Time

NASA's Pioneer 10 and 11 spacecraft carry metal plaques showing the spacecraft's time and place of origin.
--Kimm Groshong / New Scientist

I wonder how they showed time. Probably based upon the position of various celestial objects, but I found myself wondering if they could've included a radioactive sample and used half-lives. But without a common definition of "second," it could be difficult. I guess we'd have to go entirely by quantity of sample, not rate of emission, but again we'd have to somehow communicate the initial quantity of sample, a daunting task.

My favorite way would be to include some 3-10 radiactive samples with varying length half lives. These samples would all have their density of radioactive isotopes very carefully controlled, so that they all had the same percent of the radioactive material when the device was created (roughly). What I would expect to happen is that any sufficiently advanced civilization finding the samples would study each and determine their halflives (if not already known in detail). Some clever soul will put two and two together and realize that when they plotted the decay back in time, all the graphs intersected at one point: the date that the ship was launched.

This of course assumes that any civilization advanced enough to retrieve a spaceship also lives in a region of the galaxy (universe?) metal-rich enough to have observed radioactive materials previously. We'd also have to give small enough samples that they wouldn't be harmful to an ingorant culture, but strong enough that they're likely (for some values of "likely") to observe that they're radioactive.