# http section

map $http_if_modified_since $last_modified {
    "~*.+" $http_if_modified_since;
    default $date_gmt;
}

map $http_if_modified_since $response304 {
    "~*.+" 1;
}

# =========================================

# server section

if ($response304){
    return 304 "Not modified";
}

# location
add_header Last-Modified $last_modified;