JackcessDateConvert2.java

    private static final ZoneId UTC_ZONEID = ZoneId.of("UTC");

    public double toDateDouble(Object value) {
      long time = toDateLong(value);
      time += getToLocalTimeZoneOffset(time);
      final LocalDateTime ldt =
          LocalDateTime.ofInstant(Instant.ofEpochMilli(time), UTC_ZONEID);
      final long datePart = ldt.toLocalDate().toEpochDay() * (long) MILLISECONDS_PER_DAY +
          MILLIS_BETWEEN_EPOCH_AND_1900;
      final long timePart = ldt.toLocalTime().toNanoOfDay() / 1000000L;
      final long ret = (datePart >= 0) ? (datePart + timePart) : (datePart - timePart);
      return ret / MILLISECONDS_PER_DAY;
    }

    public long fromDateDouble(double value) {
      long datePart = ((long) value) * (long) MILLISECONDS_PER_DAY;
      long timePart = Math.round(Math.abs(value % 1.0) * MILLISECONDS_PER_DAY);
      long time = datePart + timePart;
      time -= MILLIS_BETWEEN_EPOCH_AND_1900;
      time -= getFromLocalTimeZoneOffset(time);
      return time;
    }