If you're super distrustful you could argue that you should never store a timestamp with a signup event, because it could potentially reveal a user's identity...
Here's a crazy thought, what if you would do this:
1. You fire off a default first event, say “init" On the server you generate a PGP key pair, store the private key with the init-event and return the public key
2. Second event (first real event) is fired by the website owner and encrypted with the PGP public key from 1
3. On the server you try decrypt event #2 with all available active private keys (stored with init-events)
4. Once a solution is found you link the 2nd event to the 1st event, delete the private key of the 1st event, generate a new PGP key pair, store private key with 2nd event, and return the new public key
5. Third event is encrypted with the public key of 4 and...
No need to store timestamps and all traffic is encrypted, now how to make step 3 fast?