# Last-Modified header > The date and time a resource was last changed, used for cache validation. Source: https://apiguide.dev/headers/last-modified Direction: response Registered: yes Defined in: RFC 9110, Section 8.8.2 Updated: 2026-10-14 ## What is the Last-Modified header? The `Last-Modified` response header, defined in RFC 9110, carries an HTTP-date indicating when the origin server believes the resource was last modified. Clients store it and use it to ask whether their cached copy is still current. ## API Usage & Best Practices * **Conditional requests**: Clients send the stored value back in `If-Modified-Since`; if the resource has not changed, the server replies with `304 Not Modified` and no body, saving bandwidth. * **Guarding writes**: Sending the value in [`If-Unmodified-Since`](/headers/if-unmodified-since) lets the server reject a request with `412 Precondition Failed` if the resource changed in the meantime. * **Weaker than ETag**: Its one-second resolution can miss rapid updates, so `ETag` is preferred when available and takes priority when both are present. ## Examples ```http HTTP/1.1 200 OK Last-Modified: Wed, 08 Jul 2026 10:15:00 GMT ```