code: def retention_policy_effective_time(self):
          
          policy = self._properties.get("retentionPolicy")
          if policy is not None:
              timestamp = policy.get("effectiveTime")
              if timestamp is not None:
                  return _rfc3339_to_datetime(timestamp)
docs: Retrieve the effective time of the bucket's retention policy.
  
          :rtype: datetime.datetime or ``NoneType``
          :returns: point-in time at which the bucket's retention policy is
                    effective, or ``None`` if the property is not
                    set locally.