QDateTime represents a specific point in time, so arithmetic with durations makes perfect sense. Moreover, we can finally equip QDateTime with a subtraction operator, to calculate the duration between two QDateTime objects. [ChangeLog][QtCore][QDateTime] QDateTime now supports arithmetic between QDateTime objects and std::chrono::duration objects. A duration can be added to or subtracted from a QDateTime, yielding another QDateTime; and two QDateTime objects can be subtracted from each other, yielding the duration between them. Change-Id: I656419f3bb9418c49f0e2fd0800c3dbaaf6aff32 Reviewed-by: Edward Welbourne <edward.welbourne@qt.io> Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>