In 1999, for the 35th birthday celebration of the MIT Laboratory for Computer Science, MIT dedicated a time capsule to be opened on the 70th anniversary of the lab … “or upon solution of a cryptographic puzzle … designed to take approximately 35 years to solve.”
Cool, right? A time-lock is basically a cipher that can only be de-ciphered on one processor (i.e. not in parallel) and which is easy to encrypt and orders of magnitude more difficult to decrypt. For my use, it takes me only a second to time-lock a document where the inverse decryption takes at least a day.