def date_in_yymmdd(date_string):
    """
    Convert date string YYYYMMDD into YY_MM_DD format to be used for
    drive log file names.

    Parameters
    ----------
    date_string: in format YYYYMMDD

    Returns
    -------
    yy_mm_dd: date_string in format YY_MM_DD

    """
    date = list(date_string)
    year = "".join(date[2:4])
    month = "".join(date[4:6])
    day = "".join(date[6:8])
    return f"{year}_{month}_{day}"