We just recently had this come up and I did some testing with a user in the instructor role in one browser and a user in the student role in another browser. The quiz was for 5 minutes.
As soon as the student started the quiz, as the instructor I moderated the quiz, clicked on the clock icon for the user and added 5 minutes. I tested both options (End the quiz 10 minutes from the current end time and End the quiz 10 minutes from now).
In each case the student was kicked out of the quiz after the initial 5 minutes, yet in the instructor's browser, the timer is still counting down with the extended time.
I created a video of this and filed a ticket with Instructure.
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