public function lastShard()
{
$key = file_get_contents($this->lastShardFile());
if (!array_key_exists($key, $this->_shards)) {
$date = new DateTime($key, self::UTC());
$shard = Shard::createFromDate($this, $date);
} else {
$shard = $this->_shards[$key];
}
return $shard;
}