my %datehash;
@datehash{ qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec) } =
    map sprintf( '%02d', $_), 1 .. 12;

my $mon_re = join '|', keys %datehash;

while ( <DATA> ) {
    my ( $mon, $day, $year) = /($mon_re)\s+(\d\d?)\s+(\d{4})/;
    $day = sprintf '%02d', $day;
    print "$year$datehash{ $mon}$day\n";
}


__DATA__
Oct 16 2004 11:09:19:943AM
Mar 3 2007 10:30:31:170PM