timelock is a timed release encryption utility that encrypts files so
that they can only be opened in the future.

Cryptography has no notion of time. So, this is done by creating a
computationally difficult problem (known as a timelock puzzle) whose
solution is the secret key that was used to encrypt the message.

http://theory.lcs.mit.edu/~rivest/RivestShamirWagner-timelock.ps