Overview

Last-Modified is a response header telling the client when the server believes the resource last changed. The client stores this timestamp and uses it in conditional requests to reuse its cache if nothing changed since.

The value is an HTTP-date in GMT with one-second resolution.

Details

Cache revalidation: the client sends it back as If-Modified-Since: <date>; if the resource hasn't changed since that time the server replies 304 Not Modified (no body), otherwise 200 with a fresh body. It is mainly used with GET and HEAD.

Conditional modification: If-Unmodified-Since: <date> permits an update or delete only if the resource hasn't changed since that time, otherwise rejecting with 412 Precondition Failed to prevent concurrent-edit conflicts.

Because Last-Modified is second-granular, resources that change several times within a second can slip through, and rewriting a file with unchanged content still bumps the time. These limits make ETag more reliable for precise validation; provide both when you can.

Syntax

Last-Modified: <HTTP-date>

e.g. Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

Notes

• When the server sends both ETag and Last-Modified, the client can include If-None-Match and If-Modified-Since together, and the server evaluates the ETag first.
• A Last-Modified in the future or later than the Date header may not be trusted. The server clock must be accurate.
• Dynamically generated responses often lack a stable modification time, making Last-Modified unsuitable; a content-hash ETag is better there.

Related headers

Related status codes

Specification