use Time::Local;
my $str = '2007/07/17 13:21';
my @t   = $str =~ m!(\d{4})/(\d{2})/(\d{2})\s(\d{2}):(\d{2})!;
$t[1]--;
my $timestamp = timelocal 0,@t[4,3,2,1,0];
# verify...
print scalar localtime $timestamp;
__END__
Tue Jul 17 13:21:00 2007