The Voyager Golden Record carries a small sample of uranium on its cover, placed there so that whoever finds it can measure the decay and work out how long it has been drifting — a built-in clock for a message engineered to last around a billion years.

I love the ingenuity shown here. Scientists came up with a means to let any life form to have a simple means to understand intelligent life sent this craft, and included a means to understand how long it had been sent. 

https://spacedaily.com/t-the-voyager-golden-record-carries-a-small-sample-of-uranium-on-its-cover-placed-there-so-that-whoever-finds-it-can-measure-the-decay-and-work-out-how-long-it-has-been-drifting-a-built-in-cl/