change_week || change_mon) {
            if (minute == -1 
                 && (hour != -1 || day != -1 || day_of_week != -1 || month != -1)) {
              prev.tm_min = 59;
            }
          } 
          if (change_day || change_week || change_mon) {
            if (hour == -1 && (day != -1 || day_of_week != -1 || month != -1)) {
              prev.tm_hour = 23;
            }
          }
          if (change_mon) {
            if (day == -1 && month != -1) { // last day in this month
              prev.tm_mday = last_day_of_month(prev);// [1,31]
            }
          }
          if (change_week || change_mon) {
            if (month != -1 && day_of_week != -1) {  // last week-day in this month
              prev.tm_mday = last_day_of_month(prev); // last day in this month
              add(prev, std::chrono::seconds(0));
              while(true) {
                if (prev.tm_wday == day_of_week) {
                  break;
                }
                add(prev, std::chrono::hours(-24));
              }
            }
          }
          // telling mktime to figure out dst
          prev.tm_isdst = -1;
          return Clock::from_time_t(std::mktime(&prev));
        }

        // http://*.com/a/322058/1284550
        Clock::time_point cron_to_next(const Clock::time_point) const {
          int loop_times = 0;
          auto now = Clock::to_time_t(from);
          std::tm next(*std::localtime(&now));
          // it will always at least run the next minute
          next.tm_sec = 0;
          add(next, std::chrono::minutes(1));
          while (true) {
            if (loop_times++ > MAX_LOOP_TIMES) {
              printf("!!! seems dead loop in cron_to_next !!!\n");
              return Clock::time_point(Clock::duration(0));
            }
            if (month != -1 && next.tm_mon != month) {

              if (next.tm_mon + 1 > 11) {
                next.tm_mon = 0;
                next.tm_year++;
              } else {
                next.tm_mon++;
              }
              next.tm_mday = 1;
              next.tm_hour = 0;
              next.tm_min = 0;
              add(next, std::chrono::seconds(0)); // bug fix
              continue;
            }

            if (day != -1 && next.tm_mday != day) {
              add(next, std::chrono::hours(24));
              next.tm_hour = 0;
              next.tm_min = 0;
              continue;
            }
            if (day_of_week != -1 && next.tm_wday != day_of_week) {
              add(next, std::chrono::hours(24));
              next.tm_hour = 0;
              next.tm_min = 0;
              continue;
            }
            if (hour != -1 && next.tm_hour != hour) {
              add(next, std::chrono::hours(1));
              next.tm_min = 0;
              continue;
            }
            if (minute != -1 && next.tm_min != minute) {
              add(next, std::chrono::minutes(1));
              continue;
            }
            break;
          }
          
          // telling mktime to figure out dst
          next.tm_isdst = -1;
          return Clock::from_time_t(std::mktime(&next));
        }


    Bosma::Cron cron("3 * 1 10 *");
    auto fromTp = try_parse("2010-03-06 10:10:00", "%Y-%m-%d %H:%M:%S");