If your test is timed your student may be timed out–ie, their allotted time has expired. There may be situations (internet outages, technical problems) that would warrant resetting this time button.
To do reset the timer for an individual student:(April -May 2020 : make sure you're on our back up server where this functionality lives: https://lbtest.cns.utexas.edu/instructor )
navigate to the specific assignment, answer key (on the left side bar that is revealed after clicking the three horizontal lines at the top), select specific student, and click 'reset timer'
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Double check that the additional time still falls within the due date of the assignment. If it doesn't, the student will also need a late submit authorization.