How to save the date in oracle?

You can define a beforeSave function in your model to format the date as you need.

Something like this or what would be the best way to validate the format

public function beforeSave()
{
    // Set the modification date
    $this->modified_in = date('Y-m-d H:i:s');
}

Yes that's the way to validate data and more with validators :
https://docs.phalcon.io/en/latest/reference/models.html

public function afterFetch() { $this->start = new DateTime($this->start); }

  public function beforeSave()
  {
      $this->start = $this->start->format("Y-m-d H:i:s");
  }

That should work, or you can use Phalcon\Mvc\Model\Behavior\Timestampable :

public function initialize()
{
    $this->addBehavior(new Timestampable([
        'beforeSave'  => [
            'field'     => 'start',
            'format'    => 'Y-m-d H:i:s'
        ]
    ]));
}