def split_date_column(df):
    """Split date column into date and time, and reorder columns.

    Parameters
    ----------
    df: pandas.Dataframe

    Returns
    -------
    df: pandas.Dataframe

    Example
    -------
    >>> filename= "./data/WIN$N_1M_2021.01.20_2020.04.24.csv"
    >>> df = read_csv(filename)
    >>> df = split_date_column(df)
    """
    columns = [s.strip() for s in df.columns]
    if "hour" in columns:
        return df
    elif "date" in columns:
        df[["date", "hour"]] = df['date'].str.split(' ', 1, expand=True)
        cols = df.columns
        cols = cols[:-1].insert(1, cols[-1])
        return df[cols]