I'm so sad roughtime never got popular. It can be used to piggyback a "proof of known hash at time" mechanism, without blockchain waste.

https://www.imperialviolet.org/2016/09/19/roughtime.html

https://int08h.com/post/to-catch-a-lying-timeserver/

https://blog.cloudflare.com/roughtime/

https://news.ycombinator.com/item?id=12599705