[robga]

On the macro scale, would measuring sidereal time or pulsars meet your requirement? On the micro scale, atomic clocks?

[azatom]

No, I meant by a timestamp a thing, that assures other people, that another thing certainly does not happened after a specific time. There are authorities, whose in a simplified way: - I take crypto-hash of a massage, send to a timestamp authority - That authority digitally sign it with adding the current time.

So other people can check with the authority's public key, that is valid, so what I hashed, I cannot created after that time.

edit: so there is a problem with sunspots, it assures that it happened not before