public function addCalendarType($type, $calendarType)
	{
		if (is_string($calendarType) and !class_exists($calendarType)) {
			throw new \InvalidArgumentException("Class {$calendarType} does not exist.");
		} elseif (!in_array('Plummer\Calendarful\Calendar\CalendarInterface', class_implements($calendarType, false))) {
			throw new \InvalidArgumentException('File or File path required.');
		}

		$this->calendarTypes[$type] = is_string($calendarType) ?
			$calendarType :
			get_class($calendarType);
	}