When did these files exist?
Every build of this site, with the Lean files behind its proofs, is fingerprinted and the fingerprint is written into Bitcoin. Nobody can change a file afterward without the fingerprint showing it. You can check it yourself, without trusting us.
I.The two files
manifest.txt: one line per file, its sha256 fingerprint and its path.
manifest.txt.ots: the OpenTimestamps proof that the list existed by a Bitcoin block.
II.Check it in three steps
Download both files into one folder.
Install the free OpenTimestamps client: pip install opentimestamps-client.
Or, with no install, drop the .ots file on the OpenTimestamps website (opentimestamps.org).
Run ots verify manifest.txt.ots. It names the Bitcoin block and its time.
A new build can say "pending" for a few hours, until Bitcoin confirms it.
What this proves: the list, and so every file named in it, existed no later than that block. What it does not prove: who wrote a file, or that a file is right. Being right is what the labels and the checkers are for.