Is it possible to validate a property of a model class dependent on another property of the same class?

For example, I have this class:

```
class Conference
{
    /** $startDate datetime */
    protected $startDate;

    /** $endDate datetime */
    protected $endDate;
}

```

and I want that Symfony 2.0 validates, that `$startDate` has to be after `$endDate`.

Is this possible by annotations or do I have to do this manually?