Source Code : Adding timestamps to Drupal stylesheets to ensure an up to date browser cache

You need to edit /includes/common.inc and add the following text to the drupal_get_css() function.

function drupal_get_css($css = NULL) {
    $output = '';
    if (!isset($css)) {
      $css = drupal_add_css();
    }
      $preprocess_css = variable_get('preprocess_css', FALSE);
    $directory = file_directory_path();
    $is_writable = is_dir($directory) && is_writable($directory)
       && (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC)
       == FILE_DOWNLOADS_PUBLIC);  

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Courtesy:- webcoda.co.uk