The spec currently says the source timestamp should be the last time the value changed. This implies that a UA server should maintain a seperate timestamp for each readable attribute for every node. This is a lot of extra data to store for limited use.

The spec needs enumerate four situations.

If the underlying system provides a timestamp then the server should use this timestamp.

If the server is polling the data and has a cached value then the source timestamp should be the last time the cache was updated.

If the data is exception based and the server is caching the value whenever it changes then the source timestamp should be the time when the value changed.

If the server is not polling and does not know when the value changed then the source timestamp should be the same as the server timestamp.

Source timestamps are intended primarily for use with the value attributes of Varables. When the server is returning the source timestamp for other attributes then it is up to server to decide if it wants to mainatain seperate timestamps for each attribute. If the server does not maintain seperate timestamps then the source timestamp should be the same as the server timestamp.