If-Modified-Since

Date-based cache revalidation avoids downloading unchanged resources. The HTTP If-Modified-Since request header makes a GET or HEAD 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.

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

Crawlers and If-Modified-Since

Googlebot sends If-Modified-Since when recrawling a URL previously served with Last-Modified, and answering 304 on unchanged content conserves crawl budget.

Google supports the mechanism while recommending ETag and If-None-Match instead, since dates carry more room for error. Timestamps regenerated on every request defeat the exchange entirely, because the resource appears modified on each visit and the crawler receives a full body every time. Setting the max-age field of the Cache-Control response header to the seconds the content stays unchanged additionally helps crawlers decide when to recrawl a URL.

Verified crawler probe

Probe data from verified crawler traffic (September 2026 snapshot) shows which bots send If-Modified-Since when requesting pages. Yes marks the header on at least nine of ten sampled requests from the bot, Sometimes marks a smaller share, and No marks absence.

Crawler Sends If-Modified-Since
Amazonbot No
Applebot No
Baiduspider No
Bingbot No
ClaudeBot No
DuckDuckBot No
GPTBot No
Googlebot No
Googlebot (smartphone) Sometimes
Googlebot-Image No
Meta-ExternalAgent No
Meta-WebIndexer No
OAI-SearchBot No
SeznamBot Sometimes
YandexBot No
YandexFavicons No

See also

Last updated: September 21, 2026