# Last-Modified header

Category: [Technical SEO](https://seo-geo-glossary.com/en/glossary/category/technical-seo/) › [Servers, HTTP & caching](https://seo-geo-glossary.com/en/glossary/category/technical-seo/servers-http-caching/) · Also known as: If-Modified-Since

The **Last-Modified header** is an HTTP response header with the date and time the server believes a resource was last changed. The client can send that date back in an **If-Modified-Since** request header to ask whether anything changed.

## How do Last-Modified and If-Modified-Since work together?
The server sends `Last-Modified` in GMT, for example `Wed, 21 Oct 2015 07:28:00 GMT`. On the next request, the client adds `If-Modified-Since` with this value. If the resource has not changed, the server answers 304 Not Modified without a body. If it changed, the server sends 200 with the full content. This works for GET and HEAD requests only. When `If-None-Match` is present, it takes precedence.

MDN calls Last-Modified less precise than ETag, and RFC 9110 notes a granularity of one second. Google's crawling infrastructure supports both validators, and Google recommends ETag, in part to avoid date formatting problems.

## Example
```http
HTTP/1.1 200 OK
Last-Modified: Wed, 21 Oct 2015 07:28:00 GMT

GET /page/ HTTP/1.1
If-Modified-Since: Wed, 21 Oct 2015 07:28:00 GMT
```

## Common mistakes
The header shows the current time on every request. The client then never gets a 304.

## At a glance

- **Type:** HTTP response header
- **Affects:** conditional requests with If-Modified-Since
- **Values:** date in HTTP format, GMT
- **Docs:** [RFC 9110, Section 8.8.2](https://www.rfc-editor.org/rfc/rfc9110.html#name-last-modified)

Last checked: October 8, 2026

## Cache headers and 304 compared

| Header / status | Request counterpart | What it does | Relevance for Google's crawlers |
|---|---|---|---|
| Cache-Control | – (directives can also appear in requests) | lists directives for caches, e.g. max-age (fresh for N seconds), no-cache (reuse only after revalidation), no-store (do not store) | max-age is optional and helps crawlers decide when to recrawl the URL; Google does not support other caching directives |
| ETag | If-None-Match | identifier for the current version of a resource | supported and recommended by Google because it has no date formatting issues; takes precedence when both validators are present |
| Last-Modified | If-Modified-Since | date and time at which the server believes the resource was last modified | supported; the date must follow the HTTP format, e.g. Fri, 4 Sep 1998 19:15:56 GMT |
| 304 Not Modified | response to a conditional GET or HEAD request | signals that the stored copy is still valid; the response has no body | the crawler tells the next system that the content is the same as at the last crawl; Search may recalculate signals, otherwise no effect on indexing |

Sources: [Google Crawling Infrastructure: Overview of Google crawlers and fetchers, HTTP caching (last updated 2026-06-12)](https://developers.google.com/crawling/docs/crawlers-fetchers/overview-google-crawlers); [Google Search Central Blog: Crawling December: HTTP caching (2024-12-09)](https://developers.google.com/search/blog/2024/12/crawling-december-caching); [Google Crawling Infrastructure: HTTP status codes, network and DNS errors (last updated 2026-02-04)](https://developers.google.com/crawling/docs/troubleshooting/http-status-codes); [MDN: HTTP caching](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Caching); [RFC 9111: HTTP Caching](https://www.rfc-editor.org/rfc/rfc9111.html); [RFC 9110: HTTP Semantics (8.8.2 Last-Modified, 8.8.3 ETag, 15.4.5 304)](https://www.rfc-editor.org/rfc/rfc9110.html). Last checked: October 8, 2026

## Related terms

- [304 Not Modified](https://seo-geo-glossary.com/en/glossary/304-not-modified/index.md)
- [ETag](https://seo-geo-glossary.com/en/glossary/etag/index.md)
- [Cache-Control](https://seo-geo-glossary.com/en/glossary/cache-control/index.md)
- [HTTP header](https://seo-geo-glossary.com/en/glossary/http-header/index.md)

## Sources

- [Last-Modified header – MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Last-Modified)
- [If-Modified-Since header – MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/If-Modified-Since)
- [HTTP caching of Google's crawlers – Google Search Central](https://developers.google.com/search/docs/crawling-indexing/overview-google-crawlers)
- [RFC 9110, Section 8.8.2 Last-Modified](https://www.rfc-editor.org/rfc/rfc9110.html#name-last-modified)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/last-modified-header/