code: def _event_duration(vevent):
          
          if hasattr(vevent, 'dtend'):
              return vevent.dtend.value - vevent.dtstart.value
          elif hasattr(vevent, 'duration') and vevent.duration.value:
              return vevent.duration.value
          return timedelta(0)
docs: unify dtend and duration to the duration of the given vevent