Setting expiry on static resources with htaccess

just got to know that setting expiry dates for static resources make your site go fast and jump in rankings.

My question is:

Does browser checks create date of the static resource?
If I make change to a css before 1 month, does server sends fresh copy and browser refreshes its cache?

This is the code I added to htaccess:

<IfModule mod_expires.c>
# Enable expirations
ExpiresActive On 
# Default directive
ExpiresDefault "access plus 1 month"
# My favicon
ExpiresByType image/x-icon "access plus 1 year"
# Images
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpg "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
# CSS
ExpiresByType text/css "access plus 1 month"
# Javascript
ExpiresByType application/javascript "access plus 1 year"
</IfModule>

Solution

From mod_expires documentation:

The expiration date can set to be relative to either the time the source file was last modified, or to the time of the client access.

Here is the syntax:

ExpiresByType type/encoding "base[plus num type] [num type] ..."

where base is one of:

access

now (equivalent to 'access')

modification

The plus keyword is optional. num should be an integer value [acceptable to atoi()], and type is one of:

years

months

weeks

days

hours

minutes

seconds

So if you write:

ExpiresByType image/gif "access plus 1 month"

Your file will be cached as long as visitors keep requesting it without interruption for more than a month, even if you modify it.

You should then write instead:

ExpiresByType image/gif "modification plus 1 month"

Then your file will be cached for a month as long as it is not modified, otherwise the cache will be updated.