QDateTime values with a UTC offset are not correctly serialized with
QDataStream::Qt_4_0. So use a newer QDataStream format for this type and
mark it with "@DateTime" instead of "@Variant".
Task-number: QTBUG-46551
Change-Id: I211c89e8cd0211c949ec993e6ffd5192d0eebbb3
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>