protected function calculateSeconds( $matches )
	{
		$value	= $matches[1];
		$format	= $matches[2];
		switch( $format )
		{
			case 's': 	return $value;
			case 'm': 	return $value * 60;
			case 'h': 	return $value * 60 * 60;
			case 'D': 	return $value * 60 * 60 * 24;
			case 'W': 	return $value * 60 * 60 * 24 * 7;
			case 'M': 	return $value * 60 * 60 * 24 * 30.4375;
			case 'Y': 	return $value * 60 * 60 * 24 * 365.25;
		}
		throw new Exception( 'Unknown date format "'.$format.'"' );
	}