Request

HTTP If-Modified-Since Header

Makes the request conditional. The server responds with 304 Not Modified if the resource has not been modified since the specified date.

Purpose

Cache validation using timestamps. Tells the server to only return the full response if it has changed since the given date.

If-Modified-Since: <day-name>, <day> <month> <year> <hour>:<min>:<sec> GMT Example: If-Modified-Since: Mon, 18 May 2026 12:00:00 GMT

Code Examples

See how to use the If-Modified-Since header in different tools and languages.

curl -I -H 'If-Modified-Since: Mon, 18 May 2026 12:00:00 GMT' \
  https://api.example.com/resource
# Response: 304 Not Modified (if unchanged)

Related Headers

If-None-Match

Makes the request conditional. The server responds with 304 Not Modified if the resource's ETag matches the provided value. Otherwise returns the full resource with 200.

Last-Modified

Indicates the date and time when the resource was last modified. Used for cache validation with If-Modified-Since.

ETag

An identifier for a specific version of a resource. Used for cache validation and conditional requests to avoid re-downloading unchanged content.

Cache-Control

Specifies caching directives for both requests and responses. Controls how and for how long content is cached by browsers and intermediate caches.

Test Your APIs

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HTTP Headers Reference

Complete reference of all HTTP request and response headers

if-modified-since headerhttp if-modified-sincehttp headers guide