public static function parsePeriod(string $annotation)
	{
		$period = NULL;
		$annotation = Strings::trim($annotation);
		if (Strings::length($annotation)) {
			if (strtotime('+ ' . $annotation) === FALSE) {
				throw new InvalidParameterException(
					"Given period parameter '" . $annotation . "' must be valid for strtotime() with '+' sign as its prefix (added by Cronner automatically)."
				);
			}
			$period = $annotation;
		}

		return $period ?: NULL;
	}