In the Due Date Modifier, each quiz item has a date and time field for Show Answers and Hide Answers.  However, dates entered in these fields will not save if the quiz settings do not allow students to see the correct answers.  


To fix this issue, go to the quiz page, edit the quiz, and check the boxes to allow students to see the correct answers.  Then the dates entered in the Due Date Modifier will save.  


screenshot of quiz options for a classic quiz, with red arrows pointing to checked boxes called "Let Students See Their Quiz Responses (Incorrect Questions Will Be Marked in Student Feedback)" and, "Let Students See The Correct Answers"