I think it's in Vernor Vinge's "A Deepness in the Sky", space faring humans far in the future are still using the Unix Epoch in their time keeping code, but it'

I think it's in Vernor Vinge's "A Deepness in the Sky", space faring humans far in the future are still using the Unix Epoch in their time keeping code, but it's so ancient by then that the exact origins are lost and they are conflating it with the Moon Landing, a more concrete seminal event for such a civilization.