Blockchain timestamps prove a file existed by a date, but not when it was created

A timestamp anchored on-chain shows that a specific file existed no later than the block containing its hash, but it does not reveal when the file was actually made. Because the file could have existed long before anyone recorded its hash, the timestamp functions only as an upper bound. This matters a great deal for proofs that a document was produced after a certain point, since someone could prepare several versions in advance and anchor only the one that turns out to be correct. The article suggests embedding an unpredictable value, such as a recent Bitcoin block hash, inside the file before hashing it to establish a lower bound as well. Placement and unpredictability of that value are essential, since it must be part of the hashed bytes and cannot be a value anyone could guess in advance.