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The settings for an assignment's due date, as well as the open and close dates, are now located in the "Assign to" side panel.
There are different ways to access this side panel.
In the Assignment's Settings
If you're creating a new assignment or editing the settings for an existing assignment, you'll find "Assign Access" at the bottom of the page. Select "Manage Assign to" to open the panel.
From the Assignments List
You can also adjust due dates quickly without entering the full assignment settings. In the Assignments list, select the three dots to the right of an assignment and choose "Assign to."
NOTE: If an assignment is in a module, you can also use this method from the Modules list.