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function wpb_copyright() {
global $wpdb;
$copyright_dates = $wpdb->get_results("
SELECT
YEAR(min(post_date_gmt)) AS firstdate,
YEAR(max(post_date_gmt)) AS lastdate
FROM
$wpdb->posts
WHERE
post_status = 'publish'
");
$output = '';
if($copyright_dates) {
$copyright = "© " . $copyright_dates[0]->firstdate;
if($copyright_dates[0]->firstdate != $copyright_dates[0]->lastdate) {
$copyright .= '-' . $copyright_dates[0]->lastdate;
}
$output = $copyright;
}
return $output;
}

<?php echo wpb_copyright(); ?>