No need to spoof the GPS signal, pop open the lid and replace the GPS chip with one that outputs an attacker-controlled time signal. I suppose it could encrypt this signal with some certificate burned into the silicon, but that could still (slowly and painfully) be reverse-engineered.

GP’s point stands, you can’t enforce a time lock with cryptography primitives. That’s the fundamental issue with DRM - you’re trying to restrict someone from getting your ciphertext, while at the same time allowing them to get to it if they meet certain (non-cryptographic) conditions, which standard cryptography just can’t do.