def retention_policy_effective_time(self):
        """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.
        """
        policy = self._properties.get("retentionPolicy")
        if policy is not None:
            timestamp = policy.get("effectiveTime")
            if timestamp is not None:
                return _rfc3339_to_datetime(timestamp)