Well why couldn't it? If you used a regular unix timestamp or even mysql's datetime, which you could then translate into the timestamp, as your dynamic hash and had it stored in a field which was called say, "registered_on", how do they know you're using that as a salt? It just looks like you're recording the timestamp of when they signed up.

Security isn't just locking the door, sometimes it's making the door hard to find.
-m