<?php

class Timestamp
{
    private $value;

    public function __construct($value)
    {
        $this->value = $value;
    }

    public function updateTimestamps()
    {
        $this->value = new DateTime();
        $this->touch();
    }

    public function touch()
    {
        $this->value = new DateTime();
    }

    public function getValue()
    {
        return $this->value;
    }
}

class User
{
    /**
     * @var Timestamp
     */
    protected $createdAt;
    /**
     * @var Timestamp
     */
    protected $updatedAt;

    public function setCreateAt(Timestamp $timestamp)
    {
        $this->createdAt = $timestamp;
    }

    poublic function getCreateAt()
    {
        return $this->createdA;
    }
}

$user = new User();
$user->setCreateAt(new Timestamp(3123121));
echo $user->getCreateAt()->touch()->getValue();