The default value for timestamp is "0000-00-00 00:00:00", so
$e = explode('T', trim($value))
should be changed to
$e = explode(' ', trim($value))

public function validate($value)
{
if (is_null($value))

$e = explode('T', trim($value));
$date = isset($e[0]) ? $e[0]:null;
$time = isset($e[1]) ? $e[1]:null;

$dateValidator = Doctrine_Validator::getValidator('date');
$timeValidator = Doctrine_Validator::getValidator('time');

if ( ! $dateValidator->validate($date))

return true;
}