public function dateAdd($date, $interval, $datePart)
	{
		if (substr($interval, 0, 1) !== '-')
		{
			return "timestamp " . $date . " + interval '" . $interval . " " . $datePart . "'";
		}

		return "timestamp " . $date . " - interval '" . ltrim($interval, '-') . " " . $datePart . "'";
	}