[campboy]

Maybe you could engrave the relative positions of some nearby stars onto a plaque or something like that? Ceramic lasts basically forever, right? Maybe you wouldn’t get “Tuesday, Deceduary 110th 2132” level of precision but you could perhaps get something pretty precise with the right choice of stars?

[jameshart]

We know what the positions of those stars were back into the ancient past and forwards into the distant future. We can engrave positions corresponding to any date we like onto such an object. It provides no proof of date.