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Last Modifed Date
See how to use Last Modifed Date
The last modified tool simply checks the HTTP header that is returned by each URL. One of the header fields that is returned is the last-modified time.
The time returned in the header is not always accurate. CMSes and caching techniques often cause the last-modified time to show as the current time.
Websites that are built with static HTML files generally do display the correct time that the file was last modified.
Here is the w3c.org listing on the last-modified header field from RFC-2616 which defines the rules on how it operates