Document robustness of Keybase timestamps under various threat models

**Context:** There are a variety of models, standards and services for secure / verifiable / trusted timestamp protocols such as
* [RFC 3161 \- Internet X\.509 Public Key Infrastructure Time\-Stamp Protocol \(TSP\)](https://tools.ietf.org/html/rfc3161)
* [PGP Digital Timestamping Service](http://www.itconsult.co.uk/stamper.htm)
* [OriginStamp](https://originstamp.org/home)
* Various blockchains
* Etc....

But Keybase also provides visible timestamps on chats, and timestamps other content in a variety of ways.

So how does Keybase stack up as a trusted timestamp service? I've searched the documentation that seemed relevant and see no discussion of this.

* What guarantees are there on the validity of timestamps? Are they generated based on clocks on on local devices, or checked for being in sequence with nearby chat messages, or ??
* Are there ways for users to subvert the timestamps of their own content, and pre- or post-date it? (E.g. deleting a public object such that Keybase or archived data can't be used to document its data and existence in the future, or modifying a timestamp?)
* At what granularity might someone be able to prove a given timestamp?
* How can timestamps on kbfs files, proofs, signatures and other content be conveniently uncovered by users, and related guarantees on validity, granularity etc be communicated?
* What denial of service attacks are there? Can I publish something in a way that a timestamp on it is guaranteed to be verifiable, even if the Keybase server goes away some day or tries to violate any of these guarantees?

**Issue:** Can you answer these sorts of questions in the Keybase documentation, or point to existing answers?

**Reference use case:** I've been looking for public, easy-to-verify timestamps for a long time, and posted this example use case focused on publication and auditing of election results in 2010:
* [What good, standard digital signature / timestamp verification clients are widely or easily deployed? \- Information Security Stack Exchange](https://security.stackexchange.com/questions/1270/what-good-standard-digital-signature-verification-clients-are-widely-or-easily)

Start by reviewing the existing Keybase documentation for chats, KBFS files, proofs, and signatures, then compare its timestamp behavior with RFC 3161 and the other referenced services. Document the threat models, validity and granularity guarantees, user verification methods, and denial-of-service limitations, with clear answers or pointers for each question.

Written by the indexing model from the issue text.