When importing resources that should have a calendar event, the calendar event is not preserved and imported into the new course. Example:
Create a course (Course A) with a file resource and set activity completion on it to be completed on view with an expected completion date of tomorrow. Note that in this course there is an event on the calendar stating the expected completion date.
Create a second course (Course B) and then import the same file resource from Course A, being sure to also import calendar events. Notice that in Course B there is no calendar event for that resource even though there should be.
Then if you edit the resource in Course B and hit save (not making any actual changes) the event then appears on the calendar like it should.