Person entity with mandatory birthdate that can either be set or uknown

BirthDate.php

<?php

final class BirthDate
{
    private $date;

    private function __construct(\DateTimeInterface $date = null)
    {
        $this->date = $date;
    }

    public static function on(\DateTimeInterface $date) : self
    {
        return new self($date);
    }

    public static function unknown() : self
    {
        return new self();
    }

    public function toString() : string
    {
        return $this->date ? $this->date->format('Y-m-d') : 'unknown';
    }
}

PersonV1.php

<?php

final class Person
{
    private $firstName;

    private $lastName;

    private $birthDate;

    public function __construct(Firstname $firstName, Lastname $lastName, BirthDate $birthDate)
    {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
        $this->birthDate = $birthDate;
    }

    public function getFirstName() : Firstname
    {
        return $this->firstName;
    }

    public function getLastName() : Lastname
    {
        return $this->lastName;
    }

    public function getBirthDate() : BirthDate
    {
        return $this->birthDate;
    }
}

PersonV2.php

<?php

final class Person
{
    private $firstName;

    private $lastName;

    private $birthDate;

    public function __construct(Firstname $firstName, Lastname $lastName, BirthDate $birthDate = null)
    {
        $this->firstName = $firstName;
        $this->lastName = $lastName;
        $this->birthDate = $birthDate;
    }

    public function getFirstName() : Firstname
    {
        return $this->firstName;
    }

    public function getLastName() : Lastname
    {
        return $this->lastName;
    }
  
    public function hasBirthDate() : bool
    {
        return null !== $this->birthDate;
    }

    public function getBirthDate() : BirthDate
    {
        if (! $this->hasBirthDate()) {
            throw new BirthDateNotSetException();
        }
        
        return $this->birthDate;
    }
}