core-bundle/src/Resources/contao/dca/tl_member.php
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, $dc->id);
	}
public function storeDateAdded($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);
	}
Store the date when the account has been added

@param Contao\DataContainer|Contao\FrontendUser $dc