As far as I understand there is a race condition in QDateTimePrivate::refreshDateTime.

QDateTimePrivate::refreshDateTime is called by const QDateTime methods. Therfore if there are copies of a QDateTime instance due to COW all point to the same data.

Now when e.g. QDateTime::isValid is called for multiple objects in multiple threads, all pointing to the same data, then the following lines will lead to a race condition.

case Qt::TimeZone:
  // If already cached then don't need to refresh as tz won't change
  if (isTimeZoneCached())
    return;
  // Flag that will have a cached result after calculations
  setTimeZoneCached();
  break;

For example:

1. Thread A enters QDateTimePrivate::refreshDateTime first and calls setTimeZoneCached()
2. Now Thread B enters the function and leaves it again, since the time zone is already marked as cached.
3. Thread B uses m_offsetFromUt, though it has not been calculated yet.
4. Thread A calculates m_offsetFromUt

Other than that with the way it is handled now there can be issues with cpu caches.