QDateTimeEdit cannot support general time-zones

The date-time editor we provide does have a timeSpec property (which should probably really live on QDateTimeParser rather than QDateTimeEditPrivate), but it can only sensibly be set to Qt::LocalTime (its default) or Qt::UTC; it has no provision for recording an offset from UTC (even a fixed one), much less taking a QTimeZone to identify a specific zone.

Modern calendar apps, although they default to local time, do permit selecting other time-zones; in particular, when scheduling a call with a colleague in another zone, the user may find it constructive to select that zone in their date-time dialogs. It is thus a fairly major deficiency of QDateTimeEdit that it cannot be configured to use an arbitrary time-zone, not even by specifying it as an offset from UTC.