use strict;
use warnings;

my $n = 0;
my @YMDAY = map $n += $_, qw<0 0 31 28 31 30 31 30 31 31 30 31 30 31>;

my $today = days(2013, 10, 3);
my $bday  = days(1970, 3, 9);

printf "You are %d days old.\n", $today - $bday;

sub days {
    my ($year, $month, $day) = @_;
    my $days = $year * 365 + $YMDAY[$month] + $day - 1;
    $days-- if $month <= 2
           and !($year % 4) && $year % 100 || !($year % 400);
    $year++, $days-- if $year < 0;
    $days + int($year / 4) - int($year / 100) + int($year / 400);
}

__END__
You are 15914 days old.