def date_pad(date_string):
    """
    accept a day or month like 2 or 5,
    then pad it, prefix it with a "0" string.
    input: 2018.2.8
    ouput: 2018.02.08
    """
    if len(date_string) < 2:
        return "0" + date_string
    else:
        return date_string