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
Return mattress last seen time.