private function AddSitemapCacheLifetimeField()
{
$name = 'SitemapCacheLifetime';
$value = $this->site->Exists() ? $this->site->GetSitemapCacheLifetime() : 24 * 60 * 60;
$field = Input::Text($name, $value);
$this->AddField($field);
$this->AddValidator($name, Integer::PositiveOrNull());
}