Conditional · Request header

If-Modified-Since

Send the resource only if it changed after this date.

What is If-Modified-Since?

If-Modified-Since is the older, date-based conditional GET. The server compares this date to the resource's Last-Modified and either returns fresh content or 304 Not Modified.

Typical usage

Used when a resource has Last-Modified but no ETag.

Examples

If-Modified-Since: Wed, 21 Oct 2025 07:28:00 GMT

Specification

RFC 9110 §13.1.3

Related headers

Last-Modified

When the resource was last modified.

If-None-Match

Send the resource only if its ETag has changed.