Hello @bobbieoh
Thank you for contacting the Instructure Community. Are the dates not working when you try to change them to show correct answers at a certain time? Do you get an error or what happens? Did the students already try to come back in to review the quiz and were denied or they haven't had the chance yet? This guide should explain in depth how it works and how to change it:
What options can I set in a quiz? - Instructure Community
However, if you still have questions or things aren't working as intended you're going to want to speak with Canvas Support.
Because of the complexity of the issue, and the concerns for privacy, it would be best if you contacted Canvas Support directly to pinpoint the source of the issue and provide a resolution:
How do I contact Canvas Support? - Instructure Community