For static documents, the server actually outputs the date of the last change. However, if the formation of your pages has a dynamic nature (for example, using PHP), then you should form this parameter yourself.
You can do this in the following ways:
In PHP – add this line to the script
header(“Last-Modified: “.gmdate(“D, d M Y H:i:s”).” GMT +0200″);
This code gives the date of the last change as the current date, which is generally true – searchers will feel that the document has literally just been changed.
Or, using the capabilities of .htaccess, write the following lines:
ExpiresByType text/html “access plus 3 days”
Header set Last-Modified “15 Jun 2004 12:00:00 GMT”