Memento-Datetime

When a web archive returns a stored snapshot, the Memento-Datetime response header indicates the exact datetime when the resource was captured.

Table of Contents

Usage

Web archives like the Wayback Machine, national libraries, and other preservation services store snapshots of web pages over time. The Memento-Datetime header appears in responses from these archives to communicate exactly when a particular snapshot was recorded. This is part of the Memento framework, which provides time-based access to prior versions of web resources.

When a client requests an archived version of a page, the archive server returns the stored snapshot along with Memento-Datetime set to the capture timestamp. This allows clients and tools to confirm which point in time the snapshot represents. The value follows the same HTTP-date format used by Date and Last-Modified, making parsing consistent with existing HTTP date handling.

The Memento framework introduces several related concepts: an original resource (the live page), a memento (an archived snapshot), a TimeGate (a resource negotiating by date), and a TimeMap (a listing of available mementos). The Memento-Datetime header identifies a response as a memento and pins the snapshot to a specific moment.

Values

The value is a single HTTP-date in the IMF-fixdate format, identical to the format used by the Date header.

Memento-Datetime: <day-name>, <day> <month> <year>
<hour>:<minute>:<second> GMT

Example

A response from the Wayback Machine for a page captured on September 29, 2010. The Memento-Datetime value records the exact moment the archival crawl stored this snapshot.

Memento-Datetime: Wed, 29 Sep 2010 08:58:15 GMT

A more recent capture from 2025 shows the same format. The timestamp confirms this is a snapshot taken during a May 2025 crawl of the target site.

Memento-Datetime: Fri, 09 May 2025 23:41:44 GMT

Archives often return Memento-Datetime alongside a Vary header indicating content negotiation by Accept-Datetime, and a Link header pointing to the TimeMap and original resource.

Memento-Datetime: Thu, 27 Dec 2012 19:25:48 GMT
Vary: Accept-Datetime
Link: <https://web.archive.org/web/timemap/link/
http://example.re/>; rel="timemap"

See also

Last updated: April 4, 2026