Last-Modified and If-Modified-Since checkChecks the server response headers for Last-Modified and the server response to a request with an If-Modified-Since header.
Checking Last-Modified and If-Modified-Since is one of the integral parts in website promotion, it is required to understand whether the server response headers are correctly formed.
Last-Modified is one of the server response headers that tells the date when the page was last modified, it helps to estimate how long ago the content changed on it.
If-Modified-Since is a header sent to the server. With the help of it, the date is transmitted to understand if the page has changed after this date. If the content has changed, then in response the server will return the page in the same way as with a normal request, with a response code of 200. But if there were no changes on the page, the server will return an empty page with the 304 Not Modified header.
If-Modified-Since is used by browsers to save bandwidth. If, for example, the requested page is present in the browser cache, then it will create a request with the If-Modified-Since header. Further, if a 304 Not Modified response is returned, then the browser will show this page from its cache.
Correct configuration of the server response to the If-Modified-Since header also has a positive effect on interaction with search engines, which save time on downloading an unmodified page and use it to process the next pages of the site.