I want to use a version element for optimistic locking, and a timestamp element to be automatic updated every time the entity is updated.
How to do it? How to get a auto-updated timestamp property but not use it for optimistic locking?

In the dtd this is not allowed. Why don't you want to use your timestamp for optimistic locking if you need a timestamp ?
Alternatively you can use a version for optimistic locking and manage your timestamp manually

Yes, I think you had telled me all the truth, but since timestamps are not unique, it's better to use version for optimistic locking.
And yes I can do any job manually, but I hope hibernate can update timestamp for me.