Just want to summarize the proposed solutions in an orderly fashion so that it remains for the future:
@Christian-Ehrlicher: use QDate::addDays() / QDate::addMonths() / QDate::addYears(). [for dates]
@JonB : write a global "utility" inline function to do a particular operation you want on a QDateTime.
@J-Hilk: make a new class and inherit from QDateTime where you add those convenient setX functions.
don't forget to convert the returning objects from the existing functions that return a QDateTime object to the new object,
or make a constructor that excepts a QDateTime object as argument, that than can be done implicitly.
Thank you all for a fruitful discussion and varied and helpful suggestions!