public static function firstDayOfQuarter($ts = null)
	{
		// default to now
		if ($ts === null) $ts = sfDateTimeToolkit::now();
		
		// variables for computation
		$month = date('m', $ts);
		$quarter = ceil($month / 3) - 1; // zero based quarter
		
		return sfTime::subtractMonth(sfTime::firstDayOfMonth($ts), $month - ($quarter * 3) - 1);
	}