QDateTime::addSecs needs to do something similar, but not identical
because it needs the number of days too. And then there are daylight
savings transitions...
Task-number: QTBUG-47717
Change-Id: I7de033f80b0e4431b7f1ffff13f976f4f5e5a059
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>