@@ -255,7 +255,14 @@ class AsyncRunner(BaseRunner):

                           "'adaptive.notebook_extension()'")

     def elapsed_time(self):

-        end_time = self.end_time if self.task.done() else time.time()

+        if self.task.done():

+            end_time = self.end_time

+            if end_time is None:

+                # task was cancelled before it began

+                assert self.task.cancelled()

+                return 0

+        else:

+            end_time = time.time()

         return end_time - self.start_time

     def status(self):