Time is evidence too.
Why every signature needs a clock that does not belong to the signer.
Almost every dispute over a signed document eventually comes down to a question about time. Was the certificate valid when the document was signed? Did the signature happen before or after the certificate was revoked? Was it signed within the deadline? Who signed first? Without a reliable answer to "when," many other answers are left hanging.
The signer's clock is not enough
Every digital signature can carry a declared date, taken from the clock of the device where it was produced. It is useful as a reference, but it proves nothing: anyone can set their computer's clock forward or back. A date that depends on one of the parties is a claim made by that party, not evidence.
That is why the timestamp exists: an independent third party, the timestamping authority (TSA), receives a cryptographic hash of the document or the signature, adds the time according to its own clock, and signs the whole. The protocol that standardized this was published in 2001 as RFC 3161 and remains the basis of almost every system in use today.
What exactly a timestamp proves
A timestamp proves that the content existed no later than the time indicated. It does not say when the content was created, who created it, or that it did not exist earlier. This precision matters: it is enough to show that a signature was produced before a certificate was revoked or before a deadline passed, which is precisely what tends to be in dispute.
- It does not reveal the content. The authority receives only a hash, not the document, so confidentiality is preserved.
- Anyone can verify it. All that is needed is the document, the timestamp and the authority's certificate.
- It becomes part of the signature. In advanced formats, the timestamp is embedded in the signature itself and travels with it.
Where the authority gets its time
A timestamp is only as good as the clock behind it. Reputable authorities synchronize their clocks with sources traceable to Coordinated Universal Time (UTC), maintained by national metrology laboratories, and document the accuracy they guarantee. European policies for timestamping authorities, such as the ETSI EN 319 421 standard, require that traceability and a declared accuracy margin. The eIDAS Regulation grants a qualified timestamp a presumption that the date and time it indicates are accurate.
The questions time answers
| Question in a dispute | What is needed to answer it |
|---|---|
| Was the certificate valid? | The time of signing, fixed by a third party, and the certificate's validity period. |
| Was it signed before revocation? | The timestamp and the revocation information from that moment. |
| Was the deadline met? | A timestamp on the signature, not on the sending. |
| In what order did the parties sign? | One timestamp per signature, or a chained and timestamped event log. |
What to do
- Timestamp every signature through an independent authority, not just the final document.
- Have more than one authority available and record which one was used, so that an outage neither halts the process nor goes unnoticed.
- Timestamp the event log as well, so that the complete sequence is fixed in time.
- Keep the timestamp inside the signed document, so that it can be verified without depending on any system.
The time declared by the signer proves nothing. A timestamp issued by a third party proves that the content existed no later than that moment, and that is enough to settle the most common disputes: validity, revocation, deadlines and order. Its value depends on the authority's clock and on the timestamp traveling with the signature.