One thing you could try to do is to use the restrict access and activity completion features.
Create a page with some kind of explanation of the steps for accessing the quiz. Set the completion setting to “students must view” to complete the activity. Set the restrict access settings to close at 8:00.
In your quiz settings go to the “restrict access settings” and set “activity completion” for the page you created to “must be marked complete”. You could also hide the quiz from people who haven’t accessed the quiz explanation page before the time deadline.
I think this should give you what you want.