The Google crew cleared up a bit about how the Googlebot caches webpages. The date at the cached page reflects the last time the page was modified.
Vanessa Fox explains:
When you look at Google’s cache of a page (for instance, by using the cache: operator or clicking the Cached link under a URL in the search results), you can see the date that Googlebot retrieved that page. Previously, the date we listed for the page’s cache was the date that we last successfully fetched the content of the page. This meant that even if we visited a page very recently, the cache date might be quite a bit older if the page hadn’t changed since the previous visit. This made it difficult for webmasters to use the cache date we display to determine Googlebot’s most recent visit. Consider the following example:
1. Googlebot crawls a page on April 12, 2006.
2. Our cached version of that page notes that “This is G o o g l e’s cache of http://www.example.com/ as retrieved on April 12, 2006 20:02:06 GMT.”
3. Periodically, Googlebot checks to see if that page has changed, and each time, receives a Not-Modified response. For instance, on August 27, 2006, Googlebot checks the page, receives a Not-Modified response, and therefore, doesn’t download the contents of the page.
4. On August 28, 2006, our cached version of the page still shows the April 12, 2006 date — the date we last downloaded the page’s contents, even though Googlebot last visited the day before.
And just in case that wasn’t clear enough, Matt Cutts explains via Google Video – with Skittles.
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