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Last-Modified

ResponseActive

The Last-Modified header provides the date and time when the server last modified the resource. Clients use this for conditional requests via If-Modified-Since – if the resource hasn't changed, the server returns 304 Not Modified without a body. Less precise than ETag but supported everywhere.

Last-Modified: <day-name>, <day> <month> <year> <hour>:<minute>:<second> GMT

Description

Last-Modified enables time-based cache validation. The value is an HTTP date (RFC 1123 format). On subsequent requests, clients send the value back via If-Modified-Since. The server compares modification times and returns 304 if unchanged. Last-Modified is less reliable than ETag for sub-second changes.

Examples

Response with Last-Modified
http
HTTP/1.1 200 OK
Last-Modified: Thu, 01 Jan 2026 12:00:00 GMT
ETag: "33a64df5"
Conditional GET
http
GET /api/data HTTP/1.1
If-Modified-Since: Thu, 01 Jan 2026 12:00:00 GMT

Related

ETagCache-Control304 Not Modified

Specification

RFC 9110 – Last-Modified specification →