The serverTimestamp is used to reflect the time that the Server received a Variable value or knew it to be accurate.
In the case of a bad or uncertain status, serverTimestamp is used to reflect the time that the Server received the status or that the Server last tried to recover from the bad or uncertain status.
In the case where the OPC UA Server subscribes to a value from another OPC UA Server, each Server applies its own serverTimestamp. This is in contrast to the sourceTimestamp in which only the originator of the data is allowed to apply the sourceTimestamp.
If the Server subscribes to the value from another Server every ten seconds and the value changes, then the serverTimestamp is updated each time a new value is received. If the value does not change, then new values will not be received on the Subscription. However, in the absence of errors, the receiving Server applies a new serverTimestamp every ten seconds because not receiving a value means that the value has not changed. Thus, the serverTimestamp reflects the time at which the Server knew the value to be accurate.
This concept also applies to OPC UA Servers that receive values from exception-based data sources. For example, suppose that a Server is receiving values from an exception-based device, and that
- the device is checking values every 0,5 seconds,
- the connection to the device is good,
- the device sent an update 3 minutes ago with a value of 1,234.
In this case, the Server value would be 1,234 and the serverTimestamp would be updated every 0,5 seconds after the receipt of the value.