QDate follows the current settings in the operating system. If that setting happens to be MM/DD/YY, then that's what you'll get. Note: this is what the user wants. A uniform date format in all applications.
The setting is also influenced by the current locale settings in the OS. In the USA for example the default would be MM/DD/YY, while in most European countries it's DD/MM/YY.
In other words: don't worry about it. It's not a bug, it's a feature. And you shouldn't try to work around it. Users don't want their applications ignoring their system settings. If my system is set to display dates in MM/DD/YY, I certainly wouldn't want your app doing its own thing and showing me dates in YYYY/MM/DD format instead.