This shall indicate the current time in POSIX Time in milliseconds, with the value taken from the same source that could populate the Timestamp field of events. This value shall only be null when any the following are true:
• The node doesn’t support the Time Synchronization cluster.
• The node’s Time Synchronization cluster instance’s UTCTime attribute is null.
This shall indicate the current System Time in milliseconds (type system-ms), with the value taken at the time of processing of the TimeSnapshot command that generated this response.
The value shall be taken from the same clock which populates the Timestamp field in events when using System Time for the field.
This command shall be generated in response to a TimeSnapshot command.
When generating this response, all fields shall be gathered as close together in time as possible, so that the time jitter between the values is minimized.
If the Time Synchronization cluster is supported by the node, the PosixTimeMs field shall NOT be null unless the UTCTime attribute in the Time Synchronization cluster is also null.
See
MatterSpecification.v13.Core § 11.12.7.3