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Open your course in which you would like to enable/change the due date or cut-off date and click on Turn editing on found at the upper-right of the course page under the settings wheel.
Navigate to the assignment you would like to modify and click on the Edit menu and then Edit settings on the right.
Scroll down to find the options for Availability and click to expand it.
To use either the due date or cut-off date make sure that it is enabled by clicking Enable on the left. Then choose the date (day, month, year) and time of the dates for this assignment.
When you have made all necessary changes to the all the due date or cut-off date, scroll to the end of the page and click Save and return to course.
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