def time_series_delta(df):
    """From a DataFrame with columns consisting of time series,
    returns a DataFrame with the changes per unit of time
    in each column.

    Parameters
    ----------
    df : DataFrame
        A DataFrame with a datetime index.

    Returns
    -------
    df_delta : DataFrame
        A Dataframe of differences per unit of time.
    """
    df_delta = df - df.shift(1)
    df_delta = df_delta.dropna()
    return df_delta