What are some options for a timestamp, like a pattern of sunpots, not a human related thing like dedicated timestamp authorities?
(Even chatgpt won't give me any idea, just forces the use of authorities, even after a long converstation said to make photos of newspaper, after my example was the daily titles of 20 bigest journals.
On the macro scale, would measuring sidereal time or pulsars meet your requirement? On the micro scale, atomic clocks?