If-Modified-Since

The HTTP If-Modified-Since request header makes the request conditional, asking the server to return the resource only if modified after the specified date and time.

Table of Contents

Usage

The If-Modified-Since header enables cache revalidation based on the modification date of a resource. The client includes the timestamp from the Last-Modified header received in a previous response. If the resource has not changed since the given date, the server returns a 304 Not Modified response with no body, saving bandwidth and processing time. If the resource has changed, the server sends a normal 200 response with the updated content.

The header applies only to GET and HEAD requests. For other methods, the companion If-Unmodified-Since header provides date-based conditional logic.

When both If-Modified-Since and If-None-Match appear in the same request, If-None-Match takes precedence. The ETag-based comparison is more reliable because timestamps have one-second resolution and clock skew between server and intermediate caches introduces edge cases.

The date value follows the HTTP-date format described in the Date header specification: day name, date, time in hours, minutes, and seconds, followed by GMT.

Note

Googlebot sends If-Modified-Since on crawl requests with the timestamp of the last content crawl. Servers returning 304 for unchanged content save processing time and resources, indirectly improving crawl efficiency. Servers are also free to return 304 proactively for any Googlebot request when content has not changed, regardless of whether the request includes conditional headers. Setting the max-age field of the Cache-Control response header to the expected number of seconds the content remains unchanged helps crawlers determine when to recrawl a URL.

Example

A client revalidates a cached resource. The server returns 304 Not Modified if the resource has not changed since June 1, 2022 at 08:00 GMT. Otherwise, the server sends the full updated resource.

If-Modified-Since: Wed, 01 Jun 2022 08:00:00 GMT

A conditional request paired with Last-Modified in the previous response. The client stores the modification date and sends the modification date on the next request to the same resource.

Last-Modified: Wed, 01 Jun 2022 08:00:00 GMT

On the next request:

If-Modified-Since: Wed, 01 Jun 2022 08:00:00 GMT

Takeaway

The If-Modified-Since header makes GET and HEAD requests conditional on the resource's modification date, enabling efficient cache revalidation through 304 responses when content has not changed.

Note

For SEO and caching assistance, contact ex-Google SEO consultants Search Brothers.

See also

Last updated: March 11, 2026