Indeed it is: if one day we want to differenciate between an "all day"
event and a start datetime at midnight, then a single QDateTime value isn't enough.

KDateTime has allDay(), while QDateTime converts null (or invalid) times to midnight.

This is probably why KCalCore still uses KDateTime. So if zanshin wants to support
todos with a specific start time, it has to either use KDateTime (i.e. kdelibs4support)
everywhere (starting with domain/task.h), or to wait until KCalCore is (somehow)
ported away from KDateTime (e.g. with its own bool - or QDateTime needs extending).