You can also use reflection techniques for building a new Post class with custom dates. Consider Mimic, a simple functional library for object prototyping, data hydration, and data exposition:

namespace Domain;

use mimic as m;

class ComputerScientist {
    private $name;
    private $surname;

    public function __construct($name, $surname) 
    {
        $this->name = $name;
        $this->surname = $surname;
    }

    public function rocks() 
    {
        return $this->name . ' ' . $this->surname . ' rocks!';
    }
}

assert(mprototype('DomainComputerScientist')
    instanceof DomainComputerScientist);

mhydrate('DomainComputerScientist', [
    'name'   =>'John' ,
    'surname'=>'McCarthy'
])->rocks(); //John McCarthy rocks!

assert(mexpose(
    new DomainComputerScientist('Grace', 'Hopper')) ==
    [
        'name'    => 'Grace' ,
        'surname' => 'Hopper'
    ]
)

If you want to know more about testing patterns and approaches, take a look at the book xUnit Test Patterns: Refactoring Test Code by Gerard Meszaros.