$date_a =~ s!(\d{2})/(\d{2})/(9\d)!$3$1$2!;
$date_b =~ s!(\d{2})/(\d{2})/(9\d)!$3$1$2!;
$date_a =~ s!(\d{2})/(\d{2})/(0\d)!1$3$1$2!;
$date_b =~ s!(\d{2})/(\d{2})/(0\d)!1$3$1$2!;