Googlebot first visits a page and records the access time, then on subsequent visits it checks the If-Modified-Since header to see if the page has changed since its last visit. If unchanged, Googlebot receives a 304 status code and knows it doesn't need to reload the content, but if changed it will load the page to get the updates. Status codes are logged to help sites understand if and when content is being recrawled by Googlebot after changes.

1. If-Modified-Since HTTP header

3. 200 status code
Google bot first visits your page.
It will see a 200 status code which means the content loaded fine.
Googlebot will take a note of when it accessed your page and then the next time it
comes to the same page something new happens.

4. 304 status code
• If the content has not changed since Googlebot last visited, it will receive a 304
status code. If it does not receive this status code Googlebot determines that
something has changed and will access your page again to get the changed content.
• Status codes are listed in your log files, and you will typically see then in your
statistics report.

5. header("Last-Modified: " .the_modified_date());
http://stackoverflow.com/questions/27589282/i
f-modified-since-header-in-wordpress