function copyrightDate() {
	global $wpdb;
	$copyright_dates = $wpdb->get_results("
		SELECT
			YEAR(min(post_date_gmt)) AS firstdate,
			YEAR(max(post_date_gmt)) AS lastdate
		FROM
			$wpdb->posts
		WHERE post_status = 'publish'
	");
	if($copyright_dates) {
		$date = date('Y-m-d');
		$date = explode('-', $date);
		$copyright = "Copyright © " . $copyright_dates[0]->firstdate;
		if($copyright_dates[0]->firstdate != $date[0]) {
			$copyright .= '-' . $date[0];
		}
		echo $copyright;
	}
}

<?php copyrightDate();?>