There isn't such a service. You have to use a dedicated item and/or field and cast the timestamp in the Data Adapter as a field value.

In order to ensure that the timestamp is taken when the Server receives the subscription request, you should use a different item for each subscription, so that the client subscription triggers a subscribe invocation on the Data Adapter. This can be achieved easily, by requesting the same item name on all clients, but then decorating the name in a unique way by the Metadata Adapter upon each invocation of getItems().

An alternative technique, in which a single dedicated item is needed, involves the use of "customizers", which are only available for in-process Java Adapters, though. We can expand on this, if needed.