def string_to_date(input):
    """Convert string to date object.

    :param input: the date string to parse
    :type input: str
    :returns: the parsed datetime object
    :rtype: datetime.datetime
    """
    # try date formats --mmdd, --mm-dd, yyyymmdd, yyyy-mm-dd and datetime
    # formats yyyymmddThhmmss, yyyy-mm-ddThh:mm:ss, yyyymmddThhmmssZ,
    # yyyy-mm-ddThh:mm:ssZ.
    for format_string in ("--%m%d", "--%m-%d", "%Y%m%d", "%Y-%m-%d",
                          "%Y%m%dT%H%M%S", "%Y-%m-%dT%H:%M:%S",
                          "%Y%m%dT%H%M%SZ", "%Y-%m-%dT%H:%M:%SZ"):
        try:
            return datetime.strptime(input, format_string)
        except ValueError:
            pass
    # try datetime formats yyyymmddThhmmsstz and yyyy-mm-ddThh:mm:sstz where tz
    # may look like -06:00.
    for format_string in ("%Y%m%dT%H%M%S%z", "%Y-%m-%dT%H:%M:%S%z"):
        try:
            return datetime.strptime(''.join(input.rsplit(":", 1)),
                                     format_string)
        except ValueError:
            pass
    raise ValueError