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 GMTAlso known as
Also commonly written as: if modified since · if-modified-since header · if modified since header · http if-modified-since · conditional get if-modified-since · If-Modified-Since ヘッダ. These variants — including plural forms, unhyphenated spellings, and Japanese (ヘッダ) — all refer to the same If-Modified-Since HTTP header.
Specification
RFC 9110 §13.1.3
Frequently asked questions
What is the If-Modified-Since HTTP header?
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.
Is If-Modified-Since a request or response header?
If-Modified-Since is a request header — the client sends it to the server. Used when a resource has Last-Modified but no ETag.
What does a If-Modified-Since header look like?
A typical If-Modified-Since header looks like: If-Modified-Since: Wed, 21 Oct 2025 07:28:00 GMT.