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  1. HTTP (Hypertext Transfer Protocol)
  1. HTTP Headers
  1. If-Modified-Since
    1. Writing requests (clients)
    2. Reading requests (servers)
    3. Overview table
    4. Syntax
    5. History

If-Modified-Since

The "If-Modified-Since" header field in a request asks the server to only perform the requested operation of the state of the resource has changed since the client's last update. The equivelant header for ETags is If-None-Match; if available, use that instead.

Writing requests (clients)

Use If-Modified-Since when freshining a cache.

The If-None-Match header is more precise; if available, use that instead.

Reading requests (servers)

If the client also specified If-None-Match, ignore this header.

If the request method is not a valid HTTP-date, ignore this header.

If the request method is not GET or HEAD, ignore this header.

If the resource's Last-Modified date is the same or older than this header, return 304 (Not Modified).

Overview table

Name
If-Modified-Since
Description
Makes the request conditional on the resource being updated after the provided date.
Direction
Request
Specification
RFC 9110: HTTP Semantics §13.3.1. If-Modified-Since

Syntax

If-Modified-Since = HTTP-date

History

  1. 1999-06: RFC 2616 §14.24. If-Modified-Since
  2. 2014-06: RFC 7232 §3.3. If-Modified-Since. Conditional requests is split into a separate document during the update.
  3. 2022-06: RFC 9110 §13.3.1. If-Modified-Since. Conditional requests was moved back into core HTTP semantics.