At the moment OPC UA only knows one single timescale (UtcTime), which is used for ReceiveTime, ServerTimestamp, SourceTimestamp, ...
The request is to introduce additional sources of time.

Use-Case:
A time master (not linked to any other time master) distributes its time to a number of CPUs which run OPCUA servers (e.g. over IEE1588)
Even if TAI is used it might come to a time jump if the time of the master is manually set (OPC UA does not forbid jumps of UtcTime).
For some cases, such as log entries, these jumps are intended to make sure the logbook entries contain the correct day of time.
For other cases, e.g. when calculations are based on the timestamps, such jumps are not allowed.

How is this usually solved?
The time master distributes two time domains to the same network.
One time domain is the timebase as mentioned above, the other domain is the WorkingClock (see links in "Additional Information").
Whether this needs to be UtcTime-Format or not is to be discussed/evaluated.

How can UPC UA support this:
OPC UA does not need to distinguish between WorkingClock/WallClock or something else.
OPC UA just should be aware that there is more than one source of time.
For example it should be possible to define somewhere that 'SoruceTimestamp' should be taken form one source, and 'ServerTimestamp' from another.

This 'awareness' will also be helpful for further use in PubSub.