<?php
  function wordpress_copyright() {
      global $wpdb;
      $post_datetimes = $wpdb->get_row($wpdb->prepare("SELECT YEAR(min(post_date_gmt)) AS firstyear, YEAR(max(post_date_gmt)) AS lastyear FROM $wpdb->posts WHERE post_date_gmt > 1970"));
      if ($post_datetimes) {
          $firstpost_year = $post_datetimes->firstyear;
          $lastpost_year = $post_datetimes->lastyear;
          $copyright = __('Copyright &copy; ') . $firstpost_year;
          if ($firstpost_year != $lastpost_year) {
              $copyright .= '-' . $lastpost_year;
          }
          $copyright .= ' ';
          echo $copyright;
      }
  }
?>