How to deal with systems where the clock looses its memory after a power outage. I.e when power is back the date and time is reset and needs external setting.

Some of the issues are:
checking certificates
creating meaningful timestamps (e.g. in alarm situations)

One solution is NTP; if there exists an NTP Server the system (device) can automatically synchronize its clock.
Other ideas / guidelines:
current time is stored periodically; the stored time is used after a power outage until a better time is provided
the server synchronizes based on date and time values passed by the OPC UA Client in OpenSecureChannelRequest (only after a power outage)
** the server disables all endpoints that require certificate checking until a reliable time is available