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If-Modified-Since

RequestActive

The If-Modified-Since header sends a date to the server on a conditional GET – the server returns 304 Not Modified if the resource hasn't changed since that date, or 200 with fresh content if it has. It is less precise than ETag/If-None-Match because it only has second-level granularity.

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

Description

If-Modified-Since performs time-based conditional validation. The client uses the Last-Modified date from a previous response. The server compares the request date to the resource's actual modification time. For resources that change multiple times per second, ETag is more reliable. If-Modified-Since is still widely used for static files.

Examples

Conditional GET
http
GET /assets/logo.png HTTP/1.1
If-Modified-Since: Thu, 01 Jan 2026 12:00:00 GMT
Not modified
http
HTTP/1.1 304 Not Modified
Last-Modified: Thu, 01 Jan 2026 12:00:00 GMT

Related

Last-ModifiedIf-None-Match304 Not Modified

Specification

RFC 9110 – If-Modified-Since specification →