I've come across inconsistent behaviour between docs and actual implementation. For assigning date here (https://phalcon-php-framework-documentation.readthedocs.org/en/latest/reference/models.html#implementing-events-in-the-model-s-class) is advised to use either event manager
public function beforeCreate()
    {
        //Set the creation date
        $this->created_at = date('Y-m-d H:i:s');
    }or Behaviors
public function initialize()
    {
        $this->addBehavior(new Timestampable(
            array(
                'beforeCreate' => array(
                    'field' => 'created_at',
                    'format' => 'Y-m-d'
                )
            )
        ));
    }But in practice it doesn't work because of the failed validation! You have to use either
public function beforeValidationOnCreate()
{
    $this->created_at = date('Y-m-d H:i:s');
}or
protected function _preSave()
{
    $this->created_at = date('Y-m-d H:i:s');
}