How to prove the date a document was signed

Why a typed date proves little

The date next to a signature is usually whatever someone typed or wrote there. On paper, anyone can write last month's date. In a PDF, a date field holds whatever text was entered, and a file's own dates (its created and modified times, and the metadata inside it) can be changed with ordinary software. Emails help, but they show when a message moved, not when a signature was made.

So when the date matters, for a deadline, an offer that expired, a price that changed or a policy that took effect, a typed date leaves the question open: who recorded this time, and could anyone have changed it since?

What makes a signing time strong

A date holds up when it has four qualities:

1. Independent. It was recorded by something other than the person signing.
2. Bound to the document. It is tied to that exact document, so it can't be moved to another one.
3. Fixed in public. It is recorded where the signing service can't change it later.
4. Checkable by the other side. The person questioning it can test it themselves.

The law recognizes this kind of evidence. Under the EU's eIDAS Regulation, an electronic time stamp, "data in electronic form which binds other data in electronic form to a particular time establishing evidence that the latter data existed at that time" (Article 3(33)), can't be refused as evidence just because it is electronic or not qualified, and only a qualified one, from a qualified trust service provider, is presumed accurate (Articles 41(1) and 41(2)). The guide to blockchain anchors and the law covers the UK, Vermont, Mexico and other places. This page is general information, not legal advice.

The three times Attest records for every signature

A fingerprint (a SHA-256 hash) is a short code calculated from a file's contents: change one character and it changes completely. A passkey is a cryptographic key only the signer controls, unlocked with Face ID, Touch ID, Windows Hello or the device PIN.

1. The signing time, sealed by the passkey. When a signer is ready, Attest's server issues the statement they will sign, with the time, valid for 5 minutes. The statement names the document's fingerprint, the fingerprint of the signer's entries, the signer and that time, and the signer's passkey signs it. Change the time afterward and the signature no longer verifies.
2. The signature's own anchor. Each signature becomes a receipt in the signer's own account, anchored on Horizen, a public blockchain, from the signer's own wallet (a blockchain account Attest creates and runs for each user). When the signer is on the Free plan, that anchor is written within about 10 minutes; on Signing, Pro and organization plans, right away.
3. The completion anchors. When the last signer signs, the signed state, which lists every signer with their signing time and the fingerprint of the statement they signed, is anchored on Horizen, and so is Attest's record of the agreement, which commits to every signer's statement and signature. Their block times prove that every signature existed no later than those moments.

The anchors are what make the time hard to dispute. Attest's registry contract on Horizen records when each value was anchored and has no function to change or remove that record, so no transaction, from Attest or anyone else, can move an anchor's time afterward. Only fingerprints go on chain, never the document, its title or anyone's email.

Where each time appears

Where	What it shows
The signature stamp	The agreement's verification code (ATT-XXXX-XXXX), the signing day in UTC and a QR code to the certificate
The certificate of completion	The completion time in UTC, each signer's name and signing time, and the Horizen anchor transaction
The audit trail page	Every step up to completion, in order, in UTC: created, sent, opened, key set up, released, signed, reminded and completed
The online certificate	Each signer's signing time, the blockchain anchor and each signature's own anchor, checked in the visitor's browser
The verifier	Attest's anchored record of the agreement on Horizen, read from a public node

Time zones and the Date signed field

Attest's evidence uses UTC. The Date signed field, which fills in by itself when the signer taps it, uses the date on the signer's phone or computer, in their time zone. A signer in Los Angeles who signs at 6 p.m. on March 3 sees March 3 in the field, while the stamp shows March 4, because it is already 02:00 on March 4 in UTC. Both are right. When a date is ever questioned, the time that counts is the one in the signed statement, which Attest's server set and the anchors back.

Effective date and signing date

Many agreements take effect on a date other than the day they are signed. Write that effective date in the document itself ("effective as of September 1"). The evidence will still show when each person actually signed, which keeps both facts honest.

Step by step: prove when your document was signed

1. Open the certificate. Scan a stamp's QR code to open the agreement's certificate, or type its code (ATT-...) at attest.datahubz.com/verify to look up the same record. The certificate shows each signer's name and signing time and the anchor, and re-checks every passkey signature in your browser. If the certificate is private, only the parties can open it.
2. Drop the signed PDF on the verifier attached inside it (attest-verifier.html, in the PDF's attachments). It checks the original document inside against what every signer signed, each signer's passkey signature and entries, Attest's anchored record of the agreement on Horizen, and that nothing was added to the PDF after it was sealed. It runs on your device, reading only a public blockchain node.
3. Show the bounds. Each signing time is sealed into its signature, and each anchor's block time is a moment by which that signature already existed.
4. If you can't share the document, a party who signed can share a zero-knowledge proof that they signed it and the date it was completed, without the document. See proving a signing date to an auditor.

When a rule names a method

Attest's time evidence is its server's time, signed by each passkey, plus Horizen's block time. Where a rule names a specific method, use that method: HM Land Registry, for example, registers a document that takes effect as a deed under section 91 of the Land Registration Act 2002 only if every signature is a qualified electronic signature with embedded timestamps and long-term validation (Notice 2 under rule 54C, paragraph 8).

Electronic signatures are recognized for most agreements in the United States (the ESIGN Act and UETA) and the European Union (eIDAS). Some documents, such as wills or certain notarized deeds, have special rules in many places: check yours.