A self-reported timestamp proves nothing about itself
A trusted timestamp is a cryptographically verifiable assertion of when a specific piece of data existed, issued by a party independent of whoever actually created the data — rather than relying on a timestamp the creator generated themselves, which they could set to whatever value suited them at the time. Because the timestamping party is independent, the resulting proof carries weight a self-reported timestamp never could: it demonstrates the data existed at or before a specific point in time, verified by someone with no stake in the outcome either way.
This closes a gap that tamper-evident logging, on its own, doesn't address: a tamper-evident log protects against altering a record after the fact, but it has nothing to say about whether the timestamp on that record was accurate to begin with — a record could be perfectly unaltered from the moment it was created and still carry a timestamp that misrepresents when it was actually made.
Where proving 'before this date' actually matters
This distinction matters wherever proving 'this existed before that date' has real consequences — establishing that a document, a piece of evidence, or a signed record genuinely existed at a specific moment, independent of whatever the system that produced it might claim about its own history. Combining trusted timestamping with tamper-evident logging closes both gaps at once: the record can't be altered afterward, and its claimed creation time can't be misrepresented from the start either.