def cache_filename(product, day):
    """Helper function to get the name of the cache

    :param product: product of the data to cache
    :type product: str
    :param day: day of the data to cache
    :type day: datetime.datetime
    :return: Filename of the cache
    :rtype: str
    """
    return f"cache-{product}-{day.isoformat()}"