def is_leap_year(year: str) -> bool:
    """
    Helper function used to determine if a string is a leap year or not.
    :param year: The year to check.
    :return: True if a leap year, false otherwise.
    """
    if int(year) % 4 == 0:
        if int(year) % 100 == 0:
            if int(year) % 400 == 0:
                return True
            return False
        return True
    return False