If it's just using Google search "before <x date>" filtering I don't think there's a way to game it... but I guess that depends on whether Google uses the date that it indexed a page versus the date that a page itself declares.
None of these documents were actually published on the web by then, incl., a Watergate PDF bearing date of Nov 21, 1974 - almost 20 years before PDF format got released. Of course, WWW itself started in 1991.
Google Search's date filter is useful for finding documents about historical topics, but unreliable for proving when information actually became publicly available online.
https://www.google.com/search?q=site%3Achatgpt.com&tbs=cdr%3...
So it looks like Google uses inferred dates over its own indexing timestamps, even for recently crawled pages from domains that didn't exist during the claimed date range.
I wonder why they do that when they could use time of first indexing instead.