How does a ProofMark indicate data integrity?

We define "data integrity" to mean that a given piece of data has not changed over a specified period of time. A ProofMark proves the data integrity of a record by comparing the message digest on the original proofmarked record with a new digest created on the spot, at the time the verification is requested. If the ProofMark can be decrypted by the public key of its purported time interval and the digests are identical, then the current record has not changed since it was originally proofmarked.