Learn in the textbookApplying Encryption and Digital Signatures
DefinitionA timestamp (タイムスタンプ) is proof that electronic data existed at a certain time and has not been rewritten since.
ExplanationWith a timestamp on an electronic contract or similar document, no one can later backdate it or deny that it existed at that time. The reliability of the time itself rests on time authentication (時刻認証), covered next.
AnalogyIt is like a postmark. The post office, not the sender, stamps the date, so it cannot be fudged afterward.