It looks like this method is a bit overhead as it will be called on one position.

As idea: `MongoDate` supports milliseconds. This could be used to make TTL more precision:
```php
$expiresMicro = microtime(true) + $ttl;
$expiresSecs  = (int)$expiresMicro;
$expiresMongo = new MongoDate($expiresSecs, $expiresMicro - $expiresSecs);
```