use strict;
use warnings;

my $date = 'Wed Dec 29 08:17:04';
$date = convert($date);
print $date;

BEGIN {
    my %c_mon = ('Jan',1,'Feb',2,'Mar',3,'Apr',4,'May',5,'Jun',6,
               'Jul',7,'Aug',8,'Sep',9,'Oct',10,'Nov',11,'Dec',12);
    sub convert {
        my ($mon, $day, $rest) = $_[0] =~ /\w+ (\w+) (\d+) (.+)/; $mon = $c_mon{$mon};
        @_ = localtime(); $_[4]++; my $year = $_[5] % 100;
        $year-- if "$mon$day" gt "$_[4]$_[3]"; $year = 99 if $year == -1;
        return sprintf('%02d', $year) . "-$mon-$day\_$rest";
    }
}

##</code><code>##

use strict;
use warnings;

@_ = localtime();
my $date = sprintf('%02d-%02d-%02d_%02d:%02d:%02d',
                    $_[5] % 100, $_[4] + 1, $_[3], $_[2], $_[1], $_[0]);
print $date;