No idea, but I was going to say that with Timestamps, it is possible that two records are added at the exact same moment, therefore not unique. But dates doesn't make sense unless they have a higher precision that can guarantee uniqueness.
Perhaps I missed something in the spec, but I see nothing in JPA disallowing having a timestamp as a primary key or part of one. I'm pretty sure it is supported in the JPA reference impl.
In general you should be careful in doing so, as different databases support different precision of timestamp values (seconds, milliseconds, microsecond). Normally a generated sequential id is a much better way to go.