Enabling the quiz setting in assignments allows you to protect the content with passwords and limit how long students can spend completing an assignment.
Add open and due dates like normal assignments, and, if you want, you can set a time limit for the students. They will be able to see the due date and how much time they have to complete it before they start the quiz.
The clock starts running after they click "Start Assignment" and will continue running regardless of active time on the screen or whether they close the quiz or leave the page.
Once the time is up, if they haven't submitted, they will be notified that they ran out of time and that the assignment will be submitted in its current state.
Granting Extensions and Resetting Time
At any time during a students progress, you can reset the timer for selected students by using the grant extension button.
If you wish to give your students less than the initial time limit, you can modify the due date.
Changing the time allocated
If you need to change the time allocated to the quiz while the quiz is open, you can do so from the edit mode, but your students will have to refresh the page to see the changes. Students who have run out of time will not get the extra time granted (if the time change results in an increase in total time) and you will have to reset the timer for them.