DateTime slow performance

There are few performances issues with QDateTime class.

• Comparison of 2 datetimes can be very slow, even if the 2 local datetime represent the same date and time.
  For example under windows :

      QDateTime d1 = QDateTime::currentDateTime();
      qint64 ms_utc = d1.toMSecsSinceEpoch();
      QDateTime d2 = QDateTime::fromMSecsSinceEpoch(ms_utc, Qt::LocalTime, 0);
      if (d1 < d2) ...
  

  For example under linux :

      QDateTime d1 = QDateTime::currentDateTime();
      QDateTime d2 = QDateTime(d1.date(), d1.time());
      if (d1 < d2) ...
  

  The main problem here is around daylightStatus that is not set...

• Check the validity of the datetime object is slow.
  This is normal for the first call of .isValid(), but the result should be cached for future call