What Does a File Timestamp Actually Prove?

Someone is using your work and claiming they made it first. You open the folder, point at the "Date modified" column, and realise it proves nothing: anyone can change that date in a few clicks, and the other side knows it. So you start searching for how to prove a file existed on a date, and you keep running into the word timestamp.

A file timestamp is strong evidence, but it proves something narrower than most people expect. Here is what a file timestamp actually proves, what it cannot, and the one habit that decides whether it holds up.

The date on your computer is not a timestamp

Every file carries dates: created, modified, sometimes "taken" for photos. They are written by your own machine, stored next to the file, and editable by anyone who has the file. As evidence, they are your word, formatted as a date.

An emailed copy is a little better, but the proof then depends on a mail provider's records and on someone trusting them.

What you want is a date that nobody can backdate, not you, not the tool that made it, not the person disputing it. That is what a cryptographic timestamp is designed to give you.

How a file timestamp works

It starts with a fingerprint. Your file is run through a hash function (SHA-256) that turns any file, whatever its size, into a 64-character string. Two properties make this useful:

• The same file always gives the same fingerprint. Copy it, rename it, move it to another computer: the content is identical, so the fingerprint is too.
• Any change gives a completely different fingerprint. Change one pixel or one comma and the result looks nothing like the original.

The fingerprint cannot be turned back into the file, so it reveals nothing about what the file contains. That fingerprint is what gets timestamped, not the file itself.

It is sent to OpenTimestamps, an open service that bundles many fingerprints and records their combined result in a Bitcoin transaction. The Bitcoin blockchain is a public ledger copied across thousands of machines, and nobody can insert an entry into its past. In return you get a small .ots file: the chain of calculations that links your fingerprint to one specific block.

Anchoring is not instant. The proof starts as pending and becomes fully confirmed once the transaction lands in a block, which can take anywhere from minutes to a few hours.

What it proves, and what it doesn't

When a verifier checks your file against its .ots proof and the blockchain, a successful result means one precise thing: this exact file, byte for byte, existed no later than the time of that Bitcoin block.

That is a strong claim a stranger can check without trusting you. It is also the whole claim. A timestamp does not prove:

• Who made the file. It records that a file existed, not whose it is. You could timestamp a stranger's photo just as easily as your own. Evidence of authorship comes from elsewhere: drafts, source files, contracts, signatures.
• When the file was created. It only sets a latest possible date. The work may be years older than its timestamp; the timestamp can only vouch for the day you stamped it.
• An exact time of day. Block times are only accurate to within a few hours. That settles "before the deadline" or "before March", not "before 3:14 pm".
• Anything about a different version. Stamp a draft, then edit it, and the proof belongs to the draft only.

So a timestamp is proof of prior existence. Alongside your other records it makes a date carry weight; on its own, it is not a certificate of ownership.

The illustrator whose design showed up in someone else's shop

An illustrator finds her fox pattern printed on tote bags in an online shop. She writes to the seller, who replies that they designed it last spring. Her source file is on her laptop, but its dates are the kind anyone can edit, and the argument turns into her word against theirs.

The one thing that breaks a timestamp

A timestamp is tied to the exact bytes you stamped, which is where most proofs quietly fail. Opening a PDF or a Word document and saving it again, even without visibly changing anything, often rewrites internal metadata. Re-exporting an image, adjusting it in a photo app, or letting a messaging app recompress it all produce a new file with a new fingerprint.

When that happens, the verifier does not report a later date. It reports a mismatch: the proof and the file no longer belong together.

The contractor whose proof stopped matching

A contractor timestamps a signed quote the day he sends it, in case the client later disputes the price. Two months on, the client does. He opens the quote, notices a typo in the address, fixes it, saves, and runs the file through a verifier with its .ots proof. The result is a mismatch, and his evidence is gone at the moment he needs it.

Stamping a file and checking it later

The practical habit is short: stamp the final version as soon as it is final, and keep that exact file untouched next to its .ots proof. If you revise the work, stamp the new version too. Each proof covers its own version.

MyTools does both halves for free. To timestamp a file and get its .ots proof, drop it into the page: the fingerprint is calculated in your browser, and only that 64-character string is sent out, never the file. To verify a timestamp against the blockchain, drop in the original file and its .ots together; the result is verified with the block date and height, still pending, or a mismatch. No account either way.

How the illustrator gets a date nobody can argue with

For the tote bags it is too late: a timestamp made today can only prove the pattern existed today. So she stamps her whole portfolio that evening, which puts a public date on every existing design, and from then on she stamps each new piece the day she finishes it. When her next pattern is copied, she sends the original file and its .ots proof, and the verifier shows a Bitcoin block months before the copy's listing.

How the contractor keeps the proof matching

The contractor now keeps two copies of every quote he sends: a working copy he is free to edit, and an archive copy he never opens, stored next to its .ots proof. A revised quote gets stamped as a new file. The next time a client questions a price, the archive copy verifies first time, with a block date from the week he sent it.

The short version

A file timestamp proves that one exact file existed no later than a public, tamper-proof date. It does not prove who made it, and it stops matching the moment the file changes. Stamp the final version early, keep it untouched beside its proof, and the date will hold.

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