protected function applyMask($unit)
{
$cron = explode(' ', $this->expression);
$mask = ['0', '0', '1', '1', '*', '*'];
$fpos = $this->fieldsPosition[$unit] - 1;
array_splice($cron, 0, $fpos, array_slice($mask, 0, $fpos));
return $this->cron(implode(' ', $cron));
}