code: public function storeDateAdded($dc)
  	{
  		// Front end call
  		if (!$dc instanceof Contao\DataContainer)
  		{
  			return;
  		}
  
  		// Return if there is no active record (override all)
  		if (!$dc->activeRecord || $dc->activeRecord->dateAdded > 0)
  		{
  			return;
  		}
  
  		// Fallback solution for existing accounts
  		if ($dc->activeRecord->lastLogin > 0)
  		{
  			$time = $dc->activeRecord->lastLogin;
  		}
  		else
  		{
  			$time = time();
  		}
  
  		$this->Database->prepare("UPDATE tl_member SET dateAdded=? WHERE id=?")
  					   ->execute($time, $dc->id);
  	}
docs: Store the date when the account has been added
  
  @param Contao\DataContainer|Contao\FrontendUser $dc