code: def duration(self):
          
          if not self.started:
              return None
          start = self.started
          end = self.completed
          if not end:
              end = datetime.utcnow()
          return end - start
docs: Return a timedelta for this task.
  
          Measure the time between this task's start and end time, or "now"
          if the task has not yet finished.
  
          :returns: timedelta object, or None if the task has not even started.