rem: DateTime& operator=(const QDateTime& dt) { mDateTime = dt;  mDateOnly = false;  mTimeValid = true;  return *this; }
add: DateTime& operator=(const QDate& d) { mDateTime.setDate(d);  mDateOnly = true;  return *this; }
context: 		DateTime& operator=(const QDateTime& dt)		                               { mDateTime = dt;  mDateOnly = false;  mTimeValid = true;  return *this; }