This is expected behavior with imports from Commons, whether you are importing an entire course, a module, or an individual course assignment, quiz, or discussion.
If you are looking to push out completed courses with specific due dates, you may want to look into Blueprinting, as creating a Blueprint course allows you to associate multiple "child" courses with a "master" Blueprint course and gives you the ability to lock specific course content, including due dates and availability dates.
You can learn more about the features of Bluepring
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