I like this solution because spammers are unlikely to try to get around it. A delay eats into their time budget and they can't introduce a human-like waiting time on every site they try to spam, better to just move on to find cheaper targets.
You could just decrease the timestamp instead of actually waiting.
I meant for general spammers who goes after tons of sites mostly blind. I agree it would not help for a targeted attack.
I'm already using this timestamp technique on my website and so far no bot operator has bothered trying to work around this. However even if some bot operator were to specifically target a website using this technique and try to decrease the timestamp, I believe you could still force a bot to wait by just changing the website to use something like a cryptographic nonce that includes a timestamp instead of just a simple timestamp that can be understood easily.