Conditional tags FTW。
// Check what posts were returned in order to find what date the archive is for
global $posts;
$archivedate = strtotime( $posts[0]->post_date);
if (is_day()) {
echo "Daily archive: ".date(\'m/d/Y\', $archivedate);
} else if (is_month()) {
echo "Monthly archive: ".date(\'F Y\', $archivedate);
} else if (is_year()) {
echo "Yearly archive: ".date(\'Y\', $archivedate);
}