docs: public static long monthFirstDay(long time) {
          ZoneId zid = ZoneId.systemDefault();
          Instant instant = Instant.ofEpochMilli(time);
          LocalDate ld = instant.atZone(zid).toLocalDate().withDayOfMonth(1);
          return ld.atStartOfDay(zid).toInstant().toEpochMilli();
      }