If you started a timed assignment and your instructor granted you extra time, the assignment due date changes until you resume work on the assignment.
After extra time is granted for a timed assignment, its due date is recalculated to reflect the current time plus the extra time you were granted each time your home page or the My Assignments page is loaded.
For example, if your instructor granted you 30 extra minutes and you reload the page at 1:30, the due date will be 2:00. If you reload the page at 2:01, the due date will be 2:31.
Resolution
Although potentially confusing, this is expected behavior and shows the correct due date if you were to resume work on the assignment as of the time the page was loaded.
When you open the assignment, the timer counts down your remaining time as expected.