I wonder if there's a way to implement this without storing the code with a central authority, e.g. by encrypting the code so that it can only be decrypted in X years. You'd probably still have to have a central authority involved to ensure people can't just fast-forward - but a system similar to TOTP codes could be a neat mechanism!
I don't think we have any way to do that. Time is abstract for algorithms. Unless you make something you know you couldn't solve in less than x years. But that assumes you can predict improvements in algorithms and computing power over a long period which could be tricky to get precisely.
You use reflective solar bodies X/2 light years away and blast them with highly redundant encrypted data such that in X years Earth will be on the receiving end of the reflected transmission.
