For setting last modified date for ur site.
Make sure your site apache or any other web server supports mod_expires, cache control

and include below in your htaccess


# Expire all files one week after they're accessed

ExpiresActive On
ExpiresDefault "access plus 1 week"
ExpiresByType image/x-icon A2419200
ExpiresByType image/gif A604800
ExpiresByType image/png A604800
ExpiresByType image/jpeg A604800
ExpiresByType text/css A604800
ExpiresByType application/x-javascript A604800
ExpiresByType text/plain A604800
ExpiresByType application/x-shockwave-flash A604800
ExpiresByType application/pdf A604800
ExpiresByType text/html A900
### Apply a Cache-Control header to index.html

Header append Cache-Control "public, must-revalidate"


 
    Header append Vary Accept-Encoding
 

  AddType application/x-httpd-php5 .htm


# below lines are used for content-type header setting for your
AddType 'text/html; charset=UTF-8' html
AddType 'text/html; charset=UTF-8' htm