The Cloudflare article from 2018: "Roughtime: Securing Time with Digital Signatures" [1] presented a very comprehensive article but it seems that Google and Cloudflare Roughtime servers don't work currently. Do you know of a list of available servers, mainly from well known organizations? I just found https://roughtime.se/ but have not tested it yet.There is an old question "Roughtime servers list?" from 2016 about this in the mailing list [2].
[1] https://blog.cloudflare.com/roughtime/
[2] https://groups.google.com/a/chromium.org/g/proto-roughtime/c/EtBGl4BUQn4