docs: def last_seen(self):
          """Return mattress last seen time."""
          """
          These values seem to be rarely updated correctly in the API.
          Don't expect accurate results from this property.
          """
          try:
              if self.side == 'left':
                  lastseen = self.device.device_data['leftPresenceEnd']
              elif self.side == 'right':
                  lastseen = self.device.device_data['rightPresenceEnd']
  
              date = datetime.fromtimestamp(int(lastseen)) \
                  .strftime('%Y-%m-%dT%H:%M:%S')
              return date
          except TypeError:
              return None