Timestamps cannot order access-control decisions in a decentralised system, because the user whose access was revoked can simply forge them.

Computer science researcher at the University of Cambridge…

if a revoked user still wants to, say, vandalize a document, they can just backdate their edits, give it an earlier timestamp. So, relying on clocks is absolutely useless here because people can forge the timestamps from those clocks and thereby then potentially undermine the access control mechanism.