If-Modified-Since Explained

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What Is If-Modified-Since? … If-Modified-Since (aka “the If-Modified-Since HTTP header”) explained.

(This is an SEO-relevant question asked by web developers who want search engines to understand and rank their webpages.)

“304 status code” or no “304”?

Every so often, the Googlebot spider will visit your website. If you have updated your page since last time it was there, you will want the spider to re-index the page, not just pass it over and think the same material is there as last time.

It takes a lot of time and energy to crawl through an entire website and index all those pages. Googlebot wants you to tell it if you have changed each page or if there is no change.

304 is a signal by which a web page tells a crawler that it has not changed. If there is no 304 signal, the crawler will crawl the page and see what is there.

So you put an “If-Modified-Since HTTP header” on each page. If the page has modified since last time, re-index it, please, you are saying to the spider. If the page has not changed, you can keep your old records in the Google Seach Engine.

How to do the If-Modified-Since header:

– Use the If-Modified-Since Tool (… not right now thought. This tool is down.)

– Communicate with your server. Ask them, “Does my web server (the server on which my sites are hosted) support the If Modified Since header?

– You can also install a plugin that does adds the If-Modified-Since HTTP header to your website. Some of these are buggy or not updated, but at least (as long as it works) you can activate it, then run a test to check (HTTP Header Checker by ForBrains.co.uk) if it’s doing “Last-Modified”, and if it is, you can see If-Modified-Since is supported. (However, you may want to code your own code for If-Modified-Since for your website and avoid a plugin, because plugins can cause problems and slow down websites.)