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.
